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This study examined morphology, physico-chemical characteristics, classification, potentials and limitations of soils developed on gneissic-granites of Rwanda. Three pedons (GSK-P1, GSK-P2 and TMB-P1) were studied and 19 soil samples analysed. All pedons were very deep, well drained, with loamy to clayey topsoils overlying dominantly clayey subsoils. Pedons GSK-P1 and GSK-P2 depicted eluviation-illuviation as dominant pedogenetic process. All topsoils had low bulk and particle densities ranging respectively from 1.02 to 1.34 g/cm3 and from 2.18 to 2.59 g/cm3. Topsoil porosity was high, ranging from 48.34 to 53.23%. Topsoil reaction was strongly to slightly acid with pH ranging from 5.49 to 6.59, whereas subsoil reaction was strongly to medium acid with pH between 5.13 and 5.86. Total nitrogen (TN), organic carbon (OC) and available phosphorus (Av. P) increased with depth. Whereas TN and OC were, respectively, very low to medium (0.15 to 0.24%), and low to medium (1.6 to 2.5%), Av. P was rated as low to medium (6.4 to 15.7 mg/kg). Topsoil and subsoil cation exchange capacity ranged from low (9.8 - 21.6 cmol (+)/kg) to medium (4.2 - 13.2 cmol (+)/kg) respectively while base saturation was low (<50%) for all horizons. Phosphorus retention capacity was < 50% and increased with depth in Pedons GSK-P1 and GSK-P2 but did not show any definite trend in Pedon TMB-P1. All pedons had suboptimal nutrient ratios implying potential nutrient imbalance and toxicity. On the basis of silt/clay ratios, degree of weathering decreased from GSK-P1>GSK-P2>TMB-P1. Using USDA Soil Taxonomy, the pedons classified as Ultisols (GSK-P1 and GSK-P2) and Inceptisols (TMB-P1), respectively, correlating to Alisols and Cambisols of WRB for Soil Resources. The soils were rated as having low fertility and only marginally suitable for major crops of the area. Specific land management and cropping systems were recommended for sustainable utilization of soils. |
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