**1. Introduction**

In soil erosion studies too much emphasis has been placed on the weight of soil loss (t/ha), while the real issue is not only about the amount of soil lost or the area of land degraded, but the effect of soil erosion on the productivity of the land. Soil erosion is rated as one of the major threats of sustainable land management, but the research data on the impact of erosion on soil properties and its effect on crop yield is grossly missing (Hudson, 1993), es‐ pecially in tropical Africa (Kaihura, *et.al.*, 1998). While the process of erosion is somewhat better understood, the resultant changes in the soil properties, the decline in yield and eval‐ uating the loss in productivity should be of concern to the researchers in this region.

On arable land, soil erosion is initiated through tillage. Tillage is the mechanical manipula‐ tion of soil for any purpose (Gill and Vanden Berg, 1967). It is an important part of the over‐ all farming system. The primary objectives of tillage, as given by Godwin (1990) and Lobb (1995) are to prepare a desirable seedbed, to control weeds, enhance soil and water storage and retention, manage crop residues and reduce erosion. Tillage can however, either con‐ serve or damage the soil depending on the intensity of inversion and the degree of exposure of the soil to weather conditions. The intensity of soil inversion also influences surface roughness, which in turn determines the sealing tendency of uncovered soil. The rougher the surface, the smaller the raindrop density per unit time and the lower the tendency to seal (Frede and Gaeth, 1995).

Conventional tillage or ploughing promotes soil organic matter loss through disruption of soil aggregates and increased aeration (Angers, N'dayegamiye and Cote, 1993; Beare, Hen‐ drix and Coleman, 1994; Reicosky, *et al.*, 1996; Salinas-Garcia, Hons and Matocha, 1997). Al‐

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so through ploughing, the crop residues are buried there-by enhancing organic matter decomposition and transformations. Where ploughing is practiced, it is practically impossi‐ ble to increase organic matter content, even when huge amounts of fertilizer are applied. Re‐ duced tillage intensity on the other hand can result in the maintenance/ increase of more labile fractions of soil organic matter (Angers, N'dayegamiye and Cote, 1993). Combining reduced tillage with surface crop residues not only inhibits the loss of soil organic matter but also improves soil aggregation.

effect of tillage on soil productivity, giving the desired effect should conservation tillage sys‐

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115

**ARABLE LAND**

 Conventional Conservation Tillage Tillage

 removal of crop residues mulching & min tillage Bare ground Climate Protected land

 high soil temperatures low soil temperatures mineralisation of OM maintenance of OM crusting enhanced aggregation low infiltration rate high infiltration rate

 poor soil structure improved soil structure reduced soil fertility improved soil fertility reduced soil moisture improved soil moisture high run-off low run-off

 **ACCELERATED EROSION MINIMUM EROSION**

**LOW SOIL PRODUCTIVITY SUSTAINED SOIL PRODUCTIVITY** 

Appropriate tillage systems should therefore, aim to maintain/ increase soil organic matter as it is the key to the productivity of the soils, as will be highlighted shortly. Organic matter content in most agricultural soils has been found to be highly correlated with their tilth, fer‐ tility and potential productivity. This soil constituent has positive effects on soil chemical, physical and biological properties that in turn contribute to improved crop yields (Bauer and Black, 1994; Gerzabek, Kirchmann and Pichlmayer, 1995). It facilitates soil aggregation and provides structural stability - improving air and water relationship - and protects soils from wind and water erosion (Godwin, 1990; Hunt, *et al.*, 1996). It is the source of plant nu‐ trients and Carbon source for micro-flora. Its loss results in reduced infiltration rates, in‐ creased crusting, decreased water holding capacity, increased resistance to root penetration, decreased nutrient availability and subsequent degradation of soil structure (Godwin, 1990). Small changes in soil organic matter of soils with low soil organic matter contents - as is the

 removal of fine particles conservation of soil removal of plant nutrients conservation of plant nutrients reduced soil tilth soil tilth maintained reduced plant available water improved plant available water

Figure 1. : Soil erosion as affected by tillage and climate and its impact on soil productivity

**Figure 1.** Soil erosion as affected by tillage and climate and its impact on soil productivity

3

tems be used.

