**3. Results**

#### **3.1. Run-off**

As slope steepness and slope length are the same for all treatments, run-off is thus expected to be mainly dependent on the amount, distribution and intensity of seasonal rainfall, infil‐ tration rate, which is directly influenced by tillage and ground cover. A tillage system that either maintains a good soil structure, or inhibits run-off velocity and raindrop impact, or forces water to pond, or has a good ground cover (mulch or crop) tends to have a higher infiltration rate and therefore, lower run-off.

The highest run-off was recorded under bare fallow, with a run-off range of 17 – 39% of total seasonal rainfall. Conventional tillage recorded the second highest average run-off ranging from 13 to 22% of total seasonal rainfall. This could be attributed to a somewhat better infil‐ tration rate at the beginning of the season, as the soil would be loose. The best treatments in conserving water were mulch ripping and tied ridging, which had run-off ranges of 9 - 15% and 1 - 11% respectively. The mulch cover has all the positive attributes that have been high‐ lighted, i.e., reducing raindrop impact thus inhibiting soil capping, reducing run-off velocity and increasing water infiltration. Under tied ridging, run-off is also contained at low levels by way of water ponding. The micro-dams force water to pond - thus increasing infiltration - until all the micro-dams are full and start overtopping along the ridges, allowing very little water to leave the system.

Table 1 shows ANOVA results between treatments and as influenced by year. Note that the year x treatment interaction was mainly due to the differences in rainfall amount. Run-off differed significantly between treatments at P < 0.001. To properly evaluate the effectiveness of the conservation tillage treatments, the mean of conventional tillage versus the mean of the two conservation tillage treatments was compared using an independent t-test. The re‐ sults of this test are given in Table 1. Despite the overall high significant variation between the treatments, it was established that this difference was only between conventional tillage and the two conservation tillage treatments (mulch ripping and tied ridging). There was, however no significant difference between the two conservation tillage treatments. This finding confirms that both mulch ripping and tied ridging treatments are effective in reduc‐ ing run-off when compared to conventional tillage.


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

*2.7.6. Nutrients dissolved in run-off*

in the case of K.

122 Research on Soil Erosion Soil Erosion

0.001 (\*\*\*).

**3. Results**

**3.1. Run-off**

infiltration rate and therefore, lower run-off.

water to leave the system.

**2.8. Statistical analyses**

Run-off was filtered and the aliquot treated as soil extract, where the nutrient concentra‐ tion was either titrated with boric acid, for N determination, read from an Atomic Ab‐ sorption Spectrophotometer for the determination of P or read from a flamephotometer

The differences in soil loss, run-off, plant growth parameters and yield attributed to treat‐ ment were analyzed with the analysis of variance (ANOVA) procedure of Genstat 5 Release 1.3 statistical package. An independent t-test was used to compare the means of different populations. Unless otherwise indicated, significance is indicated at P < 0.05 (\*), 0.01(\*\*) to

As slope steepness and slope length are the same for all treatments, run-off is thus expected to be mainly dependent on the amount, distribution and intensity of seasonal rainfall, infil‐ tration rate, which is directly influenced by tillage and ground cover. A tillage system that either maintains a good soil structure, or inhibits run-off velocity and raindrop impact, or forces water to pond, or has a good ground cover (mulch or crop) tends to have a higher

The highest run-off was recorded under bare fallow, with a run-off range of 17 – 39% of total seasonal rainfall. Conventional tillage recorded the second highest average run-off ranging from 13 to 22% of total seasonal rainfall. This could be attributed to a somewhat better infil‐ tration rate at the beginning of the season, as the soil would be loose. The best treatments in conserving water were mulch ripping and tied ridging, which had run-off ranges of 9 - 15% and 1 - 11% respectively. The mulch cover has all the positive attributes that have been high‐ lighted, i.e., reducing raindrop impact thus inhibiting soil capping, reducing run-off velocity and increasing water infiltration. Under tied ridging, run-off is also contained at low levels by way of water ponding. The micro-dams force water to pond - thus increasing infiltration - until all the micro-dams are full and start overtopping along the ridges, allowing very little

Table 1 shows ANOVA results between treatments and as influenced by year. Note that the year x treatment interaction was mainly due to the differences in rainfall amount. Run-off differed significantly between treatments at P < 0.001. To properly evaluate the effectiveness of the conservation tillage treatments, the mean of conventional tillage versus the mean of the two conservation tillage treatments was compared using an independent t-test. The re‐ sults of this test are given in Table 1. Despite the overall high significant variation between the treatments, it was established that this difference was only between conventional tillage **Table 1.** Run-off (mm) as affected by tillage and year (rainfall) and their interactions at MakoholiContill site during three seasons

The amount of run-off recorded during the different years also differed significantly at P < 0.001. This was due to the high variation in rainfall amounts received during the three sea‐ sons. Year 1 received close to twice the rainfall amount received during Year 2. Run-off in‐ creased by more than six times, due to the concentration of rainfall in January, inducing saturated conditions, which led to high run-off. As a result of this highly significant seasonal variation an independent t test was carried out on the means of the different years. The re‐ sults showed that the 100 mm difference between Year 1 and Year 2 resulted in significantly different run-off levels, at P < 0.05, while run-off from Year 3 differed significantly (P < 0.001) from the mean of that of Year 1 and Year 2. There was no significant difference for the interaction between the treatment and the year (P = 0.145).

The significant difference between the years further prompted an analysis of variance to es‐ tablish how treatments varied within the individual years (Table 1). The overall run-off treatment differences were significant at P < 0.01 for the Year 1 and Year 3. A higher overall significant treatment difference was found for Year 2 indicating that the differences in runoff become more pronounced if seasonal rainfall amount was low than during wetter sea‐ sons. During wet seasons, run-off was also higher under the conservation tillage systems as they reached saturation point faster due to the already high residual soil moisture. An inde‐ pendent t-test showed that conventional tillage differed highly significantly from the mean of conservation tillage treatments throughout the three seasons. Mulch ripping and tied ridging, however did not differ significantly in any one of the seasons. This finding further emphasizes the water conservation potential of mulch ripping and tied ridging and also shows that a lot more rain water is lost under conventional tillage.

son between the treatment means also confirmed a significant variation (P < 0.001), between conventional tillage and the mean of mulch ripping and tied ridging. There was no signifi‐ cant difference between mulch ripping and tied ridging. As soil loss is a function of run-off, the increase of soil loss with the increase in the number of years of cultivation was expected and the same range of factors that affected run-off should be responsible for these increases

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The average mechanical composition of the sediments collected over the three years is shown in Table 3. Only clay and silt fractions are given. The sediments from the conserva‐ tion tillage treatments comprised of more clay, i.e. more suspended material as compared to sludge (coarse material), while under the conventional tillage systems more sludge was lost compared to suspended material. There was very little clay/ silt found in sludge, while the suspended material hardly contained any sand fraction, i.e. over 90% of the suspended ma‐ terial was found to be clay and silt fractions. It is clear that the suspended material compris‐ es of the most reactive soil particles (clay, silt and organic matter) and thus its loss is most detrimental to the soils' productivity as compared to sludge. Furthermore, the total sedi‐ ments (sludge + suspended material) had higher clay and silt contents when compared to

The ratios between these two sediment components were worked out for the different tillage systems (Table 3). The results show that 10 - 17 times more sludge than suspended material was found under the bare fallow, while the ratio ranged between 1.5 and 5 under conven‐ tional tillage and below 1, under the conservation tillage treatments (mulch ripping and tied ridging). This is an indication that not so much soil is moved during erosion under these treatments, while under bare fallow, mass movement is realized. The impediments created under the two conservation tillage systems ensured that the run-off velocity was reduced thus allowing no sheet wash but only the suspended soil particles to leave the system.

Figure 2 shows the actual amount of clay lost with sludge, suspension and with sediments as a whole. Although there was a lower percentage of suspended material as compared to sludge, under the bare fallow and conventional tillage, the actual amount of clay lost with suspension exceeds that lost with sludge. The clay amount lost with sediments showed the

**•** the highest amount of clay was lost under the bare fallow (an average of 7 t/ha/yr.) fol‐ lowed by conventional tillage (5 t/ha/yr.) and only a negligible amount was lost under two conservation tillage treatments, i.e. 0.9 and 0.8 t/ha/yr. for mulch ripping and tied

**•** the amount of clay lost with suspension followed the same trend as that of the total sedi‐ ments although the differences between bare fallow and conventional tillage were rela‐ tively smaller (Figure 1). The significant difference among the treatments was realized in

in soil loss.

the original soil.

following trends:

ridging respectively

the clay amount lost with sludge.

**3.3. Particle size distribution of the sediments**

#### **3.2. Soil loss**

Soil losses followed the same trend as rainfall, especially under the bare fallow, where there was no ground cover (Table 2). The highest soil losses were recorded under bare fallow, averaging 93 t/ha/yr. Soil losses under conventional tillage averaged 34 t/ha/yr, while mulch ripping and tied ridging recorded soil loss averages of 1.7 and 3.3 t/ha/yr respectively. The importance of crop cover on soil erosion is shown by the different cropped treatments, espe‐ cially conventional tillage, where the reduction in erosion (34 t/ha/yr from that of bare fal‐ low, 93 t/ha/yr) is attributed to cover alone and not tillage system. Overall, the treatments differed significantly at P < 0.001. Independent t-tests showed that conventional tillage dif‐ fered highly significantly from the two conservation tillage treatments, while there was no significant difference between the conservation treatments. This finding, tallies with the runoff results and is in accordance with expectations as soil loss is a function of run-off.


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 2.** Soil losses (t/ha) as affected by tillage and year (rainfall) and their interactions at MakoholiContill site during three seasons

Year had a significant effect on soil loss (P < 0.001) due to the varied seasonal rainfall totals. The Years 1 and 2 varied significantly at P < 0.001. When the mean of these two seasons was compared with the mean of Year 3, the difference also varied significantly at P < 0.001. The influence of rainfall on soil loss is apparent, as the season with the highest rainfall also re‐ corded the highest soil loss and vice versa. The analysis of variance during the individual years, gave a significant overall treatment difference at P < 0.001 across all years. A compari‐ son between the treatment means also confirmed a significant variation (P < 0.001), between conventional tillage and the mean of mulch ripping and tied ridging. There was no signifi‐ cant difference between mulch ripping and tied ridging. As soil loss is a function of run-off, the increase of soil loss with the increase in the number of years of cultivation was expected and the same range of factors that affected run-off should be responsible for these increases in soil loss.

#### **3.3. Particle size distribution of the sediments**

of conservation tillage treatments throughout the three seasons. Mulch ripping and tied ridging, however did not differ significantly in any one of the seasons. This finding further emphasizes the water conservation potential of mulch ripping and tied ridging and also

Soil losses followed the same trend as rainfall, especially under the bare fallow, where there was no ground cover (Table 2). The highest soil losses were recorded under bare fallow, averaging 93 t/ha/yr. Soil losses under conventional tillage averaged 34 t/ha/yr, while mulch ripping and tied ridging recorded soil loss averages of 1.7 and 3.3 t/ha/yr respectively. The importance of crop cover on soil erosion is shown by the different cropped treatments, espe‐ cially conventional tillage, where the reduction in erosion (34 t/ha/yr from that of bare fal‐ low, 93 t/ha/yr) is attributed to cover alone and not tillage system. Overall, the treatments differed significantly at P < 0.001. Independent t-tests showed that conventional tillage dif‐ fered highly significantly from the two conservation tillage treatments, while there was no significant difference between the conservation treatments. This finding, tallies with the run-

off results and is in accordance with expectations as soil loss is a function of run-off.

**Year 3 (765mm)**

**Table 2.** Soil losses (t/ha) as affected by tillage and year (rainfall) and their interactions at MakoholiContill site during

Year had a significant effect on soil loss (P < 0.001) due to the varied seasonal rainfall totals. The Years 1 and 2 varied significantly at P < 0.001. When the mean of these two seasons was compared with the mean of Year 3, the difference also varied significantly at P < 0.001. The influence of rainfall on soil loss is apparent, as the season with the highest rainfall also re‐ corded the highest soil loss and vice versa. The analysis of variance during the individual years, gave a significant overall treatment difference at P < 0.001 across all years. A compari‐

CT 40.2 6.8 54.0 33.7 Treat \*\*\* MR 0.2 0.1 4.8 1.7 Year \*\*\* TR 3.0 0.1 3.5 2.2 Treat x Year \*\*\* BF 84.1 43.5 152.5 93.4 MR vs TR NS Overall mean 31.9 12.6 53.7 32.7 CT vs (MR, TR) \*\*\* n = 9 (Treatment) s.e.d. = 4.00 s2 = 71.83 Yr 1 vs Yr 2 \*\*\* n = 12 (Year) s.e.d. = 3.46 df = 24 Yr 3 vs (Yrs 1,2) \*\*\*

**Overall mean (t/ha)**

**Source of variation Soil loss**

**Year 2 (384mm)**

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

shows that a lot more rain water is lost under conventional tillage.

**3.2. Soil loss**

124 Research on Soil Erosion Soil Erosion

**Treat/Year (Rainfall) Year 1**

n = 3 (Treatment x Year)

three seasons

**(483mm)**

s.e.d. = 6.92

The average mechanical composition of the sediments collected over the three years is shown in Table 3. Only clay and silt fractions are given. The sediments from the conserva‐ tion tillage treatments comprised of more clay, i.e. more suspended material as compared to sludge (coarse material), while under the conventional tillage systems more sludge was lost compared to suspended material. There was very little clay/ silt found in sludge, while the suspended material hardly contained any sand fraction, i.e. over 90% of the suspended ma‐ terial was found to be clay and silt fractions. It is clear that the suspended material compris‐ es of the most reactive soil particles (clay, silt and organic matter) and thus its loss is most detrimental to the soils' productivity as compared to sludge. Furthermore, the total sedi‐ ments (sludge + suspended material) had higher clay and silt contents when compared to the original soil.

