**2. Materials and methods**

#### **2.1. Study site**

Zimbabwe's climate is moderated by altitude and although the country lies within the trop‐ ics its climate is sub-tropical. According to the Koeppen climate classification system, the country is thus classified as temperate Cwb, i.e. mild mid-latitude, with dry winters and hot summers (Roesenberg, 2007). The average temperatures rarely exceed 330 C in summer or drop beyond 70 C in winter (MNTR, 1987). The country has been classified into five agro-eco‐ logical regions, namely Natural Regions I, II, III, IV and V. Only Natural Regions I and II have relatively high effective rainfall and are suitable for intensive agricultural production. Natural Regions III, IV and V constitute 83% of the total land area and are not suitable for intensive, high input agriculture (Moyo et al., 1991). Zimbabwe's soils are predominantly derived from granite and the geological complexity of the granites leads to the complexity of the soils (Thompson and Purves, 1978; Nyamapfene, 1991). The clay content of these soils varies according to the degree of weathering (influenced by rainfall) and catenal position (Thompson and Purves, 1978; Nyamapfene, 1991). From among all the soils derived from granite, the sandy soils, of the fersiallitic group, comprise the majority (Thompson and Purves, 1978) and are dominant in the small-holder farming areas (Vogel, 1993). These soils are generally light to medium textured and characterized by the presence of significant amounts of coarse sands (MNRT, 1987; Nyamapfene, 1991). The agricultural potential of these soils is fair (Grant, 1981; MNRT, 1987) and their productivity is likely to decline under intensive continuous cropping (Thompson and Purves, 1978). Therefore increased produc‐ tion can only be achieved through good management as well as application of fertilizers or animal manure (MNRT, 1987).

The research work was carried out at Makoholi Research Station, situated 30 km North of Masvingo town and is the regional agricultural research centre for the sandveld soils in the medium to low rainfall areas. The station lies within Natural Region IV at an altitude of about 1200 m (Thompson, 1967; Anon, 1969). Characteristic of this region is the erratic and unreliable rainfall both between and within seasons (Anon, 1969). Average annual rainfall is between 450 and 650 mm (Thompson and Purves, 1981). The soils at Makoholi are also in‐ herently infertile, pale, coarse-grained, granite-derived sands, (Makoholi 5G) of the fersiallit‐ ic group, Ferralic Arenosols (Thompson, 1967; Thompson and Purves, 1978). Arable topsoil averages between 82 and 93% sand, 1 and 12% silt and 4 and 6% clay (Thompson and Purves, 1981; Vogel, 1993). The small amount of clay present is in a highly dispersed form and contains a mixture of 2:1 lattice minerals and kaolinite (Thompson, 1967). The organic matter content is also very low, about 0.8%, while pH (CaCl2) is as low as 4.5. The soils are generally well drained with no distinct structure (Thompson and Purves, 1981), but some sites have a stone line between 50 and 80 cm depth. The low infiltration rates and water holding capacities are due to the soil texture characteristics.

#### **2.2. Experimental design and tillage treatments**

case with the soils under study - are highly significant to the environmental and agricultural

An ideal tillage system should also promote soil water storage, reduce erosion, increase crop yield and be straight forward enough to be adopted by farmers (Cassel, Raczkowski and Denton, 1995). Tillage intensity should be reduced and mulching promoted so that erosion susceptible soils are not exposed to weather conditions (Sauerbeck, 1994). Research has shown that the most cost effective erosion control practices involve keeping crop residues

The consequence of inappropriate land-use management is accelerated soil erosion leading to soil degradation and eventually to decreased soil productivity. On-site loss of potential crop pro‐ duction due to eroding away of productive organic-enriched topsoil has always been considered a major threat to sustained food production (Lowery and Larson, 1995). On arable land, the proc‐ ess of sheet erosion is insidious and is usually irreversible. Sheet erosion depletes soil productivi‐ ty through alteration of soil physical and chemical properties. The extent to which these changes

