Properties and effects of management on selected granitic soils in Zimbabwe

Sandy soils formed from granitic rocks on Miocene-aged and older land surfaces cover a large area of central Zimbabwe. These deeply weathered soils are an important agricultural resource for the country yet susceptible to erosion with agricultural use, and decreases in productivity and sustainability have been documented. Regional-based surveys of Zimbabwe have evaluated the general extent of erosion, but site-specific data to determine soil properties related to erosional susceptibility and influence of management are lacking in this area. This study measured physical and chemical properties of five sites (10 pedons) in an area of granitic soils east of the city of Harare. Sites were from university research, commercial, and communal farms. Pedons were paired to determine soil property differences in row crop vs. pasture management. The objectives of this study were to (a) verify important soil properties that influence erosional susceptibility and productivity of these granitic soils, and (b) determine differences in those properties as related to agriculture management. Results indicate important properties that favor high erosional susceptibility in these soils are coarse textures and high degree of weathering (resulting in extractable bases and cation exchange capacity < 10 cmol kg(-1)), and low organic C (ranging from 2.5 to 23.5 g kg(-1) C in surface horizons). Pedons in pasture have 2 to 19 cm thicker A horizons, water dispersible to total clay ratios 3% to 17% lower, and greater aggregate stability relative to the paired, cropped pedons. Saturated hydraulic conductivity (K-sat) at 15 to 30 cm depth ranges from 0.2 to 23.4 cm h(-1) for all pedons. Greater K-sat in cropped pedons relative to the pasture pair is likely temporal and influenced by tillage. Bulk density and extractable-P exhibit no trends with management and such results are likely confounded by management history such as crop-pasture rotations and fertilization. This study has shown the importance of increasing and maintaining soil C levels to minimize agricultural-induced erosion of these granitic soils. Results suggest that soil textures and the weathered nature of these soils in this semitropical climate produce conditions where organic matter is generally low even in soils maintained in pasture. Therefore, use of conservation management practices will be required to maintain and build adequate soil tilth and fertility for sustainable crop production. <(c)> 2001 Elsevier Science B.V. All rights reserved.