The ratios between these two sediment components were worked out for the different tillage systems (Table 3). The results show that 10 - 17 times more sludge than suspended material was found under the bare fallow, while the ratio ranged between 1.5 and 5 under conven‐ tional tillage and below 1, under the conservation tillage treatments (mulch ripping and tied ridging). This is an indication that not so much soil is moved during erosion under these treatments, while under bare fallow, mass movement is realized. The impediments created under the two conservation tillage systems ensured that the run-off velocity was reduced thus allowing no sheet wash but only the suspended soil particles to leave the system.

Figure 2 shows the actual amount of clay lost with sludge, suspension and with sediments as a whole. Although there was a lower percentage of suspended material as compared to sludge, under the bare fallow and conventional tillage, the actual amount of clay lost with suspension exceeds that lost with sludge. The clay amount lost with sediments showed the following trends:



The clay enrichment ratios for the total sediments (clay content in soil: clay content in sedi‐ ments) show that the sediments have distinctly more clay than the original soil (Table 4). In all cases the bare fallow had enrichment ratios of less than 2, while under conventional till‐ age the ratio ranged between 2.8 and 6.0. The two conservation tillage treatments recorded the highest enrichment ratios, as expected, of between 12.4 and 14.5 for mulch ripping and 13.8 - 19.0 for tied ridging. The very high clay enrichment ratios found under conservation tillage treatments indicate the very low run-off velocity which only carries suspended mate‐ rial but has not enough energy to erode and carry coarse particles, as is the case under con‐ ventional tillage and the bare fallow. Table 4 also shows that clay content in the sediments varied highly significantly between the treatments (P < 0.001), while there was no difference

BF 6.36 5.20 1.41 CT 11.63 3.99 2.78 MR 58.83 0.10 13.68 TR 50.00 0.77 13.81

BF 5.50 2.39 1.22 CT 25.07 1.67 5.98 MR 62.50 0.05 14.53 TR 50.00 0.07 13.81

BF 8.45 12.87 1.87 CT 14.68 7.94 3.50 MR 53.39 2.60 12.42 TR 68.84 1.43 19.02

**Table 4.** Clay loss with sediments and its enrichment ratios for the different tillage systems during three seasons at

**Clay t/ha**

Quantifying Nutrient Losses with Different Sediment Fractions Under Four Tillage Systems...

\*\*\* 2.676

\*\*\* 2.320 **Clay Enrichment Ratio**

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(P = 0.966) between the sediment composition over the years

**%**

Source of variation Clay % in sediments Clay (t/ha)

\*\*\* 5.65

NS 19.23

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Treat./Year Clay in total sediments**

Year 1

Year 2

Year 3

Treatment s.e.d.

> Year s.e.d.

MakoholiContill site

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 3.** Relationship between sludge and suspended material in erosion sediments from four tillage systems over three years at MakoholiContill site

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Figure 2.** Average clay loss with sediments during three years at MakoholiContill site with sludge, suspension and to‐ tal sediments

The clay enrichment ratios for the total sediments (clay content in soil: clay content in sedi‐ ments) show that the sediments have distinctly more clay than the original soil (Table 4). In all cases the bare fallow had enrichment ratios of less than 2, while under conventional till‐ age the ratio ranged between 2.8 and 6.0. The two conservation tillage treatments recorded the highest enrichment ratios, as expected, of between 12.4 and 14.5 for mulch ripping and 13.8 - 19.0 for tied ridging. The very high clay enrichment ratios found under conservation tillage treatments indicate the very low run-off velocity which only carries suspended mate‐ rial but has not enough energy to erode and carry coarse particles, as is the case under con‐ ventional tillage and the bare fallow. Table 4 also shows that clay content in the sediments varied highly significantly between the treatments (P < 0.001), while there was no difference (P = 0.966) between the sediment composition over the years

**Year/Treat. Soil loss (t/ha) Ratio Clay content (%) Silt content (%)**

BF 74.54 7.28 10.24 1.32 58.00 2.93 37.87 CT 27.97 6.33 4.42 1.22 57.69 3.21 34.40 MR 0.00 0.17 0.00 0.00 59.93 4.10 36.75 TR 0.38 1.16 0.33 0.81 66.68 4.59 33.32

BF 41.09 2.37 17.34 2.78 52.56 1.58 29.55 CT 4.12 2.70 1.53 2.79 57.81 2.69 36.45 MR 0.00 0.08 0.00 0.00 59.75 0.00 38.02 TR 0.04 0.10 0.40 2.01 69.95 1.76 43.57

BF 139.02 13.36 10.41 3.02 64.89 1.48 27.66 CT 45.66 8.44 5.41 3.15 76.97 2.22 34.27 MR 1.84 3.03 0.61 4.00 83.43 2.44 17.25 TR 0.65 2.88 0.23 4.02 83.40 4.12 6.05

**Table 3.** Relationship between sludge and suspended material in erosion sediments from four tillage systems over

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Figure 2.** Average clay loss with sediments during three years at MakoholiContill site with sludge, suspension and to‐

Year 1 (483 mm)

126 Research on Soil Erosion Soil Erosion

Year 2 (384 mm)

Year 3 (765 mm)

three years at MakoholiContill site

tal sediments

**Sludge Susp Slud:Susp Sludge Susp Sludge Susp**


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 4.** Clay loss with sediments and its enrichment ratios for the different tillage systems during three seasons at MakoholiContill site

#### **3.4. Organic matter loss with sediments**

The original organic matter content of the virgin soils averaged approximately 0.8%, (Table 5). After continuous cultivation for five years the organic matter content was found to have declined by 25% under the bare fallow; 19% under the conventional tillage; 6% under mulch ripping and 9% under tied ridging. This finding shows that with continuous cultivation the organic matter status of these soils decreases, more so if no plant residues are left in the field, e.g. bare fallow. The higher organic matter content under conventional tillage, com‐ pared to bare fallow, is a result of roots left behind after harvest. Tied ridging combines this effect with that of soil conservation to give an even better maintenance of organic matter. The best effect is, however, achieved under mulch ripping, where roots together with plant residues and soil conservation effects contribute to better organic matter maintenance, thus only 6% was lost. The mineralization of organic matter after cultivation is expected to take place but by further addition of mulch the depreciation rate is lowered drastically. Reduced tillage in the mulch ripping treatment, as compared to other treatments, further contributes to conservation of organic matter.

Under the conventional tillage treatments organic matter losses were much higher than with conservation tillage. Whereas most organic matter in conservation tillage was lost in suspen‐ sion, losses with conventional tillage were more evenly distributed between suspension and

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**Treatm. OM content (%) Enrichment ratio**

**Soil Sediments**

BF 0.54 1.63 3.02 CT 0.68 2.24 3.29 MR 0.80 5.36 6.70 TR 0.63 2.82 4.48

BF 0.54 1.05 1.94 CT 0.68 1.27 1.87 MR 0.80 2.88 3.60 TR 0.63 1.22 1.94

BF 0.54 1.49 2.76 CT 0.68 2.13 3.13 MR 0.80 2.69 3.36 TR 0.63 2.34 3.71

**Table 6.** Organic matter contents of the soils and sediments and calculated enrichment ratios for four tillage

The amount of organic matter lost varied significantly (P < 0.001) among all treatments. Con‐ trasting the different systems against one another showed that conventional tillage did not differ significantly from the bare fallow. The mean of conventional tillage and bare fallow, however, differed significantly at P < 0.001, with that of mulch ripping and tied ridging, in‐ dicating that the two conservation tillage treatments are very effective in conserving and/or maintaining soil organic matter. When conventional tillage was also compared to the mean of the conservation tillage treatments, the difference was significant, P < 0.001. Finally the two conservation treatments were compared within the group and they were not signifi‐ cantly different. As it is important to show which of the two conservation treatments per‐ forms better than the other, an independent t-test was carried out, i.e., disregarding the other two treatments altogether and comparing the two conservation treatments only. The

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

treatments, over three seasons at MakoholiContill site

sludge, because of the very high sludge losses, see Table 7.

Year 1

Year 2

Year 3


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 5.** Soil organic matter content of the soils (0 - 25 cm depth) for different tillage systems as at opening from virgin land and five years later at MakoholiContill site

The concentration of organic matter in sediments was higher for conservation tillage sys‐ tems than for conventional tillage and bare fallow. This resulted in higher enrichment ratios (organic matter content in soil: organic matter content in sediments) for conservation tillage systems. Mulch ripping and tied ridging recorded enrichment ratios of 4.6 and 3.4 respec‐ tively, while conventional tillage recorded an enrichment ratio of 2.8 and bare fallow 2.6 (Table 6). The total amount of organic matter lost with conservation tillage was, however, only a fraction of that lost from conventional tillage and bare fallow. Under bare fallow an annual average of 424 kg/ha was lost, while under conventional tillage 299 kg/ha were lost, mulch ripping lost only 55 kg/ha and tied ridging 61 kg/ha/yr (Table 7). It is quite obvious that although the organic matter concentrations in sediments of bare fallow and convention‐ al tillage are quite low, the extensive losses of soil contributed to a tremendous total loss. Higher contents for conventional tillage as compared to bare fallow show the contribution of roots to the soil organic matter, thus a higher depreciation is apparent under bare fallow, where no crops are grown.

Under the conventional tillage treatments organic matter losses were much higher than with conservation tillage. Whereas most organic matter in conservation tillage was lost in suspen‐ sion, losses with conventional tillage were more evenly distributed between suspension and sludge, because of the very high sludge losses, see Table 7.

**3.4. Organic matter loss with sediments**

128 Research on Soil Erosion Soil Erosion

to conservation of organic matter.

**Treatment Virgin land**

virgin land and five years later at MakoholiContill site

where no crops are grown.

**OM %**

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

The original organic matter content of the virgin soils averaged approximately 0.8%, (Table 5). After continuous cultivation for five years the organic matter content was found to have declined by 25% under the bare fallow; 19% under the conventional tillage; 6% under mulch ripping and 9% under tied ridging. This finding shows that with continuous cultivation the organic matter status of these soils decreases, more so if no plant residues are left in the field, e.g. bare fallow. The higher organic matter content under conventional tillage, com‐ pared to bare fallow, is a result of roots left behind after harvest. Tied ridging combines this effect with that of soil conservation to give an even better maintenance of organic matter. The best effect is, however, achieved under mulch ripping, where roots together with plant residues and soil conservation effects contribute to better organic matter maintenance, thus only 6% was lost. The mineralization of organic matter after cultivation is expected to take place but by further addition of mulch the depreciation rate is lowered drastically. Reduced tillage in the mulch ripping treatment, as compared to other treatments, further contributes

> **After five of cultivation OM %**

BF 0.72 0.53 24.9 CT 0.84 0.68 18.9 MR 0.85 0.80 5.7 TR 0.70 0.64 9.1

**Table 5.** Soil organic matter content of the soils (0 - 25 cm depth) for different tillage systems as at opening from

The concentration of organic matter in sediments was higher for conservation tillage sys‐ tems than for conventional tillage and bare fallow. This resulted in higher enrichment ratios (organic matter content in soil: organic matter content in sediments) for conservation tillage systems. Mulch ripping and tied ridging recorded enrichment ratios of 4.6 and 3.4 respec‐ tively, while conventional tillage recorded an enrichment ratio of 2.8 and bare fallow 2.6 (Table 6). The total amount of organic matter lost with conservation tillage was, however, only a fraction of that lost from conventional tillage and bare fallow. Under bare fallow an annual average of 424 kg/ha was lost, while under conventional tillage 299 kg/ha were lost, mulch ripping lost only 55 kg/ha and tied ridging 61 kg/ha/yr (Table 7). It is quite obvious that although the organic matter concentrations in sediments of bare fallow and convention‐ al tillage are quite low, the extensive losses of soil contributed to a tremendous total loss. Higher contents for conventional tillage as compared to bare fallow show the contribution of roots to the soil organic matter, thus a higher depreciation is apparent under bare fallow,

**OM reduction %**


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 6.** Organic matter contents of the soils and sediments and calculated enrichment ratios for four tillage treatments, over three seasons at MakoholiContill site

The amount of organic matter lost varied significantly (P < 0.001) among all treatments. Con‐ trasting the different systems against one another showed that conventional tillage did not differ significantly from the bare fallow. The mean of conventional tillage and bare fallow, however, differed significantly at P < 0.001, with that of mulch ripping and tied ridging, in‐ dicating that the two conservation tillage treatments are very effective in conserving and/or maintaining soil organic matter. When conventional tillage was also compared to the mean of the conservation tillage treatments, the difference was significant, P < 0.001. Finally the two conservation treatments were compared within the group and they were not signifi‐ cantly different. As it is important to show which of the two conservation treatments per‐ forms better than the other, an independent t-test was carried out, i.e., disregarding the other two treatments altogether and comparing the two conservation treatments only. The results showed a significant difference at P < 0.05, where lower losses were found under the mulch ripping treatment.