Sheet erosion is a selective process that deprives the soil of its fine particles, i.e. particle size separation often takes place when soil material is eroded by water. Sediments generally con‐ tain a larger amount of the lighter elements, such as humus and higher proportions of finer soil particles than the original soil (Aylen, 1939; Massey and Jackson, 1952; Cormack, 1953; Hudson and Jackson, 1962; Shaxson, 1975; Hanotiaux, 1980; Young, 1980; Elwell and Stock‐ ing, 1984 ; Biot, 1986; Elwell, 1987). The finest particles are easily splashed out and/or carried in suspension, while the heavier particles are left behind (Poesen and Savat 1980). The soils are thus impoverished as these nutrient reservoirs are lost together with inherent and ap‐ plied plant nutrients. The bulk density of the soils is increased and plant available water is decreased. The degree with which particle size separation takes place is higher on sandy

The major significance of soil erosion therefore, lies in the movement of plant nutrients both inherent and applied (Shaxson, 1975). As a result, the eroded material is enriched with nu‐ trients, organic matter and clay particles. The enrichment ratio, defined as the concentration level of each factor (nutrient element, organic matter, clay) in eroded soil material compared to its level in the soil before erosion (Kejela, 1991), is an important parameter for the assess‐ ment of nutrient loss through erosion as well as assessing the impact of erosion on crop pro‐ ductivity. To this end therefore, this chapter seeks to assess the selective process of soil erosion and quantify the nutrient losses with each sediment fraction and the significance of

Zimbabwe's climate is moderated by altitude and although the country lies within the trop‐ ics its climate is sub-tropical. According to the Koeppen climate classification system, the

each sediment fraction in carrying plant nutrients during an erosion process.

on the surface and reducing tillage as much as possible (Reicosky*et al.*, 1996).

take place greatly depends on the soil type, crop and eco-region (Kaihura*et al.*, 1998).

potential of these soils (Hunt *et al.*, 1996).

116 Research on Soil Erosion Soil Erosion

soils than on clay soils (Hudson, 1958; 1959).

**2. Materials and methods**

**2.1. Study site**

The treatments were laid out in a randomized block design replicated three times. The blocks were located at different positions along the slope (Down-slope, Middle-slope and Up-slope). Four different tillage systems were considered namely: conventional tillage, mulch ripping, tied ridging and a bare fallow.

### *2.2.1. Conventional tillage*

The land was ox-ploughed to 23 cm depth, soon after harvest (winter ploughing), using a single-furrow mould-board plough and thereafter harrowed with a spike harrow in spring. All crop residues were removed from the plots, as is the practice in the communal areas. This tillage system is the most commonly used tillage system in the communal areas and was chosen as a standard primary tillage method, i.e. including this treatment provides a baseline for assessing the merits of other treatments (Working Document, 1990).

on-set of the rains, planting holes were made on all crop treatments, using a hand-hoe.

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Maize (*Zea mays* L.) is the staple food in Zimbabwe. For this reason, maize was chosen as a trial crop, so as to make the research project relevant to the small holder areas. Due to the dry conditions prevailing at Makoholi, maize variety R 201, which tolerates moisture stress and is short seasoned, was used. The crop spacing of 900 mm inter-row and 310 mm in-row were used resulting in a plant population of about 36 000 plants/ha. All weeding operations were done using a hand-hoe. The problems of nematodes, very common in the sandy soils and that of maize stalk borer were controlled, so as to minimize the influence of factors oth‐ er than those imposed by treatments. Carbofuran, a nematicide was applied into the plant‐ ing holes before the on-set of the rains, while Thiodan (against maize stalk borer) was

On all plots planting holes of about 10 cm depth and diameter were opened before the onset of the rains. Thereafter Carbofuran, was applied into these planting holes at a rate of 20 kg/ha. Compound D (N:P:K = 8:14:7) was also applied into the planting holes at a rate of 200 kg/ha to give a final ratio of 16 kg N: 12 kg P: 12 kg K. The nematicide and fertilizer were

