Tillage for soil and water conservation in the semi-arid Tropics

Soil tillage is the manipulation of soil which is generally considered as necessary to obtain optimum growth conditions for a crop. In the same time the resulting modification of soil structure has serious implications for the behaviour of the soil to erosive forces by water and wind. In Chapter 1 an introduction is given to the most important aspects: the objectives of tillage, the conflicting requirements set to tillage, the characteristics of soil and water conservation in the semi-arid tropics, and the nature of tillage research including modelling.Chapter 2 treats in detail the characteristics of the soils often found in the semi-arid tropics: the SCH soils (sealing, crusting and hardsetting). Sealing and crusting causes problems with emergence of seedlings and with infiltration. The hardsetting soils are difficult to manage, particularly when tillage has to be performed with limited energy inputs. Physical characteristics and low organic matter contents are primarily responsible for SCH behaviour.In case 1, research undertaken in Mali is reported. Sandy soils of the Sahel area, mainly cropped to millet (Pennisetum glaucum (L.) R.Br.) are very sensitive to crust formation. These crusts were found to strongly reduce infiltration capacity. On the typically gently (1-3%) fields runoff is a widespread phenomenon; on the average 25% of the rain (mainly in the form of a few large storms during the rainy season) is lost by runoff. Crust formation and its effect on the infiltration rate was studied in experiments using a rainfall simulator are discussed. On untilled soils the presence of a crust is a permanent feature, and the effect of superficial tillage on crust disturbance disappeared quickly under subsequent rainfall. It was established that rainfall characteristics (aggressiveness, intensity) play a key role in crust formation.Research reported in case 2 was carried out in Niger. Here, important processes of soil structural changes under rainfall were assessed, to obtain a basis for a proper development of improved soil management methods. Soil and rainfall characteristics of a millet growing area close to Niamey, were determined. Laboratory tests showed a confirmation of what was observed in the field, namely that the coarse sandy soil of the area shows a mechanical behaviour which is extremely dependent on the moisture content at the time of soil handling. Therefore, the workability range is very narrow. Special tillage under wet conditions, resulting in smearing of the surface layer caused a condition which was more resistant to wind erosion. In an extension of the analyses reported in case 1, it was found the rainfall in this region is aggressive; even small storms may fall with high intensities.The major rainfall characteristics of the Sahel differ significantly from those of other semi-arid areas (such as India). The erratic rainfall pattern in combination with the sandy nature of the soil in the region studied, leads to an extremely small number of days available for planting millet, on average around 11 for the season. Therefore, time-efficiency of soil preparation and planting methods is even more important than a positive effect on SWC and crop emergence aspects.Chapter 3 gives a review of the various tillage systems as they may be applied for soil and water conservation, based the soil characteristics and on different mechanization levels.A study carried out in Brazil is presented in case 3. In a highly mechanized farming situation, erosion problems on sloping, red soils in the state of Paraná are high. Conventional tillage is based on the use of heavy disk ploughs and repeated passes with disk harrows in order to prepare a seedbed. This system causes severe erosion damage because of reduced infiltration rates and unstable topsoil. The zero-tillage system is a promising and realistic alternative, but is not suited for all farms in the state (small fields, high capital investment for equipment required, lacking knowledge and experience of farmers). The possibilities for the use of chisel ploughs (as an alternative between these two systems) on wheat stubble in a wheat-soybean rotation were investigated.Experiments showed that, compared to conventional tillage systems with disc implements, chisel ploughing left more plant residue at the surface. Except for the duckfoot type, the chisels were able to penetrate down to the bottom of a compacted layer at 12-20 cm depth. In addition, fuel consumption was significantly lower than disc ploughing and slightly higher than heavy disc harrowing. The capacities of the chisel ploughs were comparable to the heavy disc harrow. On the other hand, weeds and large amounts of straw may cause considerable practical difficulties and require adequately dimensioned chisel ploughs. Thus when applying the alternative tillage, adapted sowing equipment, able to cope with surface residue is required.In case 4, studies on the agronomic effect of tillage systems fit for animal traction in West Africa are reported. Crop establishment is an important yield factor for pearl millet in the Sahel. Therefore, a series of experiments was conducted to determine the effects of seed size, depth and method of planting, millet variety, tillage, and soil fertilization upon seedling emergence, crop establishment, and yield. All experiments were conducted on a sandy Psammentic Paleustalf in Niger. Three millet varieties were studied, and for all of these, out of a range of sowing depths from 1 to 7 cm, a sowing depth of 3-5 cm resulted in the highest percentage emergence, the highest above-ground biomass, and most secondary roots. High soil temperatures are common during establishment, typical maximum temperatures at a depth of 1 cm exceed 46 °C. It was found that the adverse effects of wind erosion and these high temperatures were least when sowing in hills (the traditional hand-method); establishment, crop stand survival, and yield were better under hill planting than drilling seed.Tillage of the field before sowing increased initial stands and their survival, the latter also depending on fertility. Thus, improved crop yields result from better stand survival and higher yields per hill. Fertilizer application (17 kg ha -1P and 40 kg ha -1of N) caused a threefold increase in grain yields. Ridging without prior tillage and ploughing increased grain and stover yields two- to three-fold. In combination with fertilizer application, sixfold yield increases were obtained. In view of the time limitations, ridging without prior tillage was preferable to ploughing, as it is a much faster operation giving equally good results in terms of crop establishment and yield.Chapter 4 deals with simulation models for soil and water conservation and the difficulties of modelling tillage effects. A review of the most important models currently used in SWC is presented with a brief indication if and how tillage is incorporated in these models. Various options of approaching tillage effects by modelling are given.In case 5, the development and application of a model simulating the role of tillage in SWC is presented. The data used are mainly from the situation prevailing in the West African Sahel and Sudan zone, characterized by a low input (particular N and P) rainfed farming system, growing cereal crops such as millet. Two types of soil tillage are distinguished: tillage aimed at water conservation (by increasing infiltration and/or surface roughness) and tillage aimed at weed control. Various scenarios are evaluated by combining simulation models for plant production (WOFOST) and soil water movement (SWATRE), developed and adapted for application in these regions. Based on simulations of 35 years of weather data, it was found that water conserving tillage as such has a very small yield-conserving effect because of the limitations set by the nutrient status. Elimination by tillage of the competition by weeds had a larger effect on the grain yield of a millet crop.In case 6, the water balance for millet fields and for permanently crusted natural pastures is described, with special emphasis on the role of the crust in governing infiltration and runoff. This study was based on the same field experiments in Mali as described in case 1. It was tried to quantify the effect of tillage as it destroys the crust and increases the surface storage for rainwater. The crust-breaking effect was found to last for only a few rainshowers, but the increase of surface storage is more permanent. The effect of a tillage system on the water balance of a millet crop was calculated. From this calculation it was concluded that tied ridges, giving a surface storage of 20-30 mm, could completely prevent runoff, compared to about 50% loss under the conventional system. Such a savings would allow earlier sowing and thus prolong the vegetative growth by as much as 20 days, which might increase the average millet yield (500 kg ha -1) by 40%.In Chapter 5, the prospects for development of tillage systems for the difficult SCH soils are discussed. Analysis of the various options shows that no-tillage is not a solution for the semi-arid tropics with hardsetting soils. It also can be argued that the introduction of animal traction in situations with purely handlabour, in many cases is not feasible, and notwithstanding all other problems, tillage by tractors should be investigated as a serious option.