(BF) and 60 - 94% (CT) of the total organic matter lost. The soil lost under mulch ripping was almost entirely in suspended form. This resulted in almost all the organic matter (97 - 100%) being lost with suspended material. Under tied ridging most of the soil was also lost as sus‐ pension, 75% of the total soil loss, with 97% of total organic matter loss. The enrichment ra‐ tios are thus very high for the suspended soil consisting mainly of clay and silt. The sludge has lower enrichment ratios and the organic matter contents are even less than of the origi‐

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131

Many scientists have reported on the selective nature of soil erosion (Aylen, 1939; Hudson and Jackson, 1962; Shaxson, 1975; Elwell, 1987; Lal, 1988 among others), however the extent to which the soils have been impoverished or the sediments enriched have mainly been esti‐ mated. This study showed that sheet erosion is selective as sediments recorded higher con‐ tents of clay, silt and organic matter than the original soil. The particle size distribution of sludge was mainly coarse sand and had a maximum of 4% clay. Suspended material on the other hand had up to 83% clay, the rest mainly being the silt fraction. Sediments from con‐ servation tillage systems comprised of more sand than the clay fraction, bare fallow had 13 times and conventional tillage 4 times less sludge than suspension. Due to the high losses of coarse material under the conventional tillage, the clay enrichment ratio of sediments was lower than under the conventional tillage systems. The bare fallow recorded only 1.5 and conventional tillage 4.1 times more clay in the sediments. Under the conservation tillage sys‐ tems more soil was lost in suspension than as sludge. Mulch ripping recorded 0.2 times and tied ridging 0.3 times more sludge than suspension, resulting in higher clay enrichment ra‐ tios of 13.5 and 15.5 under mulch ripping and tied ridging respectively. However, these high enrichment ratios amounted to less amount of clay lost from conservation tillage compared to conventional tillage, due to the reduced total sediments lost under the conservation till‐ age systems. Under bare fallow an average of 7 t/ha of clay were lost, 5 t/ha under conven‐ tional tillage and 0.9 and 0.8 t/ha under mulch ripping and tied ridging respectively. The very high enrichment ratios under the conservation tillage systems are a reflection of the soil losses, which were lost mainly as suspended material, which constitutes fine soil particles, however the very low amount of soil led to negligible losses of total clay loss with sedi‐

This selective nature of erosion manifested itself in the high enrichment ratios of the sedi‐ ments as compared to the original soil. This means that soil fertility is affected severely by the soil lost in suspension, as it constitutes mainly of clay and organic matter fractions, which are the main sources of nutrients (Stocking, 1983). The soil structure and water hold‐ ing capacity are also affected as these soil fractions are responsible for soil aggregation and influence the water dynamics of the soil (Follet*et al*., 1987; Stocking and Peake, 1987). The sludge fraction, however, affects mainly the soil productivity through reduction in soil tilth

As was established for the clay loss, the organic matter enrichment ratios were higher under mulch ripping (4.6) and tied ridging (3.4) as compared to conventional tillage (2.8) and bare fallow (2.6). Exceptionally high organic matter losses were realized under conventional till‐ age systems as compared to conservation tillage systems, due to the high sediment losses.

nal soil.

ments.

as it contains few reactive particles.


**Table 7.** Differences in the organic matter contents of sludge and suspended soil over three seasons at MakoholiContill site

Soil organic matter is generally associated with the finer and more reactive clay and silt frac‐ tions of the soil (Follet*et al.,* 1987). It is, therefore as expected that more organic matter should be lost with suspended load than with sludge. Table 7 shows the two sediment pa‐ rameters (sludge and suspended load), which were treated as different entities. In relation to this, organic matter contents, quantities and enrichment ratios for the different parameters are given.

Under the conventional tillage treatments relatively less soil was lost as suspended load. The amount of organic matter lost with this fraction was, however, substantial, i.e. 40 - 81% (BF) and 60 - 94% (CT) of the total organic matter lost. The soil lost under mulch ripping was almost entirely in suspended form. This resulted in almost all the organic matter (97 - 100%) being lost with suspended material. Under tied ridging most of the soil was also lost as sus‐ pension, 75% of the total soil loss, with 97% of total organic matter loss. The enrichment ra‐ tios are thus very high for the suspended soil consisting mainly of clay and silt. The sludge has lower enrichment ratios and the organic matter contents are even less than of the origi‐ nal soil.

results showed a significant difference at P < 0.05, where lower losses were found under the

BF 0.23 2.59 168.25 188.86 0.43 4.80 CT 0.35 3.27 99.15 206.78 0.51 4.81 MR 0.00 5.02 0.00 8.46 0.00 6.28 TR 0.42 4.56 1.58 52.74 0.67 7.24

BF 0.04 2.06 11.04 48.20 0.07 3.81 CT 0.10 2.44 4.60 70.15 0.15 3.59 MR 0.00 2.88 0.00 2.58 0.00 3.60 TR 0.15 2.29 0.06 2.11 0.24 3.63

BF 0.37 2.61 509.49 345.07 0.69 4.83 CT 0.45 3.81 208.55 307.09 0.66 5.60 MR 0.31 5.07 4.80 148.53 0.39 6.34 TR 0.55 4.12 4.24 121.30 0.87 6.54

Treatment \*\*\* \*\*\* \*\*\* \*\*\* Year \*\*\* \*\*\* \*\*\* \*\*\*

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

Soil organic matter is generally associated with the finer and more reactive clay and silt frac‐ tions of the soil (Follet*et al.,* 1987). It is, therefore as expected that more organic matter should be lost with suspended load than with sludge. Table 7 shows the two sediment pa‐ rameters (sludge and suspended load), which were treated as different entities. In relation to this, organic matter contents, quantities and enrichment ratios for the different parameters

Under the conventional tillage treatments relatively less soil was lost as suspended load. The amount of organic matter lost with this fraction was, however, substantial, i.e. 40 - 81%

**Table 7.** Differences in the organic matter contents of sludge and suspended soil over three seasons at

**OM loss kg/ha**

**Sludge Susp. Sludge Susp. Sludge Susp.**

**Enrichment ratio**

mulch ripping treatment.

130 Research on Soil Erosion Soil Erosion

Year 1

Year 2

Year 3

ANOVA

MakoholiContill site

are given.

**Treatm. OM content**

**%**

Many scientists have reported on the selective nature of soil erosion (Aylen, 1939; Hudson and Jackson, 1962; Shaxson, 1975; Elwell, 1987; Lal, 1988 among others), however the extent to which the soils have been impoverished or the sediments enriched have mainly been esti‐ mated. This study showed that sheet erosion is selective as sediments recorded higher con‐ tents of clay, silt and organic matter than the original soil. The particle size distribution of sludge was mainly coarse sand and had a maximum of 4% clay. Suspended material on the other hand had up to 83% clay, the rest mainly being the silt fraction. Sediments from con‐ servation tillage systems comprised of more sand than the clay fraction, bare fallow had 13 times and conventional tillage 4 times less sludge than suspension. Due to the high losses of coarse material under the conventional tillage, the clay enrichment ratio of sediments was lower than under the conventional tillage systems. The bare fallow recorded only 1.5 and conventional tillage 4.1 times more clay in the sediments. Under the conservation tillage sys‐ tems more soil was lost in suspension than as sludge. Mulch ripping recorded 0.2 times and tied ridging 0.3 times more sludge than suspension, resulting in higher clay enrichment ra‐ tios of 13.5 and 15.5 under mulch ripping and tied ridging respectively. However, these high enrichment ratios amounted to less amount of clay lost from conservation tillage compared to conventional tillage, due to the reduced total sediments lost under the conservation till‐ age systems. Under bare fallow an average of 7 t/ha of clay were lost, 5 t/ha under conven‐ tional tillage and 0.9 and 0.8 t/ha under mulch ripping and tied ridging respectively. The very high enrichment ratios under the conservation tillage systems are a reflection of the soil losses, which were lost mainly as suspended material, which constitutes fine soil particles, however the very low amount of soil led to negligible losses of total clay loss with sedi‐ ments.

This selective nature of erosion manifested itself in the high enrichment ratios of the sedi‐ ments as compared to the original soil. This means that soil fertility is affected severely by the soil lost in suspension, as it constitutes mainly of clay and organic matter fractions, which are the main sources of nutrients (Stocking, 1983). The soil structure and water hold‐ ing capacity are also affected as these soil fractions are responsible for soil aggregation and influence the water dynamics of the soil (Follet*et al*., 1987; Stocking and Peake, 1987). The sludge fraction, however, affects mainly the soil productivity through reduction in soil tilth as it contains few reactive particles.

As was established for the clay loss, the organic matter enrichment ratios were higher under mulch ripping (4.6) and tied ridging (3.4) as compared to conventional tillage (2.8) and bare fallow (2.6). Exceptionally high organic matter losses were realized under conventional till‐ age systems as compared to conservation tillage systems, due to the high sediment losses. The bare fallow lost an annual average of 424 kg/ha, while conventional tillage lost 299 kg/ha, mulch ripping lost only 55 kg/ha and tied ridging 61 kg/ha/yr and the treatments dif‐ fered significantly from one another. The proximity and concentration of soil organic matter near the soil surface (< 250 mm) and its close association with plant nutrients in the soil makes erosion of soil organic matter a strong indicator of overall plant nutrient losses result‐ ing from erosion (Follet*et al.,* 1987). Thus the effectiveness of the two conservation tillage treatments can be appreciated based upon the small amount of organic matter lost with eroded sediments, compared to the conventional tillage.

**Treatment Nutrient status of the soil**

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 8.** Nutrient of the soils as at beginning of the study at Makoholi Contill site

*3.5.1. Nutrient losses with total sediments*

*Nitrogen*

reactive coarse particles.

**Year 1 (483 mm)**

n = 9 (Treatment) s.e.d. = 1.341 s2 = 8.097 n = 12 (Year) s.e.d. = 1.162 df = 24

s.e.d. = 2.323

**Year 2 (384 mm)**

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 9.** Total nitrogen loss (kg/ha) as a result of erosion under different tillage systems over three years at

**Treat/Year (Rainfall)**

n = 3 (Treatment x Year)

MakoholiContill site

BF 0.04 39.4 554.2 CT 0.05 52.0 616.7 MR 0.05 62.2 575.0 TR 0.05 91.8 487.5

Using Equation 2 to calculate the amount of N lost with erosion, the highest total nitrogen losses were realized under bare fallow, at 28 kg/ha followed by conventional tillage (16 kg/ ha), while they were least under mulch ripping (2.3 kg/ha), which was also barely different from tied ridging (2.7 kg/ha), see Table 9. Total nitrogen loss differed significantly (P < 0.001) between the different treatments, different years and for the treatment x year interaction. These results follow, as expected, the same trend that was established for soil loss (Table 2) and serve to confirm the dependence of nutrient losses with the amount of soil lost from a field. The maintenance of soil under the two conservation tillage treatments is also directly related to the lower N losses. Although nitrogen losses were highest under the bare fallow, the actual nutrient concentration in the soil was least under this treatment (Table 12) because no fertilizer was applied and the sediments under this treatment comprised mainly the non-

> **Year 3 (765 mm)**

CT 17.40 6.82 23.22 15.81 Treat \*\*\* MR 0.53 0.16 6.06 2.25 Year \*\*\* TR 3.03 0.15 4.92 2.70 Treat x Year \*\*\* BF 32.10 9.06 44.10 28.42 MR vs TR NS Overall mean 13.27 4.05 19.58 12.30 CT vs (MR, TR) \*\*\*

N % P ppm K ppm

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133

Quantifying Nutrient Losses with Different Sediment Fractions Under Four Tillage Systems...

**Overall mean Source of variation N loss**

In situ measurement of organic matter as a measure of soil erosion yielded fruitful as the organic matter levels dropped drastically after five years of cultivation, especially under the conventional tillage systems. The bare fallow lost 25%, conventional tillage 19%, tied ridging 9% and mulch ripping 6% of total organic matter found on virgin land. This is in agreement with the very high losses of fine particles lost under the conventional tillage systems as com‐ pared to conservation tillage. It is apparent that through conservation of the soil under mulch ripping and tied ridging, the organic matter status in the soil is maintained, thus the soil structure and soil productivity.

As most of the soil fertility is associated with clay and humus and these also affect microbial activity, soil structure, permeability and water holding capacity (Troeh*et al.,* 1980), it is clear that through sheet erosion the land is degraded chemically, physically and biologically. Thus not only soil fertility is reduced, but also soil productivity, which unlike fertility can‐ not be addressed by mere fertilizer application.

#### **3.5. Nutrient losses with sediments**

Before the assessment of the nutrient losses with erosion, it was important to evaluate the nutrient (N, P, K) status of the soils. From Table 8, it is apparent that the most abundant nu‐ trient in the soil is potassium, followed closely by nitrogen and the least abundant is phos‐ phorus. Since total nutrients are considered it is expected that the nutrient with the highest concentration in the soil will also result in the highest losses and vice versa. Thus, compar‐ ing the amount of different nutrients lost with the sediments may not be very meaningful but a method of evaluating and comparing the loss of different nutrients should also be based relatively upon the status of that nutrient in the soil. This method involves the deter‐ mination of nutrient concentration in the soil and in the sediments and calculating the en‐ richment ratios.