Once the profile of the ridges was wet throughout, maize was planted, two seeds per sta‐ tion. Ten days after planting, crop emergence count was carried out followed by weeding. The crop was then thinned out to one plant per station. When the crop was about six weeks, ammonium nitrate top-dressing fertilizer was applied at 100 kg/ha, amounting to 34.5 kg N/ha. The ammonium nitrate application coincided with the second weeding and the appli‐

The standard soil erosion methodology for Zimbabwe (Wendelaar and Purkis, 1979) was used, where the plots were laid out at 4.5% slope. Soil loss and run-off measurements were from 30 m x 10 m run-off plots, with 5 m border strips on either side. The length of the plots was orientated up-slope. Tillage operations were done across the slope. Polythene strips

the tied ridging treatment, the collection area was 150 m long and 5 crop rows wide (4.5 m), with 2 guard rows above and below. The crop ridges were laid at 1% slope and the length of the plots was orientated across the slope. Surface run-off and soil loss from each plot were allowed to collect in a gutter at the bottom of the plot. From the gutter these were channeled through a PVC delivery pipe into the first 1500 litre conical tank. The collection tanks were calibrated and run-off was measured using a metre-stick. Once the first tank was full its overflow passed through a divisor box with ten slots, which channeled only one tenth of the overflow into the second tank. Nine tenths of this overflow was allowed to drain away, thus increasing the capacity of the second tank. Due to the larger net plots of the tied ridging

plot (Working Document, 1990). For

then slightly covered with soil and left until adequate rainfall had been received.

Thereafter basal fertilizer and a nematicide were applied into the planting holes.

**2.3. Agronomic details**

applied six weeks after planting.

cation of Thiodan, to control maize stalk borer.

were dug in to form the boundary around each 300 m2

**2.4. Soil loss assessment**

#### *2.2.2. Mulch ripping*

Crop residues from the previous season were left to cover the ground and only rip lines, 23 cm deep, were opened between the mulch rows, using a ripper tine. The rip lines acted as crop rows and were alternated every year, to allow roots ample time to decay. Two basic conservation tillage components were used here, i.e. minimum tillage and mulching. The main aim was to maximize infiltration through rainfall interception provided by the mulch, thus minimizing run-off. According to Hudson (1992), this parameter is the most important in the semi-arid regions, where soil moisture is the most limiting factor in agricultural pro‐ duction. This treatment is one of the basic conservation tillage systems, which has shown great potential in protecting the soils, without compromising the production potential and is currently being promoted by the Institute of Agricultural Engineering.

#### *2.2.3. Tied ridging*

The land was ploughed to the recommended depth of 23 cm in the first year and crop ridges constructed at 1 in 250 grade, using a ridger. The ridges were about 900 mm apart and small ties were put at about 700-1000 mm along the furrows between the crop ridges. These ties were between one half to two thirds the height of the crop ridges allowing for the water to flow over the ties and not over the ridges (Elwell and Norton, 1988). The ridges were main‐ tained several years through re-ridging so as to maintain their correct size and shape. This treatment has been found to reduce run-off, and the soil losses are also reduced to satisfacto‐ rily low levels of 0.1 to 0.3 t/ha, much less than the tolerable limit of 5 t/ha/yr. (Elwell and Norton, 1988).

#### *2.2.4. Bare fallow*

Ploughing, up to 23 cm depth, was done using a tractor disc plough and disc harrow. The plots were kept bare and weed free, by spraying the germinating weeds during the season. This treatment is important for soil erodibility assessment and modeling purposes, as it gives the highest possible soil loss values and will probably give the lowest nutrient loss val‐ ues as no fertilizers are applied.

At the beginning of this study, all trial plots had been under cultivation and the same treat‐ ment for a period of five years, having been opened up from virgin woodland. All tillage operations were carried out soon after harvest before the soil dried out. Shortly before the on-set of the rains, planting holes were made on all crop treatments, using a hand-hoe. Thereafter basal fertilizer and a nematicide were applied into the planting holes.

