Chapter 6
Soil conservation measures

Three kinds of benifits can be obtained from soil conservation on cultivated land:

1. Long-term reduction or halting of decline in agricultural production or availability of good quality land.

2. Immediate or gradual increase in agricultural production.

3. Non-agricultural benefits such as improved dry season flow of rivers, reduced flooding and siltation of reservoirs, and reduced damage to infrastructure and farmland on lower slopes.

The soil erosion and productivity model essentially quantifies the long-term benefits (i.e. reducing or preventing further losses of agricultural land and decline in crop yields) of seven soil conservation measures for alternative uses of land. It is envisaged that the model would be extended in the future to include an estimation of other agricultural and nonagricultural benefits implied in (b) and (c) above.

Seven types of conservation measures are considered in the model: cut-off drains, narrow-based terraces, bench terraces, converse terraces (‘fanya juu’ terraces), grass strips, trash-lines, and stone terraces. Of these, narrow-based terraces and grass strips are suitable for large farms, while all measures except narrow-based terraces are suitable for small farms. Bench terraces can be used on large farms, but the costs of making them wide enough for mechanical cultivation is high. Also, as seen in Section 4, narrow-based terraces are applicable to slopes < 20%, whereas effectiveness of grass strips is reduced in low rainfall areas (LGP < 150 days) because of poor establishment. Also, trash-lines are subject to availability of crop residue whereas stone terraces are subject to availability of stones, and are feasible only on stony soils.

In the application of the soil erosion and productivity model (Figure 2.1), potential erosion losses for each desired land use (crop, livestock, fuelwood) is evaluated first on the assumption that no specific soil conservation measures are applied, i.e. protection factor P = 1. The results are compared with what is considered as acceptable rates of soil loss under the three levels of inputs circumstances, and then the required amount of conservation and associated costs are estimated.

6.1 Estimation of Conservation Need

The need for soil conservation is estimated from the protection factor (P) required to reduce soil erosion from its average rate on unprotected land to the tolerable rate estimated in Section 4. The average rate of erosion covers both the cultivated and the uncultivated parts of the crop and fallow period cycle, but the soil conservation measures described are only applied and maintained in the cultivated part of the cycle. If unacceptable rates of erosion are also occuring during the uncultivated part of the cycle, then additional protection will be needed.

The following example shows how conservation need is estimated in the model, for the cultivated part of the crop and fallow period cycle.

Soil loss reduction needed is 66 t/ha (i.e. 146–80). The total soil loss over 6 years of the crop cycle is 130 t/ha, which has to be reduced by 66 t/ ha to 64 t/ha. The P factor needed to achieve this is 64/139 = 0.49.

The protective effect of conservation measures varies according to natural conditions soil, topography, climate - and the intensity of the measure, e.g. the interval between terraces.

The equations to calculate the required spacing for a given measure, when the relevant natural conditions and the required protection - P factor - are known, are given in Mitchell (1986). These equations form the basis of cost calculations in the model for the seven types of conservation measures listed above.

These conservation measures deal with cultivated land, and in the model their benefit is assumed to last only while the land is under cultivation. If excessive erosion is taking place during the uncultivated part of a crop-fallow period cycle, the most effective way to reduce it is by improving the grass cover. The first requirement for this is to reduce or eliminate the grazing pressure of livestock. Tables 4.7 and 4.8 (Section 4) show the percent grass cover at different times of the year, and assumed rates of regeneration of grass cover after cultivation, in relation to climate and grazing intensity. These can be used to estimate the effect on soil erosion of reducing the intensity of grazing.

Other possible causes of excessive erosion on uncultivated land are poor established grass due to low rainfall, and unfavourable soil conditions. These have not been explicitly incorporated in the model at this stage in its development, but possible measures to overcome them are: pasture improvement with fertilizers; planting of improved pasture species or broadcasting seed; use of lines of cut bushes to slow runoff to protect germinating grass seeds; use of small earth banks to trap water to encourage germination of broadcast seed (Critchly 1984).

6.2 Costs of Conservation Measures

The costs of conservation measures are given in terms of man-days of labour, and the proportion of land taken out of agricultural production by the measures. Where fertilizer is used, for example in establishing grass strips, the amount of fertilizer is specified. It is assumed that all materials used are locally available and therefore not explicitly costed.

Table 6.1 presents a generalized comparison of the characteristics, effectiveness and costs of the seven types of conservation measures considered in the model.

The appropriate conservation measure for a given set of circumstances is normally the cheapest that will achieve the required measure of protection. The costs presented in Table 6.1 are based on man-days of work for manual labour. These include initial costs and maintenance costs. Initial costs are mainly based on the horizontal interval between measures, whereas annual maintenance costs are derived as fixed percentage of the initial costs. In order to compare costs directly, the annual maintenance costs over the cultivated part of the 10-year crop and fallow cycle are converted to net present value using an interest rate of 10%.

Most conservation measures involve taking some land out of production. They vary according to the type of measure, and whether the plants used to protect the terrace banks and other structures have any production value.

TABLE 6.1
Economic aspects of soil conservation measures

¹ 50% sulphate of ammonia and 50% triple superphosphate.

² Guideline ranges for physical protection factor (P) under good management only.

Sources: Derived from Mitchell (1986); Vlaanderen (1989).