The nutrient losses were calculated using the following equation:

$$\text{Nut}\_{\text{los}} = \text{Soil}\_{\text{los}} \times \text{Nut}\_{\text{conc}} \tag{2}$$

where Nutlos= any nutrient lost with sediments (kg/ha)

Soillos= mass of soil lost by erosion (kg/ha)

Nutconc= the concentration of a nutrient in the sediment (ppm or %)


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 8.** Nutrient of the soils as at beginning of the study at Makoholi Contill site

#### *3.5.1. Nutrient losses with total sediments*

#### *Nitrogen*

The bare fallow lost an annual average of 424 kg/ha, while conventional tillage lost 299 kg/ha, mulch ripping lost only 55 kg/ha and tied ridging 61 kg/ha/yr and the treatments dif‐ fered significantly from one another. The proximity and concentration of soil organic matter near the soil surface (< 250 mm) and its close association with plant nutrients in the soil makes erosion of soil organic matter a strong indicator of overall plant nutrient losses result‐ ing from erosion (Follet*et al.,* 1987). Thus the effectiveness of the two conservation tillage treatments can be appreciated based upon the small amount of organic matter lost with

In situ measurement of organic matter as a measure of soil erosion yielded fruitful as the organic matter levels dropped drastically after five years of cultivation, especially under the conventional tillage systems. The bare fallow lost 25%, conventional tillage 19%, tied ridging 9% and mulch ripping 6% of total organic matter found on virgin land. This is in agreement with the very high losses of fine particles lost under the conventional tillage systems as com‐ pared to conservation tillage. It is apparent that through conservation of the soil under mulch ripping and tied ridging, the organic matter status in the soil is maintained, thus the

As most of the soil fertility is associated with clay and humus and these also affect microbial activity, soil structure, permeability and water holding capacity (Troeh*et al.,* 1980), it is clear that through sheet erosion the land is degraded chemically, physically and biologically. Thus not only soil fertility is reduced, but also soil productivity, which unlike fertility can‐

Before the assessment of the nutrient losses with erosion, it was important to evaluate the nutrient (N, P, K) status of the soils. From Table 8, it is apparent that the most abundant nu‐ trient in the soil is potassium, followed closely by nitrogen and the least abundant is phos‐ phorus. Since total nutrients are considered it is expected that the nutrient with the highest concentration in the soil will also result in the highest losses and vice versa. Thus, compar‐ ing the amount of different nutrients lost with the sediments may not be very meaningful but a method of evaluating and comparing the loss of different nutrients should also be based relatively upon the status of that nutrient in the soil. This method involves the deter‐ mination of nutrient concentration in the soil and in the sediments and calculating the en‐

Nutlos = Soillos x Nutconc (2)

eroded sediments, compared to the conventional tillage.

soil structure and soil productivity.

132 Research on Soil Erosion Soil Erosion

**3.5. Nutrient losses with sediments**

richment ratios.

not be addressed by mere fertilizer application.

The nutrient losses were calculated using the following equation:

Nutconc= the concentration of a nutrient in the sediment (ppm or %)

where Nutlos= any nutrient lost with sediments (kg/ha)

Soillos= mass of soil lost by erosion (kg/ha)

Using Equation 2 to calculate the amount of N lost with erosion, the highest total nitrogen losses were realized under bare fallow, at 28 kg/ha followed by conventional tillage (16 kg/ ha), while they were least under mulch ripping (2.3 kg/ha), which was also barely different from tied ridging (2.7 kg/ha), see Table 9. Total nitrogen loss differed significantly (P < 0.001) between the different treatments, different years and for the treatment x year interaction. These results follow, as expected, the same trend that was established for soil loss (Table 2) and serve to confirm the dependence of nutrient losses with the amount of soil lost from a field. The maintenance of soil under the two conservation tillage treatments is also directly related to the lower N losses. Although nitrogen losses were highest under the bare fallow, the actual nutrient concentration in the soil was least under this treatment (Table 12) because no fertilizer was applied and the sediments under this treatment comprised mainly the nonreactive coarse particles.


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 9.** Total nitrogen loss (kg/ha) as a result of erosion under different tillage systems over three years at MakoholiContill site

#### *Phosphorus*

The overall phosphorus loss of 0.5 kg/ha, was as expected, much lower than nitrogen loss (12.3 kg/ha), due to the generally low P status in the sandy soils. The bare fallow had the highest P loss of 0.9 kg/ha followed by conventional tillage with 0.8 kg/ha, tied ridging 0.2 kg/ha and the least P losses were recorded under mulch ripping (0.09 kg/ha) (Table 10). This trend was to be expected, as nutrient losses are a function of soil loss. Despite the low losses, the treatments and years gave highly significant differences at P < 0.001. The two conserva‐ tion tillage treatments were not significantly different from one another.

nificant at P < 0.001. These differences show the conservation merits of the conservation tillage treatments, implying that potassium is also conserved effectively through the ability

Quantifying Nutrient Losses with Different Sediment Fractions Under Four Tillage Systems...

CT 42.3 6.7 24.5 Treat \*\*\* MR 1.0 0.2 0.6 Year \*\*\* TR 8.3 0.2 4.3 Treat x Year \*\*\* BF 66.7 12.9 39.8 MR vs TR NS Overall mean 29.6 5.0 17.3 CT vs (MR, TR) \*\*\*

**Overall mean Source of variation K loss with erosion**

http://dx.doi.org/10.5772/3402

135

**Year 2 (384 mm)**

of these treatments in reducing erosion.

**(483 mm)**

n = 6 (Treatment) s.e.d. = 5.49 s2 = 90.44 n = 12 (Year) s.e.d. = 3.88 df = 16

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 11.** Total potassium loss (kg/ha) as a result of erosion under different tillage systems over three years at

Overall the enrichment ratios (soil nutrient concentration: sediment nutrient concentration) for the different nutrients were not very different from one another (Table 12). These were as follows: N: 4.3; P: 3.8 and K: 4.2. Although the amount of P lost with erosion was only a frac‐ tion of N and K amounts, it is clear that relative to the amount of P in the soil, all nutrients were lost in near equal proportions. The highest enrichment ratios were recorded under the conservation tillage systems, where the ratios ranged between 6.0 (P) and 7.3 (K), while un‐ der conventional tillage the sediments were enriched as follows: 2.0 for N, 1.9 for P and K. The bare fallow recorded the least nutrient enrichment ratios of about 1.0 N and K, while a ratio of 2.7 was recorded for P. The difference in enrichment ratios was only recorded for the different tillage systems and not for the plant nutrients, as these showed a similar trend

The amount of nitrogen lost with run-off was very small, on average constituting less than 1% of total nitrogen lost under conventional tillage, bare fallow and tied ridging, while un‐ der mulch ripping an average of 2% was recorded over the three years, see Figure 3a. Tied ridging recorded the least N loss of 15 g/ha and conventional tillage the highest N loss of 64 g/ha, however, there was no significant difference between treatments (P = 0.076), see Table 13. A significant difference of P < 0.001 was found between the different years showing that

**Treat/Year (Rainfall) Year 1**

n = 3 (Treatment x Year) s.e.d. = 7.76

within these tillage systems.

*Nitrogen*

*3.5.2. Nutrient losses with run-off*

MakoholiContill site


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 10.** Total phosphorus (kg/ha) as a result of erosion under different tillage systems over three years at MakoholiContill site

#### *Potassium*

Potassium was, as expected, lost in greater quantities when compared to the other elements (overall 17.3 kg/ha). It has been highlighted that K is the most abundant element in the soils' mineralogy (Table 8) and this explains the high losses. The same trend that was established for N and P was also found with K, where more K was lost with bare fallow (40 kg/ha) and conventional tillage (25 kg/ha) as compared to the conservation tillage systems (0.6 and 4 kg/ha for mulch ripping and tied ridging respectively), see Table 11. The overall treatment differences were significant at P < 0.001 mainly, due to significantly higher soil losses be‐ tween the treatments. The different years also gave rise to different K losses, which were sig‐


nificant at P < 0.001. These differences show the conservation merits of the conservation tillage treatments, implying that potassium is also conserved effectively through the ability of these treatments in reducing erosion.

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 11.** Total potassium loss (kg/ha) as a result of erosion under different tillage systems over three years at MakoholiContill site

Overall the enrichment ratios (soil nutrient concentration: sediment nutrient concentration) for the different nutrients were not very different from one another (Table 12). These were as follows: N: 4.3; P: 3.8 and K: 4.2. Although the amount of P lost with erosion was only a frac‐ tion of N and K amounts, it is clear that relative to the amount of P in the soil, all nutrients were lost in near equal proportions. The highest enrichment ratios were recorded under the conservation tillage systems, where the ratios ranged between 6.0 (P) and 7.3 (K), while un‐ der conventional tillage the sediments were enriched as follows: 2.0 for N, 1.9 for P and K. The bare fallow recorded the least nutrient enrichment ratios of about 1.0 N and K, while a ratio of 2.7 was recorded for P. The difference in enrichment ratios was only recorded for the different tillage systems and not for the plant nutrients, as these showed a similar trend within these tillage systems.

#### *3.5.2. Nutrient losses with run-off*

#### *Nitrogen*

*Phosphorus*

134 Research on Soil Erosion Soil Erosion

**Treat/Year (Rainfall)**

n = 9 (Treatment)

n = 3 (Treatment x Year)

MakoholiContill site

*Potassium*

n = 12 (Year) s.e.d. = 0.0578

s.e.d. = 0.1156

**Year 1 (483 mm)**

s.e.d. = 0.0667 s2 = 0.02004

df = 24

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 10.** Total phosphorus (kg/ha) as a result of erosion under different tillage systems over three years at

Potassium was, as expected, lost in greater quantities when compared to the other elements (overall 17.3 kg/ha). It has been highlighted that K is the most abundant element in the soils' mineralogy (Table 8) and this explains the high losses. The same trend that was established for N and P was also found with K, where more K was lost with bare fallow (40 kg/ha) and conventional tillage (25 kg/ha) as compared to the conservation tillage systems (0.6 and 4 kg/ha for mulch ripping and tied ridging respectively), see Table 11. The overall treatment differences were significant at P < 0.001 mainly, due to significantly higher soil losses be‐ tween the treatments. The different years also gave rise to different K losses, which were sig‐

The overall phosphorus loss of 0.5 kg/ha, was as expected, much lower than nitrogen loss (12.3 kg/ha), due to the generally low P status in the sandy soils. The bare fallow had the highest P loss of 0.9 kg/ha followed by conventional tillage with 0.8 kg/ha, tied ridging 0.2 kg/ha and the least P losses were recorded under mulch ripping (0.09 kg/ha) (Table 10). This trend was to be expected, as nutrient losses are a function of soil loss. Despite the low losses, the treatments and years gave highly significant differences at P < 0.001. The two conserva‐

> **Year 3 (765 mm)**

CT 1.403 0.182 0.666 0.750 Treat \*\*\*

MR 0.057 0.009 0.208 0.091 Year \*\*\*

TR 0.282 0.008 0.218 0.169 Treat x Year \*\*\*

BF 1.269 0.245 1.069 0.861 MR vs TR NS

Overall mean 0.753 0.111 0.540 0.468 CT vs (MR, TR) \*\*\*

**Overall mean Source of variation P loss**

tion tillage treatments were not significantly different from one another.

**Year 2 (384 mm)**

> The amount of nitrogen lost with run-off was very small, on average constituting less than 1% of total nitrogen lost under conventional tillage, bare fallow and tied ridging, while un‐ der mulch ripping an average of 2% was recorded over the three years, see Figure 3a. Tied ridging recorded the least N loss of 15 g/ha and conventional tillage the highest N loss of 64 g/ha, however, there was no significant difference between treatments (P = 0.076), see Table 13. A significant difference of P < 0.001 was found between the different years showing that

the different rainfall regimes influence run-off amount and consequently nitrogen loss. As N loss with run-off is dissolved N, it is expected that this fraction would be more under crop‐ ped treatments where N fertilizer was applied and generally where the nutrient status in the soil is higher. The lower N loss under the bare fallow compared to conventional tillage and mulch ripping, despite higher run-off, is because of this fact. The reason for the low N con‐ centration under the tied ridging treatment is mainly due to the fact that fertilizers are pro‐ tected on the ridges, while run-off mainly takes place in the furrows.

ments as the amounts were generally very low (Table 14). However, the different years gave rise to significantly different P losses (P < 0.001), due to the different amounts of run-off real‐

Quantifying Nutrient Losses with Different Sediment Fractions Under Four Tillage Systems...

**Year 3 (765 mm)**

**Table 13.** Nitrogen loss (kg/ha) with run-off under different tillage systems over three years at MakoholiContill site

**Year 3 (765 mm)**

CT 0.0066 0.0028 0.0153 0.0083 Treat NS MR 0.0021 0.0003 0.0078 0.0034 Year \*\*\* TR 0.0034 0.0004 0.0081 0.0040 Treat x Year NS BF 0.0041 0.0026 0.0089 0.0052 MR vs TR NS Overall mean 0.0040 0.0015 0.0100 0.0052 CT vs (MR, TR) NS

**Table 14.** Phosphorus loss (kg/ha) with run-off under different tillage systems over three years at MakoholiContill site

CT 0.0469 0.0071 0.1377 0.0639 Treat NS MR 0.0042 0.0037 0.1789 0.0623 Year \*\*\* TR 0.0030 0.0012 0.0421 0.0154 Treat x Year NS BF 0.0298 0.0058 0.1208 0.0522 MR vs TR NS Overall mean 0.0210 0.0045 0.1199 0.0484 CT vs (MR, TR) NS

**Overall mean Source of variation Nitrogen in**

http://dx.doi.org/10.5772/3402

**Overall mean Source of variation P with run-**

**run-off**

137

**off**

ized during these years.

n = 9 (Treatment) s.e.d. =

n = 12 (Year) s.e.d. =

n = 3 (Treatment x Year)

> **Treat/Year (Rainfall)**

n = 9 (Treatment) s.e.d. =

n = 12 (Year) s.e.d. =

n = 3 (Treatment x Year)

**Year 1 (483 mm)**

0.01986

0.01720

s.e.d. = 0.03440

**Year 1 (483 mm)**

0.000973

0.000843

s.e.d. = 0.001685

**Year 2 (384 mm)**

s2 = 0.001775

df = 24

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Year 2 (384 mm)**

s2 = 0.00004259

df = 24

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Treat/Year (Rainfall)**


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 12.** Nutrient concentrations in the sediments and enrichment ratios for different tillage systems at MakoholiContill site

#### *Phosphorus*

The amount of P lost with run-off constituted a slightly higher percentage of total P loss with sediments than was the case with nitrogen. The conservation tillage treatments realized a higher ratio of dissolved P losses, averaging 4% under mulch ripping and 2% under tied ridging. An average of 1% was recorded under conventional tillage and the lowest percent‐ age loss was found under the bare fallow, where P in run-off only constituted 0.6% of total P lost, (Figure 3b). As was the case with N, there were no significant differences between treat‐ ments as the amounts were generally very low (Table 14). However, the different years gave rise to significantly different P losses (P < 0.001), due to the different amounts of run-off real‐ ized during these years.