#### **2.3. Agronomic details**

*2.2.1. Conventional tillage*

118 Research on Soil Erosion Soil Erosion

*2.2.2. Mulch ripping*

*2.2.3. Tied ridging*

Norton, 1988).

*2.2.4. Bare fallow*

ues as no fertilizers are applied.

The land was ox-ploughed to 23 cm depth, soon after harvest (winter ploughing), using a single-furrow mould-board plough and thereafter harrowed with a spike harrow in spring. All crop residues were removed from the plots, as is the practice in the communal areas. This tillage system is the most commonly used tillage system in the communal areas and was chosen as a standard primary tillage method, i.e. including this treatment provides a

Crop residues from the previous season were left to cover the ground and only rip lines, 23 cm deep, were opened between the mulch rows, using a ripper tine. The rip lines acted as crop rows and were alternated every year, to allow roots ample time to decay. Two basic conservation tillage components were used here, i.e. minimum tillage and mulching. The main aim was to maximize infiltration through rainfall interception provided by the mulch, thus minimizing run-off. According to Hudson (1992), this parameter is the most important in the semi-arid regions, where soil moisture is the most limiting factor in agricultural pro‐ duction. This treatment is one of the basic conservation tillage systems, which has shown great potential in protecting the soils, without compromising the production potential and is

The land was ploughed to the recommended depth of 23 cm in the first year and crop ridges constructed at 1 in 250 grade, using a ridger. The ridges were about 900 mm apart and small ties were put at about 700-1000 mm along the furrows between the crop ridges. These ties were between one half to two thirds the height of the crop ridges allowing for the water to flow over the ties and not over the ridges (Elwell and Norton, 1988). The ridges were main‐ tained several years through re-ridging so as to maintain their correct size and shape. This treatment has been found to reduce run-off, and the soil losses are also reduced to satisfacto‐ rily low levels of 0.1 to 0.3 t/ha, much less than the tolerable limit of 5 t/ha/yr. (Elwell and

Ploughing, up to 23 cm depth, was done using a tractor disc plough and disc harrow. The plots were kept bare and weed free, by spraying the germinating weeds during the season. This treatment is important for soil erodibility assessment and modeling purposes, as it gives the highest possible soil loss values and will probably give the lowest nutrient loss val‐

At the beginning of this study, all trial plots had been under cultivation and the same treat‐ ment for a period of five years, having been opened up from virgin woodland. All tillage operations were carried out soon after harvest before the soil dried out. Shortly before the

baseline for assessing the merits of other treatments (Working Document, 1990).

currently being promoted by the Institute of Agricultural Engineering.

Maize (*Zea mays* L.) is the staple food in Zimbabwe. For this reason, maize was chosen as a trial crop, so as to make the research project relevant to the small holder areas. Due to the dry conditions prevailing at Makoholi, maize variety R 201, which tolerates moisture stress and is short seasoned, was used. The crop spacing of 900 mm inter-row and 310 mm in-row were used resulting in a plant population of about 36 000 plants/ha. All weeding operations were done using a hand-hoe. The problems of nematodes, very common in the sandy soils and that of maize stalk borer were controlled, so as to minimize the influence of factors oth‐ er than those imposed by treatments. Carbofuran, a nematicide was applied into the plant‐ ing holes before the on-set of the rains, while Thiodan (against maize stalk borer) was applied six weeks after planting.

On all plots planting holes of about 10 cm depth and diameter were opened before the onset of the rains. Thereafter Carbofuran, was applied into these planting holes at a rate of 20 kg/ha. Compound D (N:P:K = 8:14:7) was also applied into the planting holes at a rate of 200 kg/ha to give a final ratio of 16 kg N: 12 kg P: 12 kg K. The nematicide and fertilizer were then slightly covered with soil and left until adequate rainfall had been received.

Once the profile of the ridges was wet throughout, maize was planted, two seeds per sta‐ tion. Ten days after planting, crop emergence count was carried out followed by weeding. The crop was then thinned out to one plant per station. When the crop was about six weeks, ammonium nitrate top-dressing fertilizer was applied at 100 kg/ha, amounting to 34.5 kg N/ha. The ammonium nitrate application coincided with the second weeding and the appli‐ cation of Thiodan, to control maize stalk borer.

