The influence of tillage on semi-arid soil-water regimes in Zimbabwe.
Little is known about the in situ hydrological properties of soils in the semi-arid regions of the world where subsistence agriculture is the common land use. The work reported here concentrates on three major soil types, considered representative of the agriculturally important Zimbabwean fersiallitic group of soils, and describes the water regimes that develop under conventional (flat) and improved (tied ridge) practices in response to natural and simulated rainfall. The first soil, typical of those used by small scale farmers, is a deep coarse-grained granitic sand with a low available water capacity. The second is a silty clay loam derived from mafic rocks and typical of the red clays associated with Zimbabwean commercial farming areas. The third is a sandy loam that readily compacts and crusts under natural rainfall, prone to runoff and thus drought sensitive. The climate at each site has a five month rainy season, when most of the rain falls as heavy convectional storms and a dry spell from April to October. However, total seasonal rainfall at each site is unreliable (200 to 1100 mm) with unpredictable dry periods of 1-3 weeks, especially at the beginning and middle of the wet season when rain occurs as intense storms of short duration. Long term monitoring of the three soil types has shown that at the end of the dry season, the quantity of water stored in the top 0.3 m of the profile is negligible, typically below permanent wilting point. Water stored in the 0.9 m profile at the end of the dry season is more variable and depends on total seasonal rainfall, its distribution, the type of crop grown and the tillage management. At the beginning of the wet season, the recharge of the soil profile is of fundamental importance for subsequent crop establishment and can benefit from improved soil water storage in the previous season and tillage practices that help to catch the first rains of the wet season. In general terms the tillage treatment that impounds the most water (tied ridges) was the wettest, whatever the soil type. However, the incomplete wetting up of the tied ridges early in the wet season, frequently resulted in poor crop establishment or a delay in planting until the ridge structure was fully wetted. This was in contrast to the conventionally tilled soils, which were more uniformly wet in the top 200 mm of the soil profile and significantly wetter than the ridged soils early in the wet season. Irrespective of tillage technique, good weed management was critical to maximize the availability of water for crop growth.
Twomlow, S.; Bruneau, P. The influence of tillage on semi-arid soil&#8211;water regimes in Zimbabwe. Geoderma (2000) 95 (1-2) 33-51. [DOI: 10.1016/S0016-7061(99)00071-3]