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

the different rainfall regimes influence run-off amount and consequently nitrogen loss. As N loss with run-off is dissolved N, it is expected that this fraction would be more under crop‐ ped treatments where N fertilizer was applied and generally where the nutrient status in the soil is higher. The lower N loss under the bare fallow compared to conventional tillage and mulch ripping, despite higher run-off, is because of this fact. The reason for the low N con‐ centration under the tied ridging treatment is mainly due to the fact that fertilizers are pro‐

N % P ppm K ppm N P K

BF 0.05 39.8 803.9 1.3 1.0 1.5 CT 0.07 104.6 1351.1 1.4 2.0 2.2 MR 0.41 570.9 5397.7 8.2 9.2 9.39 TR 0.28 447.6 5110.6 5.7 4.9 10.5

BF 0.03 14.4 318.7 1.0 0.8 0.6 CT 0.12 61.9 961.5 3.0 2.5 1.6 MR 0.40 156.6 1875.0 8.0 5.2 4.0 TR 0.25 104.8 1813.5 5.06 2.5 4.0

BF 0.05 15.0 - 1.7 0.9 - CT 0.06 33.4 - 1.5 1.2 - MR 0.22 124.4 - 4.3 4.8 - TR 0.52 326.1 - 10.3 10.3 -

**Enrichment ratios: nutrient in soil: nutrient in sediments**

tected on the ridges, while run-off mainly takes place in the furrows.

**in the sediments**

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 12.** Nutrient concentrations in the sediments and enrichment ratios for different tillage systems at

The amount of P lost with run-off constituted a slightly higher percentage of total P loss with sediments than was the case with nitrogen. The conservation tillage treatments realized a higher ratio of dissolved P losses, averaging 4% under mulch ripping and 2% under tied ridging. An average of 1% was recorded under conventional tillage and the lowest percent‐ age loss was found under the bare fallow, where P in run-off only constituted 0.6% of total P lost, (Figure 3b). As was the case with N, there were no significant differences between treat‐

**Treat Nutrient concentration**

Year 1

136 Research on Soil Erosion Soil Erosion

Year 2

Year 3

MakoholiContill site

*Phosphorus*



**Table 13.** Nitrogen loss (kg/ha) with run-off under different tillage systems over three years at MakoholiContill site

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 14.** Phosphorus loss (kg/ha) with run-off under different tillage systems over three years at MakoholiContill site

#### *Potassium*

Dissolved potassium loss with run-off, as was the case with the other elements, constituted a smaller percentage of the total K lost with erosion. The highest percentage was found under the mulch ripping treatment (15 - 20% of total K), followed by tied ridging (5%), then con‐ ventional tillage with 2 - 3% and the bare fallow had the least percentage averaging 1% of total K lost, (Figure 3c). The treatments however, did not differ significantly from one anoth‐ er but the different years differed significantly at P < 0.001 (Table 15). Unlike the other ele‐ ments the loss of dissolved K was highest under the bare fallow, indicating that K is abundant in the soil and highly soluble in water. This also gives an indication on the availa‐ bility of K in these soils.

*Phosphorus*

**Treat/Year (Rainfall)**

n = 3 (Treatment x Year)

MakoholiContill site

**Treat/Year (Rainfall)**

n = 3 (Treatment x Year)

MakoholiContill site

Most of the P was also lost with suspended material under the cropped treatments (Table 17). Conventional tillage lost 62 - 83% of total P with suspended material, while the losses ranged between 93 and 97% under mulch ripping and between 91 and 97% under tied ridg‐ ing. Under the bare fallow, this phenomenon was less pronounced, with the P lost with this

> **Year 3 (765 mm)**

CT 10.16 5.62 11.38 9.05 Treat \*\*\* MR 0.52 0.16 5.23 1.97 Year \*\*\* TR 2.94 0.14 4.68 2.59 Treat x Year \*\*\* BF 12.33 2.66 22.03 12.34 MR vs TR NS Overall mean 6.49 2.14 10.83 6.49 CT vs (MR, TR) \*\*\*

**Overall mean**

Quantifying Nutrient Losses with Different Sediment Fractions Under Four Tillage Systems...

**Source of variation Nitrogen in**

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139

**Source of variation P in susp.**

**susp.**

fraction accounting for 27 - 61% of total P lost (Figure 3b).

**Year 2 (384 mm)**

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Year 2 (384 mm)**

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 17.** Phosphorus loss (kg/ha) with suspended material under different tillage systems over three years at

**Table 16.** Nitrogen loss (kg/ha) with suspended material under different tillage systems over three years at

**Year 3 (765 mm)**

CT 1.0541 0.1512 0.4157 0.540 Treat \*\*\* MR 0.0549 0.0087 0.1939 0.086 Year \*\*\* TR 0.2739 0.0073 0.2049 0.162 Treat x Year \*\*\* BF 0.7739 0.0664 0.5244 0.455 MR vs TR NS Overall mean 0.539 0.058 0.335 0.311 CT vs (MR, TR) \*\*\*

**Overall mean**

**Year 1 (483 mm)**

n = 9 (Treatment) s.e.d. = 1.307 s2 = 7.686 n = 12 (Year) s.e.d. = 1.132 df = 24

s.e.d. = 2.264

**Year 1 (483 mm)**

n = 9 (Treatment) s.e.d. = 0.0597 s2 = 0.01602) n = 12 (Year) s.e.d. = 0.0517 df = 24

s.e.d. = 0.1033


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 15.** Dissolved K loss (kg/ha) with run-off under different tillage systems over three years at MakoholiContill site

#### *3.5.3. Nutrient losses with suspended material*

#### *Nitrogen*

For all the cropped treatments, most of the N was lost with suspended material and ranged from 49 - 82% of total nitrogen loss under conventional tillage; 86 - 99% under mulch rip‐ ping and 93 - 97% under tied ridging. The percentage was lower under the bare fallow and ranged from 29 - 50%, due to the extra-ordinarily high losses of sludge as compared to sus‐ pended material (Figure 3a). Analysis of variance showed that nitrogen loss with suspended material differed significantly (P < 0.001) between the different treatments, as a result of the significant treatment differences in the loss of suspended material (Table 16). The conserva‐ tion tillage treatments did not differ significantly from each other. The different years also gave significant differences in nitrogen loss (P < 0.001). This finding indicates that the sus‐ pended material is by far the most important medium for overland transport of nitrogen from arable lands, as a result of erosion. The nitrogen concentration in suspended material ranged from 0.2 - 0.65% compared to 0.04 – 0.05% in the soil.

#### *Phosphorus*

*Potassium*

138 Research on Soil Erosion Soil Erosion

bility of K in these soils.

**Year 1 (483 mm)**

n = 9 (Treatment) s.e.d. = 0.663 s2 = 1.976 n = 12 (Year) s.e.d. = 0.574 df = 24

s.e.d. = 1.148

*3.5.3. Nutrient losses with suspended material*

**Year 2 (384 mm)**

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

ranged from 0.2 - 0.65% compared to 0.04 – 0.05% in the soil.

**Treat/Year (Rainfall)**

n = 3 (Treatment x Year)

*Nitrogen*

Dissolved potassium loss with run-off, as was the case with the other elements, constituted a smaller percentage of the total K lost with erosion. The highest percentage was found under the mulch ripping treatment (15 - 20% of total K), followed by tied ridging (5%), then con‐ ventional tillage with 2 - 3% and the bare fallow had the least percentage averaging 1% of total K lost, (Figure 3c). The treatments however, did not differ significantly from one anoth‐ er but the different years differed significantly at P < 0.001 (Table 15). Unlike the other ele‐ ments the loss of dissolved K was highest under the bare fallow, indicating that K is abundant in the soil and highly soluble in water. This also gives an indication on the availa‐

> **Year 3 (765 mm)**

**Table 15.** Dissolved K loss (kg/ha) with run-off under different tillage systems over three years at MakoholiContill site

For all the cropped treatments, most of the N was lost with suspended material and ranged from 49 - 82% of total nitrogen loss under conventional tillage; 86 - 99% under mulch rip‐ ping and 93 - 97% under tied ridging. The percentage was lower under the bare fallow and ranged from 29 - 50%, due to the extra-ordinarily high losses of sludge as compared to sus‐ pended material (Figure 3a). Analysis of variance showed that nitrogen loss with suspended material differed significantly (P < 0.001) between the different treatments, as a result of the significant treatment differences in the loss of suspended material (Table 16). The conserva‐ tion tillage treatments did not differ significantly from each other. The different years also gave significant differences in nitrogen loss (P < 0.001). This finding indicates that the sus‐ pended material is by far the most important medium for overland transport of nitrogen from arable lands, as a result of erosion. The nitrogen concentration in suspended material

CT 0.87 0.18 3.81 1.62 Treat NS MR 0.20 0.03 3.25 1.16 Year \*\*\* TR 0.38 0.01 1.85 0.74 Treat x Year NS BF 0.91 0.02 6.19 2.38 MR vs TR NS Overall mean 0.59 0.06 3.77 1.47 CT vs (MR, TR) NS

**Overall mean**

**Source of variation K in run-off**

Most of the P was also lost with suspended material under the cropped treatments (Table 17). Conventional tillage lost 62 - 83% of total P with suspended material, while the losses ranged between 93 and 97% under mulch ripping and between 91 and 97% under tied ridg‐ ing. Under the bare fallow, this phenomenon was less pronounced, with the P lost with this fraction accounting for 27 - 61% of total P lost (Figure 3b).


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 16.** Nitrogen loss (kg/ha) with suspended material under different tillage systems over three years at MakoholiContill site


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 17.** Phosphorus loss (kg/ha) with suspended material under different tillage systems over three years at MakoholiContill site

Due to this variation in the different treatments, analysis of variance among the different treatments gave a significant difference at P < 0.001. The different years and the interaction between treatment and year were also significantly different. The finding also shows the sig‐ nificance of suspended material in transporting P from arable lands as a result of erosion.

*3.5.4. Nutrient losses with sludge*

Under all the cropped treatments, the amount of nitrogen lost with sludge was significantly lower than that lost with suspended load (Figure 3a). This phenomenon was more pro‐ nounced under the conservation tillage treatments. Conventional tillage recorded 18 - 50%, while mulch ripping recorded 0 - 11% and tied ridging 3 - 7 % of total N loss with sludge. The bare fallow recorded more N loss with sludge (50 - 71%) due to the very high sludge loss. These findings indicate that less nitrogen is associated with coarse soil particles and this is further implicated by the less nitrogen concentration in sludge, ranging between 0.00 and 0.05% as compared to that in suspended material (0.2 - 0.65%). The amount of N lost with sludge differed highly significantly (P < 0.001) between the different treatments and it differed significantly at P < 0.01 among the different years. As expected, there was no signifi‐

Quantifying Nutrient Losses with Different Sediment Fractions Under Four Tillage Systems...

N % P ppm K ppm N P K

BF 0.18 267.6 4634.8 4.5 6.8 8.4 CT 0.24 426.2 5233.2 4.8 8.2 8.5 MR 0.41 570.9 5397.7 8.2 9.2 9.4 TR 0.37 584.2 6660.3 7.4 6.4 13.7

BF 0.18 73.1 1671.2 6.0 4.3 3.0 CT 0.26 129.5 1922.5 6.5 5.2 3.1 MR 0.40 156.6 1875.0 8.0 5.3 3.3 TR 0.35 142.8 2505.6 7.0 3.4 5.1 Year 3 - BF 0.16 73.5 - 5.3 4.6 - CT 0.21 125.3 - 5.3 4.6 - MR 0.32 192.2 - 6.4 7.4 - TR 0.27 163.6 - 5.4 5.1 -

**Enrichment ratios: nutrient in soil: nutrient in suspension**

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141

cant difference between the two conservation tillage treatments (Table 20).

**in suspension**

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

enrichment ratios for different tillage systems at MakoholiContill site

**Table 19.** Nutrient concentration in the soil (0 - 25 cm) versus nutrient concentration in suspended material and

**Treat Nutrient concentration**

*Nitrogen*

Year 1

Year 2

#### *Potassium*

The suspended material accounted for most of the potassium losses under all the cropped treat‐ ments, see Table 18. The conservation tillage treatments realized the highest percentages that ranged between 80 and 85% for mulch ripping and 90 and 93% for tied ridging, while convention‐ al tillage lost 66 - 79% of total potassium with this sediment fraction. The bare fallow was the only exception, with the losses as low as 28 - 51% (Figure 3c). Once again this is an indication of the ra‐ tio between suspended material and coarse material under the bare fallow. The analysis of var‐ iance gave highly significant differences (P < 0.001) between treatments and years and a significant difference of P < 0.01 for the treatment x year interaction. Although somewhat lower the relationship between K and fine soil particles is clearly indicated.


