Deteriorating environmental conditions are being experienced by farmers in the Sahel region with the lacustrine clay plains, south of Lake Chad, experiencing extensive changes as the lake area has contracted. Coupled with increased population pressures, these are driving the agronomic experimentation in this region. The objective of the work undertaken in this thesis has been to characterise the development, physical and nutrient status, within the soils found under an experimental agroforestry system. For the first time in semi-arid Africa, this has been accomplished over a range of spatial scales (i.e. plot (<25 m) - site (25-200 m) - region (<200 km)), and is a baseline for future studies.
At the regional scale, pedogenesis in four profiles was examined. Field observations of physical soil characteristics (cracking, gilgai) were consistent with a chronosequence of exposure from Ustic Aquepts to Usterts. In. more detailed measurements, pedogenic trends were masked by the heterogeneity of the parent materials.
At the plot (320 samples) and site (480 samples) levels, the measurements of
chemical (Tot N, Tot P, OM, pH, EC, Loi, %CaC03) and physical (BO, texture,
microtopography, penetration resistance, soil structure by image analysis, depth of sand layer, colour, infiltration) variables were ana lysed stati sticall y, including
principal component analysis to produce a summary variate, and by geostatistical techniques producing semi-variograms. A trend across the site in soil texture(% sand, silt, clay, fine clay) was mirrored by other parameters (pH, EC, Loi, microtopography) suggesting that the surface soil chemistry is dominated by the overlying aeolian sand layer. Significant (P<0.05) effects of tree planting were revealed at the plot level, in the image analysis of soil structure measurements and in the subsequent infiltration on sandy sites.
The soil-structure differences found under agroforestry are seen to be developing in parallel with natural pedogenesis and are expected to become more pronounced as the trees mature with increased organic input and physical interaction between trees and soil. Whilst general trends are found in site level measurements, at the plot level a variable heterogeneity is found. For local farrners the management of this variability is paramount to overcome the marginal climatic conditions for rain-fed cropping whilst researchers must either consider longer time periods for soil changes to become apparent, or undertake sampling at grreater density at the smaller scales.