#### **2.4. Soil loss assessment**

The standard soil erosion methodology for Zimbabwe (Wendelaar and Purkis, 1979) was used, where the plots were laid out at 4.5% slope. Soil loss and run-off measurements were from 30 m x 10 m run-off plots, with 5 m border strips on either side. The length of the plots was orientated up-slope. Tillage operations were done across the slope. Polythene strips were dug in to form the boundary around each 300 m2 plot (Working Document, 1990). For the tied ridging treatment, the collection area was 150 m long and 5 crop rows wide (4.5 m), with 2 guard rows above and below. The crop ridges were laid at 1% slope and the length of the plots was orientated across the slope. Surface run-off and soil loss from each plot were allowed to collect in a gutter at the bottom of the plot. From the gutter these were channeled through a PVC delivery pipe into the first 1500 litre conical tank. The collection tanks were calibrated and run-off was measured using a metre-stick. Once the first tank was full its overflow passed through a divisor box with ten slots, which channeled only one tenth of the overflow into the second tank. Nine tenths of this overflow was allowed to drain away, thus increasing the capacity of the second tank. Due to the larger net plots of the tied ridging treatment, three tanks were installed, so as to capture the anticipated larger volume of sedi‐ ments.

#### **2.5. Sampling eroded material**

Tanks were emptied at the end of each storm unless the interval between storms was too short to allow emptying. Sediments and run-off (including the suspended material) collect‐ ed from run-off plots were treated as two different entities. Suspension was pumped out and sub-sampled for the determination of soil concentration in run-off, using the Hach spec‐ trophotometer DL/2000. Later the sludge was transferred into 50 l milk churns, topped up with water to a volume of 50 litres and weighed. The mass of oven dry soil, Mo (kg) was calculated using the following equation (Wendelaar and Purkis, 1979; Vogel, 1993):

$$\mathcal{M}\_o = 1.7 \times \left( \mathcal{M}\_s - \mathcal{M}\_w \right) \tag{1}$$

*2.7.1. Texture*

*2.7.2. Organic carbon*

*2.7.3. Total nitrogen*

with a standard mineral acid.

*2.7.4. Total phosphorous*

at 500 - 600 0

measured.

*2.7.5. Total potassium*

ume and filtered. K was read from a flamephotometer.

and then read on a spectrophotometer.

Texture was determined using the hydrometer method as described by Gee and Bauder (1986), where 100g of air dried soil in 15 ml of calgon and 500 ml of water were stirred for 15 minutes using an electrical stirrer. The mixture was then transferred into 1 litre cylinders and diluted with water to 1 litre. After shaking the cylinder, time and temperature readings were taken and hydrometer readings were taken after 5 minutes (clay and silt) and five hours (clay). The sand fraction was determined by transferring the contents of the cylinder on a 50 micron sieve and washing away all the silt and clay fractions and then drying.

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The Walkley and Black method as described by Nelson and Sommers (1982) was used. One g of soil was digested with 10 ml of 1N potassium dichromate solution and 20 ml of concen‐ trated sulphuric acid. After ten minutes 100 ml of water were added, the mixture shaken

Nitrogen was determined using the microkjeldahl method as described by Bremner and Mulvaney (1982). The methodology, in brief, was as follows: The soil was digested with con‐ centrated sulfuric acid and hydrogen peroxide in the presence of a selenium catalyst. Organ‐ ic nitrogen was converted into ammonium sulfate. The solution was made alkaline and the liberated ammonia (NH3) was distilled and trapped in boric acid. The boric acid was titrated

The ignition method as described by Olsen and Sommers (1982)was used. Air dried soil was weighed into a crucible and the crucible placed into a muffle oven. The sample was ignited

and the mixture shaken on a reciprocating shaker for three hours. The mixture was filtered and 0.5 ml of 3M sulphuric acid were added to 5 ml of the aliquot. Twenty ml of water were added together with 4 ml of Reagent P and ascorbic acid. After 20 minutes P absorbance was