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 18.** Potassium loss (kg/ha) with suspended material under different tillage systems over three years at MakoholiContill site

The nutrient enrichment ratios for suspended material were generally higher than those re‐ corded for the total sediments, due to the high proportion of fine soil particles (Table 19). The following overall enrichment ratios were found: N: 6.2; P: 5.9 and K: 6.8. Once again the enrichment ratios show that nutrients were lost, relative to their nutrient status in the soil. Although the conservation tillage systems generally had high enrichment ratios, the differ‐ ence between the treatments was not as distinct as was the case with total sediments. This is as a result of the similar composition of the suspended material, regardless of tillage treat‐ ment. The nutrient enrichment ratios were similar across all the nutrients.

#### *3.5.4. Nutrient losses with sludge*

#### *Nitrogen*

Due to this variation in the different treatments, analysis of variance among the different treatments gave a significant difference at P < 0.001. The different years and the interaction between treatment and year were also significantly different. The finding also shows the sig‐ nificance of suspended material in transporting P from arable lands as a result of erosion.

The suspended material accounted for most of the potassium losses under all the cropped treat‐ ments, see Table 18. The conservation tillage treatments realized the highest percentages that ranged between 80 and 85% for mulch ripping and 90 and 93% for tied ridging, while convention‐ al tillage lost 66 - 79% of total potassium with this sediment fraction. The bare fallow was the only exception, with the losses as low as 28 - 51% (Figure 3c). Once again this is an indication of the ra‐ tio between suspended material and coarse material under the bare fallow. The analysis of var‐ iance gave highly significant differences (P < 0.001) between treatments and years and a significant difference of P < 0.01 for the treatment x year interaction. Although somewhat lower

CT 28.1 5.3 16.7 Treat \*\*\* MR 0.8 0.2 0.5 Year \*\*\* TR 7.7 0.2 4.0 Treat x Year \*\* BF 34.0 3.6 18.8 MR vs TR NS Overall mean 17.7 2.3 10.0 CT vs (MR, TR) \*\*\*

**Overall mean Source of variation K in susp.**

the relationship between K and fine soil particles is clearly indicated.

**Year 2 (384 mm)**

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

ment. The nutrient enrichment ratios were similar across all the nutrients.

**Table 18.** Potassium loss (kg/ha) with suspended material under different tillage systems over three years at

The nutrient enrichment ratios for suspended material were generally higher than those re‐ corded for the total sediments, due to the high proportion of fine soil particles (Table 19). The following overall enrichment ratios were found: N: 6.2; P: 5.9 and K: 6.8. Once again the enrichment ratios show that nutrients were lost, relative to their nutrient status in the soil. Although the conservation tillage systems generally had high enrichment ratios, the differ‐ ence between the treatments was not as distinct as was the case with total sediments. This is as a result of the similar composition of the suspended material, regardless of tillage treat‐

**Year 1 (483 mm)**

n = 6 (Treatment) s.e.d. = 3.60 s2 = 38.77 n = 12 (Year) s.e.d. = 2.54 df = 16

s.e.d. = 5.08

*Potassium*

140 Research on Soil Erosion Soil Erosion

**Treat/Year (Rainfall)**

n = 3 (Treatment x Year)

MakoholiContill site

Under all the cropped treatments, the amount of nitrogen lost with sludge was significantly lower than that lost with suspended load (Figure 3a). This phenomenon was more pro‐ nounced under the conservation tillage treatments. Conventional tillage recorded 18 - 50%, while mulch ripping recorded 0 - 11% and tied ridging 3 - 7 % of total N loss with sludge. The bare fallow recorded more N loss with sludge (50 - 71%) due to the very high sludge loss. These findings indicate that less nitrogen is associated with coarse soil particles and this is further implicated by the less nitrogen concentration in sludge, ranging between 0.00 and 0.05% as compared to that in suspended material (0.2 - 0.65%). The amount of N lost with sludge differed highly significantly (P < 0.001) between the different treatments and it differed significantly at P < 0.01 among the different years. As expected, there was no signifi‐ cant difference between the two conservation tillage treatments (Table 20).


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 19.** Nutrient concentration in the soil (0 - 25 cm) versus nutrient concentration in suspended material and enrichment ratios for different tillage systems at MakoholiContill site


that P is associated with fine soil particles and not with the non-reactive coarse material as is evidenced by the low P concentrations in sludge, ranging from 0 - 34 ppm compared to 73 -

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143

The sludge fraction constituted the lowest losses of K under the cropped treatments, ranging from 0 - 5% under the conservation tillage treatments, a maximum of 32% under conven‐ tional tillage (Figure 3c). Under the bare fallow 48 - 72 % of total K was lost with this sedi‐ ment fraction. These obvious differences between both treatment and year factors were significant at P < 0.001 (Table 22). K is therefore associated with the suspended material than with sludge, the somewhat higher percentages lost under conventional tillage and bare fal‐ low are merely in relation to the very high coarse material lost under these treatments as the actual nutrient concentration in the sludge is very low compared to that in suspended mate‐

CT 13.35 1.17 7.26 Treat \*\*\* MR 0.00 0.00 0.00 Year \*\*\* TR 0.17 0.01 0.09 Treat x Year \*\*\* BF 31.75 9.32 20.54 MR vs TR NS Overall mean 11.32 2.62 6.97 CT vs (MR, TR) \*\*\*

**Table 22.** Potassium loss (kg/ha) with sludge under different tillage systems over three years at MakoholiContill site

Due to the high proportion of non-reactive coarse particles in sludge, the nutrients concen‐ tration was low compared to total sediments and suspended material. Generally all the nu‐ trients under all tillage systems recorded lower nutrient concentrations in the sludge compared to nutrient concentrations in the original soil, resulting in enrichment ratios less than 1.0, with the exception of N under bare fallow, which recorded 1.0. The overall nutrient enrichment ratios in sludge were as follows: N: 0.6; P: 0.5 and K: 0.5, an indication that this sediment fraction is impoverished in plant nutrients compared to the original soil (Table 23). Furthermore, there is no report on the association of coarse soil particles and the fertility of a soil, as is the case with fine soil particles. Generally, the sandier the soil the lower its nu‐

**Overall mean Source of variation K in sludge**

rial, a range of 0 - 472 ppm in sludge and 1671 - 6660 ppm in suspended material.

**Year 2 (384 mm)**

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

584 ppm in suspended material.

*Potassium*

**Treat/Year (Rainfall)**

n = 3 (Treatment x Year)

**Year 1 (483 mm)**

n = 6 (Treatment) s.e.d. = 2.126 s2 = 13.56 n = 12 (Year) s.e.d. = 1.503 df = 16

trient status and/or soil productivity.

s.e.d. = 3.006

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 20.** Nitrogen loss (kg/ha) with sludge under different tillage systems over three years at MakoholiContill site


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 21.** Phosphorus loss (kg/ha) with sludge under different tillage systems over three years at MakoholiContill site

#### *Phosphorus*

The amount of P lost with sludge was, as expected, lower than that lost with suspended ma‐ terial for all the cropped treatments (Figure 3b). During the two out of three seasons, there was no phosphorus loss with sludge under mulch ripping and during the last year, the P lost with this sediment fraction constituted only 3% of total P lost. Under tied ridging the losses were nearly the same, ranging from 2 - 4%. The conventional tillage treatment realiz‐ ed significantly higher losses between 16 and 35% of total P lost and once again the bare fal‐ low recorded, during two of the three years, more than 50% of total P lost. The overall treatment and year differences were significant at P < 0.001 (Table 21). It is clear once again that P is associated with fine soil particles and not with the non-reactive coarse material as is evidenced by the low P concentrations in sludge, ranging from 0 - 34 ppm compared to 73 - 584 ppm in suspended material.

#### *Potassium*

**Treat/Year (Rainfall)**

142 Research on Soil Erosion Soil Erosion

n = 3 (Treatment x Year)

> **Treat/Year (Rainfall)**

n = 9 (Treatment) s.e.d. =

n = 12 (Year) s.e.d. =

n= 3 (Treatment x Year)

*Phosphorus*

**Year 1 (483 mm)**

n = 9 (Treatment) s.e.d. = 1.962 s2 = 17.32 n = 12 (Year) s.e.d. = 1.699 df = 24

s.e.d. = 3.398

**Year 1 (483 mm)**

0.02192

s.e.d. = 0.04384

**Year 2 (384 mm)**

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Year 2 (384 mm)**

0.02531 s2 = 0.002883

df = 24

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Year 3 (765 mm)**

**Table 20.** Nitrogen loss (kg/ha) with sludge under different tillage systems over three years at MakoholiContill site

**Year 3 (765 mm)**

**Table 21.** Phosphorus loss (kg/ha) with sludge under different tillage systems over three years at MakoholiContill site

The amount of P lost with sludge was, as expected, lower than that lost with suspended ma‐ terial for all the cropped treatments (Figure 3b). During the two out of three seasons, there was no phosphorus loss with sludge under mulch ripping and during the last year, the P lost with this sediment fraction constituted only 3% of total P lost. Under tied ridging the losses were nearly the same, ranging from 2 - 4%. The conventional tillage treatment realiz‐ ed significantly higher losses between 16 and 35% of total P lost and once again the bare fal‐ low recorded, during two of the three years, more than 50% of total P lost. The overall treatment and year differences were significant at P < 0.001 (Table 21). It is clear once again

CT 0.3423 0.0280 0.2350 0.4009 Treat \*\*\* MR 0.0000 0.0000 0.0063 0.0033 Year \*\*\* TR 0.0047 0.0003 0.0050 0.0021 Treat x Year \*\*\* BF 0.4910 0.1760 0.5357 0.2018 MR vs TR NS Overall mean 0.2095 0.0511 0.1955 0.1520 CT vs (MR, TR) \*\*\*

CT 7.19 1.20 11.70 6.70 Treat \*\*\* MR 0.00 0.00 0.65 0.22 Year \*\* TR 0.09 0.01 0.20 0.10 Treat x Year \* BF 19.75 6.39 21.95 16.03 MR vs TR NS Overall mean 6.76 1.90 8.62 5.76 CT vs (MR, TR) \*\*\*

**Overall mean Source of variation N in sludge**

**Overall mean Source of variation P in sludge**

The sludge fraction constituted the lowest losses of K under the cropped treatments, ranging from 0 - 5% under the conservation tillage treatments, a maximum of 32% under conven‐ tional tillage (Figure 3c). Under the bare fallow 48 - 72 % of total K was lost with this sedi‐ ment fraction. These obvious differences between both treatment and year factors were significant at P < 0.001 (Table 22). K is therefore associated with the suspended material than with sludge, the somewhat higher percentages lost under conventional tillage and bare fal‐ low are merely in relation to the very high coarse material lost under these treatments as the actual nutrient concentration in the sludge is very low compared to that in suspended mate‐ rial, a range of 0 - 472 ppm in sludge and 1671 - 6660 ppm in suspended material.


Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 22.** Potassium loss (kg/ha) with sludge under different tillage systems over three years at MakoholiContill site

Due to the high proportion of non-reactive coarse particles in sludge, the nutrients concen‐ tration was low compared to total sediments and suspended material. Generally all the nu‐ trients under all tillage systems recorded lower nutrient concentrations in the sludge compared to nutrient concentrations in the original soil, resulting in enrichment ratios less than 1.0, with the exception of N under bare fallow, which recorded 1.0. The overall nutrient enrichment ratios in sludge were as follows: N: 0.6; P: 0.5 and K: 0.5, an indication that this sediment fraction is impoverished in plant nutrients compared to the original soil (Table 23). Furthermore, there is no report on the association of coarse soil particles and the fertility of a soil, as is the case with fine soil particles. Generally, the sandier the soil the lower its nu‐ trient status and/or soil productivity.


*3.5.5. Eroded nutrients versus soil loss and sediment fraction*

loss as well as sediment fraction (Tables 24 and 25).

**kg/1t SL**

kg/1tSusp.

kg/1t Sludge

**Treat/Year Element Element**

Treat/Year Element Element

Treat/Year Element Element

SL = Soil loss; Susp = suspended material

site

low for P.