In Zimbabwe soil tillage can be divided into three broad categories namely: Conventional Tillage, Reduced Tillage and Strip Tillage (Willcocks and Twomlow, 1992). Ploughing with a single furrow ox-drawn mould-board plough (conventional tillage) is the most widely used tillage practice in the communal areas of Zimbabwe and is estimated to be practiced on 73 - 90% of the cultivated area. The remainder of the land is ploughed using hired tractor (5 - 25%) and by hand (1 - 15%). Less than 1% is under tillage systems, which conserve soil, moisture, nutrients and/or energy inputs (Working Document, 1990). Reduced tillage in‐ volves mainly tied ridging, ripping and hand-hoeing. The tied ridging system is a useful compromise between drainage and storage (Hudson, 1992). Rainwater is retained in the ba‐ sins to soak into the soil, so very little run-off occurs (Elwell, 1986). The hand-hoeing system is labour intensive and practiced mainly in areas infested with tsetse flies or in cases of ex‐ treme lack of draft power (Working document, 1990). Under this treatment, the ground usu‐ ally has poor cover, the soil tends to compact and no significant soil conservation potential over conventional tillage has been observed (Vogel, 1992).

The ripping system saves on draft power as only the crop rows are opened and no tillage takes place between the crop rows. This means that the timeliness of operations is improved and yields may be improved as according to Oliver and Norton, (1988), low yields in the communal areas are also largely a result of late ploughing/ planting. Two types of ripping systems are currently under research in Zimbabwe, namely ripping into residues and clean ripping, where all crop residues are removed after crop harvest. Clean ripping reduces till‐ age and draft power requirement, however, the soil and water conservation potential of this system is low. Vogel, (1992) found no significant differences between this system and con‐ ventional tillage in terms of run-off and soil loss.

Mulch ripping has a lot of potential in conserving soil and water. Mulching has not yet been promoted in the communal areas as most of the stover is fed to cattle; however, the advan‐ tages of the system have been observed. When mulch is left on the soil surface, the soil is protected from high intensity raindrops (Adams, 1966; Elwell, 1986). Run-off, soil loss and subsequent nutrient loss are reduced (Elwell, 1986; Reicosky*et al.*, 1996). The underlying soil retains its high infiltration rate and most of this infiltrated moisture is protected from evapo‐ ration (Adams, 1966). The disadvantages are mainly weeds and the carryover of pests and diseases (Braithwaite, 1976; Elwell, 1986).

The effects of conventional tillage on the soil are generally known and can be summed up in a cause/effect relationship as shown in Figure 1. The conservation tillage systems ideally have to be designed in such a way that they reduce the effects of conventional tillage by gen‐ erally protecting the land and sustaining crop production. Figure 1 tries to summarize the effect of tillage on soil productivity, giving the desired effect should conservation tillage sys‐ tems be used.

so through ploughing, the crop residues are buried there-by enhancing organic matter decomposition and transformations. Where ploughing is practiced, it is practically impossi‐ ble to increase organic matter content, even when huge amounts of fertilizer are applied. Re‐ duced tillage intensity on the other hand can result in the maintenance/ increase of more labile fractions of soil organic matter (Angers, N'dayegamiye and Cote, 1993). Combining reduced tillage with surface crop residues not only inhibits the loss of soil organic matter