The wet digestion method using perchloric acid as described by Knusden, Peterson and Pratt, (1982) was used. The mixture of finely ground soil, hydrofluoric acid and perchloric acid was heated and cooled. Some more hydrofluoric acid was added and the contents were evaporated in a sand bath. After cooling, 6N HCl and water were added and the mixture further heated until it boiled gently. The contents were transferred to a flask, diluted to vol‐

C for three hours after which it was allowed to cool. Sulfuric acid was added

Where Ms = mass of fixed volume of sludge (kg)

Mw = mass of the same volume full of water (kg)

1.7 = constant for the soil type

For clay, organic matter and plant nutrient assessment of the eroded soil, the collected sedi‐ ments were thoroughly mixed and a sample taken by driving a hollow plastic tube into the sludge "profile" in the churn. Suspension was pumped out into 55 litre plastic containers, left to stand for 3 days, a water sample taken and the settled material at the bottom of the container sampled. Both soil samples were then air dried and analyzed individually i.e. for each storm, thus the averages given for the different treatments refer to twenty-one effective storms recorded during the season 1; nine storms during season 2 and twenty-two storms in season 3.

#### **2.6. Soil sampling**

Soil sampling on trial plots was carried out at the end of each season. Composite soil sam‐ ples were taken (8-10 samples per plot) within the plough depth of 0-250 mm, using a split auger. They were then air dried and sieved.

#### **2.7. Laboratory analysis**

An analysis of the sediments for macro-nutrients was carried out, where the different sedi‐ ment fractions (water, suspended material and sludge) were treated as different entities. The main aim being to quantify nutrient losses as a result of erosion and to ascertain which sedi‐ ment component carries the most nutrients. Total nutrients were determined in an effort to capture all forms of nutrients and therefore give a clear picture of how much was lost with erosion, rather than giving a mere fraction of the available form. Soil samples from the trial plots, as well as eroded material, were analyzed using the following methods:

### *2.7.1. Texture*

treatment, three tanks were installed, so as to capture the anticipated larger volume of sedi‐

Tanks were emptied at the end of each storm unless the interval between storms was too short to allow emptying. Sediments and run-off (including the suspended material) collect‐ ed from run-off plots were treated as two different entities. Suspension was pumped out and sub-sampled for the determination of soil concentration in run-off, using the Hach spec‐ trophotometer DL/2000. Later the sludge was transferred into 50 l milk churns, topped up with water to a volume of 50 litres and weighed. The mass of oven dry soil, Mo (kg) was

For clay, organic matter and plant nutrient assessment of the eroded soil, the collected sedi‐ ments were thoroughly mixed and a sample taken by driving a hollow plastic tube into the sludge "profile" in the churn. Suspension was pumped out into 55 litre plastic containers, left to stand for 3 days, a water sample taken and the settled material at the bottom of the container sampled. Both soil samples were then air dried and analyzed individually i.e. for each storm, thus the averages given for the different treatments refer to twenty-one effective storms recorded during the season 1; nine storms during season 2 and twenty-two storms in

Soil sampling on trial plots was carried out at the end of each season. Composite soil sam‐ ples were taken (8-10 samples per plot) within the plough depth of 0-250 mm, using a split

An analysis of the sediments for macro-nutrients was carried out, where the different sedi‐ ment fractions (water, suspended material and sludge) were treated as different entities. The main aim being to quantify nutrient losses as a result of erosion and to ascertain which sedi‐ ment component carries the most nutrients. Total nutrients were determined in an effort to capture all forms of nutrients and therefore give a clear picture of how much was lost with erosion, rather than giving a mere fraction of the available form. Soil samples from the trial

plots, as well as eroded material, were analyzed using the following methods:

1.7 ( ) *M xM M o sw* = - (1)

calculated using the following equation (Wendelaar and Purkis, 1979; Vogel, 1993):

ments.