Regression analysis was carried out to relate nutrient loss with the amount of soil lost and sediment fraction. Firstly, a general regression analysis was carried out, where all the data collected was pooled, i.e. without specifying the treatments or the years and soil loss, sus‐ pended material and sludge were considered independently (Table 24). Data was then split according to the different treatments (disregarding years) and again the different elements were regressed with soil loss and sediment fractions. From the regression output, each ele‐ ment was then calculated in relation to a tonne of lost soil and/ or sediment fraction. Corre‐ lation coefficients were also worked out for the relationship between each element and soil

**Standard error % variance**

Standard error % variance

Standard error % variance

Pooled N 0.360 0.019700 94.5 \*\*\* 0.980 Pooled P 0.010 0.002090 38.3 \*\*\* 0.719 Pooled K 0.767 0.104000 80.0 \*\*\* 0.908

Pooled N 1.589 0.0416 95.4 \*\*\* 0.977 Pooled P 0.058 0.00722 40.6 \*\*\* 0.654 Pooled K 4.201 0.271 86.5 \*\*\* 0.932

Pooled N 0.186 0.0137 76.5 \*\*\* 0.879 Pooled P 0.005 0.000302 80.0 \*\*\* 0.904 Pooled K 0.390 0.0198 92.1 \*\*\* 0.960

**Table 24.** Nutrient loss as affected by soil loss, sludge and suspended material over three seasons at MakoholiContill

The results of the regression analysis show that pooling the data gave moderate nutrient losses for every tonne of soil lost. All the nutrients were below 1 kg for every 1 tonne of soil lost, i.e., total sediments (ranging from 0.01 for P to 0.7 kg for K). The amounts of the nu‐ trient losses were related to the losses under bare fallow but these amounts would under es‐ timate the losses under cropped treatments. Generally for the pooled estimates, K was the most abundant element in the sediments and the sequence could be summed up as follows: K > N > P. The variance accounted for in the estimates was also very high for N and K and

**accounted for**

Quantifying Nutrient Losses with Different Sediment Fractions Under Four Tillage Systems...

accounted for

accounted for

**P value Correlation**

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145

P value Correlation

P value Correlation

**SL:Element**

Susp:Element

Sludge:Element

**Table 23.** Nutrient concentrations in the soil (0 - 25 cm) versus nutrient concentrations in the sludge and enrichment ratios for different tillage systems at MakoholiContill site

Key: R-O = Run-off; Susp = Suspended material

**Figure 3.** (a) Nitrogen, (b) phosphorus and (c) potassium losses as influenced by different erosion fractions under four tillage systems at MakoholiContill site (averages over three years)

#### *3.5.5. Eroded nutrients versus soil loss and sediment fraction*

**Treat Nutrient concentration**

ratios for different tillage systems at MakoholiContill site

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Key: R-O = Run-off; Susp = Suspended material

tillage systems at MakoholiContill site (averages over three years)

N loss with each sediment fraction

Year 1

144 Research on Soil Erosion Soil Erosion

Year 2

Year 3

**in sludge**

Key: CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

CT MR TR BF

Treatments

**K loss with each sediment fraction**

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

N % P ppm K ppm N P K

BF 0.04 17.5 429.8 1.0 0.4 0.8 CT 0.03 31.8 472.8 0.6 0.6 0.8 MR 0.00 0.0 0.0 0.0 0.0 0.0 TR 0.02 33.6 414.7 0.4 0.4 0.9

BF 0.02 11.0 240.8 0.7 0.6 0.4 CT 0.03 17.7 333.3 0.8 0.7 0.5 MR 0.00 0.0 0.0 0.0 0.0 0.0 TR 0.01 10.0 83.3 0.2 0.2 0.2

BF 0.04 9.3 - 1.3 0.6 - CT 0.03 16.4 - 0.8 0.6 - MR 0.05 13.2 - 1.0 0.5 - TR 0.04 26.0 - 0.8 0.8 -

**Table 23.** Nutrient concentrations in the soil (0 - 25 cm) versus nutrient concentrations in the sludge and enrichment

**P loss with each sediment fraction**

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

CT MR TR BF

**Treatments** R-O Susp Sludge

**Figure 3.** (a) Nitrogen, (b) phosphorus and (c) potassium losses as influenced by different erosion fractions under four

CT MR TR BF

Sludge Susp R-O

**Treatments**

(c)

(a) (b)

**Enrichment ratios: nutrient in soil: nutrient in sludge**

Regression analysis was carried out to relate nutrient loss with the amount of soil lost and sediment fraction. Firstly, a general regression analysis was carried out, where all the data collected was pooled, i.e. without specifying the treatments or the years and soil loss, sus‐ pended material and sludge were considered independently (Table 24). Data was then split according to the different treatments (disregarding years) and again the different elements were regressed with soil loss and sediment fractions. From the regression output, each ele‐ ment was then calculated in relation to a tonne of lost soil and/ or sediment fraction. Corre‐ lation coefficients were also worked out for the relationship between each element and soil loss as well as sediment fraction (Tables 24 and 25).


SL = Soil loss; Susp = suspended material

**Table 24.** Nutrient loss as affected by soil loss, sludge and suspended material over three seasons at MakoholiContill site

The results of the regression analysis show that pooling the data gave moderate nutrient losses for every tonne of soil lost. All the nutrients were below 1 kg for every 1 tonne of soil lost, i.e., total sediments (ranging from 0.01 for P to 0.7 kg for K). The amounts of the nu‐ trient losses were related to the losses under bare fallow but these amounts would under es‐ timate the losses under cropped treatments. Generally for the pooled estimates, K was the most abundant element in the sediments and the sequence could be summed up as follows: K > N > P. The variance accounted for in the estimates was also very high for N and K and low for P.

The sediment composition also influenced the amount of nutrients per unit of soil loss, with more nutrients lost with suspended material than with coarse material (Table 24). This table shows that an average of 1.589 kg N was lost with one tonne of suspended material com‐ pared to 0.186 kg N lost with one tonne of sludge, i.e. (8.5 times). About 12 times more P was lost with one tonne of suspended material than with sludge, while K was 11 times more in suspended material than in sludge. This information further consolidates the fact that much more nutrients are lost with suspended material regardless of tillage treatment and plant element. The loss of coarse soil particles should have implications on soil productivity mainly due to the reduction of soil tilth and not soil fertility.

There is evidence that a substantial amount of nutrients is lost with erosion, as shown by the overall averages of 12.3 kg/ha N; 0.5 kg/ha P and 17.3 kg/ha K. The amount of nutrient lost was found to be strongly dependent on the nutrient status of the soil, i.e. the higher the sta‐ tus of a particular nutrient in the soil, the higher its loss with erosion. The nutrient status of the soils showed the following trend K > N > P and the overall nutrient loss with erosion also showed exactly the same trend. This explains why soils with higher fertility status lose much more nutrients relative to those with a lower fertility status (Stoorvogel and Smaling, 1990). According to Rose *et al.* (1988), the amount of a nutrient lost with erosion is depend‐ ent upon the soil type, tillage practice and the type of erosion. From this study it was found that the amount of soil loss and the sediment fraction − including run-off − were also impor‐ tant in determining the amount of nutrient loss, especially on sandy soils, where the amount

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147

The sediment fraction on sandy soils is very important in determining the amount of nu‐ trient loss due to the selective nature of sheet erosion on these soils. Nutrient losses in the water portion of the run-off were small, almost negligible compared to the losses with solid sediments, ranging from 1 – 2 % of total N; 0.6 – 4% of total P and 1 – 20% of total K. This was expected, as these nutrients have to be dissolved in water. Even in the original soil, the nutrients in the soil solution are only a small fraction of nutrients sorbed in the soil, ranging from 0.001% for P to 25% for Ca (Brady, 1984; Stevenson, 1985; Singer and Munns, 1987). The solid fraction is therefore, the major source of plant nutrient loss (Barisas et al., 1978; Kejela, 1991). Suspended material is the main source of nutrient loss from agricultural lands, as evidenced by the very high percentages of nutrient losses with this sediment fraction, es‐ pecially under conservation tillage, > 90%. Although the ratio between sludge and suspen‐ sion under the conventional tillage was between 1.5 and 5, about 25% of total sediments, it accounted for 63% of total N, 74% of total P and 73% of total K lost with erosion. With a sludge suspension ratio of between 10 and 17, the bare fallow also recorded proportionally higher nutrients with suspended than with sludge, shown by the following percentages: 39% of total N and K and 46% of total P being lost with this 8% suspended material. This finding certainly shows that much more nutrients are lost with suspended material than

Furthermore, the N concentration in sludge ranged from 0 – 0.5%, while in suspended mate‐ rial it ranged from 0.2 – 0.65 %, P concentration in sludge ranged from 0 – 34 ppm compared to 73 – 584 ppm and K recorded a concentration of 0 – 472 ppm in sludge and 1671 – 6660 ppm in suspended material. This is because clay and organic matter are the sorption sites for much of the nutrients and organic matter is also crucial in the cycle of P and N. Brady (1984) reported that organic matter was the major indigenous source of N while 65% of total P in the soil was found in the form of organic compounds. Clay more than organic matter, is the main source of fixed K and other cations and K losses are therefore associated with clay loss. Due to the selective nature of sheet erosion, high affinity of P to adsorption, fixation of K and ammonium ions, as well as the presence of K ions in clay minerals, erosion is the main

of clay and organic matter are critical as sources of plant nutrients.

with any other sediment fraction.

source of nutrient and productivity loss in agricultural lands.

The different treatments also showed that the conservation tillage treatments lost more nu‐ trients per unit soil loss than conventional tillage systems (Table 25), due to the low sludge: suspension ratio in the former. For the same reason, conventional tillage also lost more nu‐ trients (all elements) per tonne of soil loss than the bare fallow. Between the two conserva‐ tion tillage treatments, more nutrients (N, P and K) were lost under tied ridging than under mulch ripping. The differences though, were not significant. All the treatments showed a similar trend as that of pooled data. P losses were highly correlated to soil loss under mulch ripping, followed by bare fallow, whereas under conventional tillage and tied ridging the correlation was rather low, although still significant. The poor correlation may be as a result of the very low P losses, which may affect the accuracy of such measurements.


SL = Soil loss; Susp = suspended material; CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare Fallow

**Table 25.** The relationship between nutrient loss and soil loss under different tillage systems at MakoholiContill site

There is evidence that a substantial amount of nutrients is lost with erosion, as shown by the overall averages of 12.3 kg/ha N; 0.5 kg/ha P and 17.3 kg/ha K. The amount of nutrient lost was found to be strongly dependent on the nutrient status of the soil, i.e. the higher the sta‐ tus of a particular nutrient in the soil, the higher its loss with erosion. The nutrient status of the soils showed the following trend K > N > P and the overall nutrient loss with erosion also showed exactly the same trend. This explains why soils with higher fertility status lose much more nutrients relative to those with a lower fertility status (Stoorvogel and Smaling, 1990). According to Rose *et al.* (1988), the amount of a nutrient lost with erosion is depend‐ ent upon the soil type, tillage practice and the type of erosion. From this study it was found that the amount of soil loss and the sediment fraction − including run-off − were also impor‐ tant in determining the amount of nutrient loss, especially on sandy soils, where the amount of clay and organic matter are critical as sources of plant nutrients.

The sediment composition also influenced the amount of nutrients per unit of soil loss, with more nutrients lost with suspended material than with coarse material (Table 24). This table shows that an average of 1.589 kg N was lost with one tonne of suspended material com‐ pared to 0.186 kg N lost with one tonne of sludge, i.e. (8.5 times). About 12 times more P was lost with one tonne of suspended material than with sludge, while K was 11 times more in suspended material than in sludge. This information further consolidates the fact that much more nutrients are lost with suspended material regardless of tillage treatment and plant element. The loss of coarse soil particles should have implications on soil productivity

The different treatments also showed that the conservation tillage treatments lost more nu‐ trients per unit soil loss than conventional tillage systems (Table 25), due to the low sludge: suspension ratio in the former. For the same reason, conventional tillage also lost more nu‐ trients (all elements) per tonne of soil loss than the bare fallow. Between the two conserva‐ tion tillage treatments, more nutrients (N, P and K) were lost under tied ridging than under mulch ripping. The differences though, were not significant. All the treatments showed a similar trend as that of pooled data. P losses were highly correlated to soil loss under mulch ripping, followed by bare fallow, whereas under conventional tillage and tied ridging the correlation was rather low, although still significant. The poor correlation may be as a result

**Standard error % variance**

SL = Soil loss; Susp = suspended material; CT = Conventional Tillage; MR = Mulch Ripping; TR = Tied Ridging; BF = Bare

**Table 25.** The relationship between nutrient loss and soil loss under different tillage systems at MakoholiContill site

BF N 0.305 0.030000 70.9 \*\*\* 0.842 BF P 0.008 0.001270 29.9 \*\*\* 0.614 BF K 0.700 0.105000 72.0 \*\*\* 0.958 CT N 0.434 0.044200 54.2 \*\*\* 0.891 CT P 0.017 0.005070 very low \*\* 0.339 CT K 1.199 0.073400 95.1 \*\*\* 0.977 MR N 1.242 0.041400 98.7 \*\*\* 0.994 MR P 0.028 0.002420 89.5 \*\*\* 0.966 MR K 4.600 0.659000 80.1 \*\*\* 0.951 TR N 1.437 0.150000 79.0 \*\*\* 0.900 TR P 0.059 0.016700 11.7 \* 0.496 TR K 5.155 0.359000 95.7 \*\*\* 0.981

**accounted for**

**P value Correlation**

**SL:Element**

of the very low P losses, which may affect the accuracy of such measurements.

mainly due to the reduction of soil tilth and not soil fertility.

**Treat. Element Element**

146 Research on Soil Erosion Soil Erosion

Fallow

**kg/1t SL**

The sediment fraction on sandy soils is very important in determining the amount of nu‐ trient loss due to the selective nature of sheet erosion on these soils. Nutrient losses in the water portion of the run-off were small, almost negligible compared to the losses with solid sediments, ranging from 1 – 2 % of total N; 0.6 – 4% of total P and 1 – 20% of total K. This was expected, as these nutrients have to be dissolved in water. Even in the original soil, the nutrients in the soil solution are only a small fraction of nutrients sorbed in the soil, ranging from 0.001% for P to 25% for Ca (Brady, 1984; Stevenson, 1985; Singer and Munns, 1987). The solid fraction is therefore, the major source of plant nutrient loss (Barisas et al., 1978; Kejela, 1991). Suspended material is the main source of nutrient loss from agricultural lands, as evidenced by the very high percentages of nutrient losses with this sediment fraction, es‐ pecially under conservation tillage, > 90%. Although the ratio between sludge and suspen‐ sion under the conventional tillage was between 1.5 and 5, about 25% of total sediments, it accounted for 63% of total N, 74% of total P and 73% of total K lost with erosion. With a sludge suspension ratio of between 10 and 17, the bare fallow also recorded proportionally higher nutrients with suspended than with sludge, shown by the following percentages: 39% of total N and K and 46% of total P being lost with this 8% suspended material. This finding certainly shows that much more nutrients are lost with suspended material than with any other sediment fraction.