In Zimbabwe soil tillage can be divided into three broad categories namely: Conventional Tillage, Reduced Tillage and Strip Tillage (Willcocks and Twomlow, 1992). Ploughing with a single furrow ox-drawn mould-board plough (conventional tillage) is the most widely used tillage practice in the communal areas of Zimbabwe and is estimated to be practiced on 73 - 90% of the cultivated area. The remainder of the land is ploughed using hired tractor (5 - 25%) and by hand (1 - 15%). Less than 1% is under tillage systems, which conserve soil, moisture, nutrients and/or energy inputs (Working Document, 1990). Reduced tillage in‐ volves mainly tied ridging, ripping and hand-hoeing. The tied ridging system is a useful compromise between drainage and storage (Hudson, 1992). Rainwater is retained in the ba‐ sins to soak into the soil, so very little run-off occurs (Elwell, 1986). The hand-hoeing system is labour intensive and practiced mainly in areas infested with tsetse flies or in cases of ex‐ treme lack of draft power (Working document, 1990). Under this treatment, the ground usu‐ ally has poor cover, the soil tends to compact and no significant soil conservation potential

The ripping system saves on draft power as only the crop rows are opened and no tillage takes place between the crop rows. This means that the timeliness of operations is improved and yields may be improved as according to Oliver and Norton, (1988), low yields in the communal areas are also largely a result of late ploughing/ planting. Two types of ripping systems are currently under research in Zimbabwe, namely ripping into residues and clean ripping, where all crop residues are removed after crop harvest. Clean ripping reduces till‐ age and draft power requirement, however, the soil and water conservation potential of this system is low. Vogel, (1992) found no significant differences between this system and con‐

Mulch ripping has a lot of potential in conserving soil and water. Mulching has not yet been promoted in the communal areas as most of the stover is fed to cattle; however, the advan‐ tages of the system have been observed. When mulch is left on the soil surface, the soil is protected from high intensity raindrops (Adams, 1966; Elwell, 1986). Run-off, soil loss and subsequent nutrient loss are reduced (Elwell, 1986; Reicosky*et al.*, 1996). The underlying soil retains its high infiltration rate and most of this infiltrated moisture is protected from evapo‐ ration (Adams, 1966). The disadvantages are mainly weeds and the carryover of pests and

The effects of conventional tillage on the soil are generally known and can be summed up in a cause/effect relationship as shown in Figure 1. The conservation tillage systems ideally have to be designed in such a way that they reduce the effects of conventional tillage by gen‐ erally protecting the land and sustaining crop production. Figure 1 tries to summarize the

but also improves soil aggregation.

114 Research on Soil Erosion Soil Erosion

over conventional tillage has been observed (Vogel, 1992).

ventional tillage in terms of run-off and soil loss.

diseases (Braithwaite, 1976; Elwell, 1986).

**Figure 1.** Soil erosion as affected by tillage and climate and its impact on soil productivity

Figure 1. : Soil erosion as affected by tillage and climate and its impact on soil productivity

3 Appropriate tillage systems should therefore, aim to maintain/ increase soil organic matter as it is the key to the productivity of the soils, as will be highlighted shortly. Organic matter content in most agricultural soils has been found to be highly correlated with their tilth, fer‐ tility and potential productivity. This soil constituent has positive effects on soil chemical, physical and biological properties that in turn contribute to improved crop yields (Bauer and Black, 1994; Gerzabek, Kirchmann and Pichlmayer, 1995). It facilitates soil aggregation and provides structural stability - improving air and water relationship - and protects soils from wind and water erosion (Godwin, 1990; Hunt, *et al.*, 1996). It is the source of plant nu‐ trients and Carbon source for micro-flora. Its loss results in reduced infiltration rates, in‐ creased crusting, decreased water holding capacity, increased resistance to root penetration, decreased nutrient availability and subsequent degradation of soil structure (Godwin, 1990). Small changes in soil organic matter of soils with low soil organic matter contents - as is the case with the soils under study - are highly significant to the environmental and agricultural potential of these soils (Hunt *et al.*, 1996).

country is thus classified as temperate Cwb, i.e. mild mid-latitude, with dry winters and hot

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

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

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,

C in winter (MNTR, 1987). The country has been classified into five agro-eco‐

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

C in summer or

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summers (Roesenberg, 2007). The average temperatures rarely exceed 330

drop beyond 70

animal manure (MNRT, 1987).

holding capacities are due to the soil texture characteristics.

**2.2. Experimental design and tillage treatments**

mulch ripping, tied ridging and a bare fallow.

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

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

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

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