120 Research on Soil Erosion Soil Erosion

**2.5. Sampling eroded material**

Where Ms = mass of fixed volume of sludge (kg)

Mw = mass of the same volume full of water (kg)

auger. They were then air dried and sieved.

1.7 = constant for the soil type

season 3.

**2.6. Soil sampling**

**2.7. Laboratory analysis**

Texture was determined using the hydrometer method as described by Gee and Bauder (1986), where 100g of air dried soil in 15 ml of calgon and 500 ml of water were stirred for 15 minutes using an electrical stirrer. The mixture was then transferred into 1 litre cylinders and diluted with water to 1 litre. After shaking the cylinder, time and temperature readings were taken and hydrometer readings were taken after 5 minutes (clay and silt) and five hours (clay). The sand fraction was determined by transferring the contents of the cylinder on a 50 micron sieve and washing away all the silt and clay fractions and then drying.

#### *2.7.2. Organic carbon*

The Walkley and Black method as described by Nelson and Sommers (1982) was used. One g of soil was digested with 10 ml of 1N potassium dichromate solution and 20 ml of concen‐ trated sulphuric acid. After ten minutes 100 ml of water were added, the mixture shaken and then read on a spectrophotometer.

#### *2.7.3. Total nitrogen*

Nitrogen was determined using the microkjeldahl method as described by Bremner and Mulvaney (1982). The methodology, in brief, was as follows: The soil was digested with con‐ centrated sulfuric acid and hydrogen peroxide in the presence of a selenium catalyst. Organ‐ ic nitrogen was converted into ammonium sulfate. The solution was made alkaline and the liberated ammonia (NH3) was distilled and trapped in boric acid. The boric acid was titrated with a standard mineral acid.

#### *2.7.4. Total phosphorous*

The ignition method as described by Olsen and Sommers (1982)was used. Air dried soil was weighed into a crucible and the crucible placed into a muffle oven. The sample was ignited at 500 - 600 0 C for three hours after which it was allowed to cool. Sulfuric acid was added and the mixture shaken on a reciprocating shaker for three hours. The mixture was filtered and 0.5 ml of 3M sulphuric acid were added to 5 ml of the aliquot. Twenty ml of water were added together with 4 ml of Reagent P and ascorbic acid. After 20 minutes P absorbance was measured.

#### *2.7.5. Total potassium*

The wet digestion method using perchloric acid as described by Knusden, Peterson and Pratt, (1982) was used. The mixture of finely ground soil, hydrofluoric acid and perchloric acid was heated and cooled. Some more hydrofluoric acid was added and the contents were evaporated in a sand bath. After cooling, 6N HCl and water were added and the mixture further heated until it boiled gently. The contents were transferred to a flask, diluted to vol‐ ume and filtered. K was read from a flamephotometer.

#### *2.7.6. Nutrients dissolved in run-off*

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 in the case of K.

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‐

> **Year 3 (765 mm)**

CT 94.9 48.7 169.5 104.4 Treat \*\*\* MR 4.6 3.6 111.4 39.8 Year \*\*\* TR 16.0 4.5 81.8 34.1 Treat x Year NS BF 122.7 65.3 295.3 161.1 MR vs TR NS Overall mean 59.5 30.5 164.5 84.9 CT vs (MR, TR) \*\*\* n = 9 (Treatment) s.e.d. = 8.07 s2 = 340.4 Yr 1 vs Yr 2 \* n = 12 (Year) s.e.d. = 7.53 df = 24 Yr 3 vs (Yrs 1, 2) \*\*\*

**Overall mean (mm)**

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

**Source of variation Run-off**

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123

ing run-off when compared to conventional tillage.

**Year 2 (384 mm)**

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

interaction between the treatment and the year (P = 0.145).

**Table 1.** Run-off (mm) as affected by tillage and year (rainfall) and their interactions at MakoholiContill site during

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

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

**Year 1 (483 mm)**

s.e.d. = 15.06

**Treat/Year (Rainfall)**

n = 3 (Treatment x Year)

three seasons

#### **2.8. Statistical analyses**

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 0.001 (\*\*\*).