Furthermore, the N concentration in sludge ranged from 0 – 0.5%, while in suspended mate‐ rial it ranged from 0.2 – 0.65 %, P concentration in sludge ranged from 0 – 34 ppm compared to 73 – 584 ppm and K recorded a concentration of 0 – 472 ppm in sludge and 1671 – 6660 ppm in suspended material. This is because clay and organic matter are the sorption sites for much of the nutrients and organic matter is also crucial in the cycle of P and N. Brady (1984) reported that organic matter was the major indigenous source of N while 65% of total P in the soil was found in the form of organic compounds. Clay more than organic matter, is the main source of fixed K and other cations and K losses are therefore associated with clay loss. Due to the selective nature of sheet erosion, high affinity of P to adsorption, fixation of K and ammonium ions, as well as the presence of K ions in clay minerals, erosion is the main source of nutrient and productivity loss in agricultural lands.

The affinity of the nutrients to the fine soil particles cannot be doubted. The exchange sites on the clay minerals and organic matter are the basis for this affinity, as nutrients are held at these exchange sites (Brady, 1984; Stevenson, 1985). According to Singer and Munns (1987), the different clay minerals and humus are most important in holding nutrients due to their specialized surface properties, capable of chemically retaining individual nutrients. Tiessen, Cuevas and Salcedo (1998) and Stocking (1984) also reported that soil organic matter provid‐ ed plant nutrients in low-input agriculture and that N and P release depended on the miner‐ alization of organic matter while cation exchange depended on the maintenance of organic matter. This is why the loss of top soil is detrimental to any soils' productivity as there is a close association between clay, organic and the plant nutrients. The proximity and concen‐ tration of organic matter near the soil surface and close association with plant nutrients, make the erosion of soil organic matter a strong indicator of overall plant nutrients resulting from erosion (Follet*et al.*, 1987).

amount of nutrients lost. The conservation tillage systems dramatically reduced losses of soil and total nutrients when compared to conventional tillage systems, however the nu‐ trient concentrations per unit soil loss are higher than for conventional tillage systems.

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The concentration of nutrients in the sediments was much higher under the conservation tillage systems as compared to conventional tillage, obviously as a result of a high percent‐ age of fine particles in the sediments compared to the later. Very high enrichment ratios of all nutrients were thus recorded in the sediments of the conservation tillage systems as a re‐ sult of the high affinity of nutrients to fine soil particles (Barisas*et al.* 1978). However, the advantage of low amount of sediments in conservation tillage also resulted in lower average

The different years lost significantly different amounts of nutrients, which depended on the amount of rainfall received and amount of soil lost. For all the nutrients, nutrient losses in‐ creased with the increase in rainfall amount, i.e. with increased sediments. The regression analysis that was carried out to find the relationship between soil loss ad nutrient loss showed that nutrient losses are highly dependent on soil losses and that if soil losses are known, nutrient losses can be confidently predicted. The conservation tillage systems lose more nutrients per unit soil loss than conventional tillage systems because their sediments are predominantly fine particles, e.g., per tonne of soil lost 1.4 and 1.2 kg/ha of N were pre‐ dicted to be lost under TR and MR respectively, while BF and CT were predicted to lose 0.3 and 0.4 kg/ha respectively. The point on the high affinity of all the nutrient elements to the

By conserving the soil, nutrients are conserved and nutrient replacement costs of erosion are drastically reduced, especially if the value of sustainable production is also taken into con‐ sideration. It should be emphasized, however, that the loss of organic matter and clay and resultant physical degradation of the soil, leading to poor tilth, low available water holding capacity and high bulk density, was not evaluated. This means that the value of nutrient

Sheet erosion is a selective process that robs the soil of its fine particles, i.e. clay and organic matter. The high enrichment ratios of clay and organic matter found in sediments as com‐ pared to the original soil, serve to support this fact. Of the two sediment fractions, the soil lost in suspension is the most detrimental as it comprises of clay and organic matter parti‐ cles, which are known to be the soils' plant nutrient reservoirs. There is a very high associa‐ tion between nutrients and fine soil particles as shown by the high amount of nutrients lost with a unit mass of suspended material as compared to those lost with the same unit mass of sludge and/ or total sediments. This makes the suspended material the most detrimental sediment fraction, negatively affecting the soils' fertility status as well as impacting nega‐ tively on the soil's physical condition. The suspended material recorded high concentrations of clay, organic matter and nutrients when compared to sludge. However, the total loss of

losses under this system.

**4. Conclusions**

fine soil particles has also been emphasized.

losses is but a fraction of total loss (Kejela, 1991).

The results across all cropped treatments and for all the elements show that most of the nu‐ trients lost with erosion are associated with suspended material. Under conservation tillage systems it is arguable that the high percentages may be due to the fact that most of the soil lost was in suspended form, however, the high losses attributed to this fraction under con‐ ventional tillage indicate otherwise. This finding proves beyond doubt that although less suspended material may be lost from a field, it carries most of the soil nutrients with it. The conservation tillage systems obviously have higher percentages of nutrient losses with sus‐ pended material, however the quantities of nutrients lost are negligible when compared to those lost under conventional tillage and bare fallow because of reduced soil losses under conservation tillage.

The nutrient losses with sludge are minimal when compared to those lost with suspended material. While as high as 92% of total soil loss under bare fallow is sludge, the percentages of nutrients lost with this fraction are not as high (56% N; 23% P and 52% K). It should be noted also that there was no distinct separation of sludge and suspension, due to the fact that some suspended material would settle with the sludge, during a storm, before sampling was carried out. This explains the presence of 0.81 - 4.02% clay and 0.04 - 0.55% organic mat‐ ter in the sludge. The nutrients in the sludge can be attributed to the fine soil particles and not the coarse material.

Nutrient losses (N, P and K) varied significantly among the different treatments. The conser‐ vation tillage treatments lost significantly less nutrients compared to the conventional tillage systems. Here, 2.3 and 2.7 kg/ha N; 0.09 and 0.2 kg/ha P; 0.6 and 4.3 kg/ha K were lost under mulch ripping and tied ridging respectively compared to 15.8 and 28.4 kg/ha N; 0.8 and 0.9 kg/ha P; 24.5 and 39.8 kg/ha K under conventional tillage and bare fallow respectively. As these treatments lost significantly different amounts of sediments, also following the same trend, this indicates that the nutrient losses with erosion are closely associated with the rate of soil loss (Elwell and Stocking 1988; Kejela 1991). The tillage systems in this study also showed their effect on the amount of nutrients lost by determining the amount of soil loss. Due to the fact that plant nutrients sorbed to the soil are transported with eroding sedi‐ ments, the amount of soil lost with erosion becomes very important in determining the amount of nutrients lost. The conservation tillage systems dramatically reduced losses of soil and total nutrients when compared to conventional tillage systems, however the nu‐ trient concentrations per unit soil loss are higher than for conventional tillage systems.

The concentration of nutrients in the sediments was much higher under the conservation tillage systems as compared to conventional tillage, obviously as a result of a high percent‐ age of fine particles in the sediments compared to the later. Very high enrichment ratios of all nutrients were thus recorded in the sediments of the conservation tillage systems as a re‐ sult of the high affinity of nutrients to fine soil particles (Barisas*et al.* 1978). However, the advantage of low amount of sediments in conservation tillage also resulted in lower average losses under this system.

The different years lost significantly different amounts of nutrients, which depended on the amount of rainfall received and amount of soil lost. For all the nutrients, nutrient losses in‐ creased with the increase in rainfall amount, i.e. with increased sediments. The regression analysis that was carried out to find the relationship between soil loss ad nutrient loss showed that nutrient losses are highly dependent on soil losses and that if soil losses are known, nutrient losses can be confidently predicted. The conservation tillage systems lose more nutrients per unit soil loss than conventional tillage systems because their sediments are predominantly fine particles, e.g., per tonne of soil lost 1.4 and 1.2 kg/ha of N were pre‐ dicted to be lost under TR and MR respectively, while BF and CT were predicted to lose 0.3 and 0.4 kg/ha respectively. The point on the high affinity of all the nutrient elements to the fine soil particles has also been emphasized.

By conserving the soil, nutrients are conserved and nutrient replacement costs of erosion are drastically reduced, especially if the value of sustainable production is also taken into con‐ sideration. It should be emphasized, however, that the loss of organic matter and clay and resultant physical degradation of the soil, leading to poor tilth, low available water holding capacity and high bulk density, was not evaluated. This means that the value of nutrient losses is but a fraction of total loss (Kejela, 1991).

### **4. Conclusions**

The affinity of the nutrients to the fine soil particles cannot be doubted. The exchange sites on the clay minerals and organic matter are the basis for this affinity, as nutrients are held at these exchange sites (Brady, 1984; Stevenson, 1985). According to Singer and Munns (1987), the different clay minerals and humus are most important in holding nutrients due to their specialized surface properties, capable of chemically retaining individual nutrients. Tiessen, Cuevas and Salcedo (1998) and Stocking (1984) also reported that soil organic matter provid‐ ed plant nutrients in low-input agriculture and that N and P release depended on the miner‐ alization of organic matter while cation exchange depended on the maintenance of organic matter. This is why the loss of top soil is detrimental to any soils' productivity as there is a close association between clay, organic and the plant nutrients. The proximity and concen‐ tration of organic matter near the soil surface and close association with plant nutrients, make the erosion of soil organic matter a strong indicator of overall plant nutrients resulting

The results across all cropped treatments and for all the elements show that most of the nu‐ trients lost with erosion are associated with suspended material. Under conservation tillage systems it is arguable that the high percentages may be due to the fact that most of the soil lost was in suspended form, however, the high losses attributed to this fraction under con‐ ventional tillage indicate otherwise. This finding proves beyond doubt that although less suspended material may be lost from a field, it carries most of the soil nutrients with it. The conservation tillage systems obviously have higher percentages of nutrient losses with sus‐ pended material, however the quantities of nutrients lost are negligible when compared to those lost under conventional tillage and bare fallow because of reduced soil losses under

The nutrient losses with sludge are minimal when compared to those lost with suspended material. While as high as 92% of total soil loss under bare fallow is sludge, the percentages of nutrients lost with this fraction are not as high (56% N; 23% P and 52% K). It should be noted also that there was no distinct separation of sludge and suspension, due to the fact that some suspended material would settle with the sludge, during a storm, before sampling was carried out. This explains the presence of 0.81 - 4.02% clay and 0.04 - 0.55% organic mat‐ ter in the sludge. The nutrients in the sludge can be attributed to the fine soil particles and

Nutrient losses (N, P and K) varied significantly among the different treatments. The conser‐ vation tillage treatments lost significantly less nutrients compared to the conventional tillage systems. Here, 2.3 and 2.7 kg/ha N; 0.09 and 0.2 kg/ha P; 0.6 and 4.3 kg/ha K were lost under mulch ripping and tied ridging respectively compared to 15.8 and 28.4 kg/ha N; 0.8 and 0.9 kg/ha P; 24.5 and 39.8 kg/ha K under conventional tillage and bare fallow respectively. As these treatments lost significantly different amounts of sediments, also following the same trend, this indicates that the nutrient losses with erosion are closely associated with the rate of soil loss (Elwell and Stocking 1988; Kejela 1991). The tillage systems in this study also showed their effect on the amount of nutrients lost by determining the amount of soil loss. Due to the fact that plant nutrients sorbed to the soil are transported with eroding sedi‐ ments, the amount of soil lost with erosion becomes very important in determining the

from erosion (Follet*et al.*, 1987).

148 Research on Soil Erosion Soil Erosion

conservation tillage.

not the coarse material.

Sheet erosion is a selective process that robs the soil of its fine particles, i.e. clay and organic matter. The high enrichment ratios of clay and organic matter found in sediments as com‐ pared to the original soil, serve to support this fact. Of the two sediment fractions, the soil lost in suspension is the most detrimental as it comprises of clay and organic matter parti‐ cles, which are known to be the soils' plant nutrient reservoirs. There is a very high associa‐ tion between nutrients and fine soil particles as shown by the high amount of nutrients lost with a unit mass of suspended material as compared to those lost with the same unit mass of sludge and/ or total sediments. This makes the suspended material the most detrimental sediment fraction, negatively affecting the soils' fertility status as well as impacting nega‐ tively on the soil's physical condition. The suspended material recorded high concentrations of clay, organic matter and nutrients when compared to sludge. However, the total loss of clay, organic matter and plant nutrients in the sediments is not dependent upon their con‐ centrations in the eroded soil but rather on the total amount of soil lost. Thus mulch ripping and tied ridging proved to be effective in maintaining clay and organic matter levels and thus significantly reducing nutrient losses from agricultural lands due to their ability to re‐ duce soil erosion.

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### **Acknowledgements**

I would like to express my gratitude to GTZ for providing the much needed funding through CONTILL (Conservation Tillage), a collaborative project between GTZ and the Government of Zimbabwe (GoZ). Further acknowledgement goes to the GoZ, for providing me with the opportunity and research facilities. I would like to thank all the CONTILL members from both Domboshawa and especially Makoholi site for their relentless support and input towards the success of this project.
