Pedological characterization of soils developed on gneissic - granites in the congo nile watershed divide and central plateau zones, Rwanda

Pedological characterization was carried out on soils developed on gneissic granites of Southern Province of Rwanda. Three representative soil profiles (GSK-P1, GSK-P2 and TMB-P1) were identified, described and sampled using standard manuals. A total of nineteen soil samples were analyzed in the laboratory for physico-chemical properties. Soil classification was carried out using two international systems of classification. The potentials and limitations of the soils were also identified. All studied pedons were very deep, well drained with loamy to clayey topsoil textures overlying dominantly clayey subsoil textures. The deeper subsoil of Pedon TMB-P1 consisted of loamy and sandy textures. Pedons GSK-P1 and GSK-P2 depicted eluviation-illuviation process as indicated by presence of clay cutans in subsoils. Analytical data showed low bulk and particle densities and high porosity in topsoils ranging from 1.02 to 1.34 g/cm3, from 2.18 to 2.59 g/cm3 and from 48.34 to 53.23%, respectively. Topsoilsof studied soils were strongly to slightly acidic with pH ranging from 5.49 to 6.59 while subsoils were strongly acidic to medium acidic with pH range between 5.13 and 5.86. The studied pedons had very low to medium total nitrogen and organic carbon respectively ranging from 0.15 to 0.24% and from 1.6 to 2.5%, both decreasing with depth. Topsoil and subsoil cation exchange capacity of the soils respectively ranged from low (9.8 - 21.6 cmol (+)/kg) to medium (4.2 - 13.2 cmol (+)/kg). Base saturation of the studied pedons was rated as low (< 50%) throughout all horizons of studied pedons. Whereas topsoil available phosphorus ranged from low to medium (6.4 - 15.7 mg/kg) and decreased with depth, phosphorus retention capacity was < 50% and increased with depth in Pedons GSK-P1 and GSK-P2 but didnot show any definite trend in PedonTMB-P1. All studied pedons indicated suboptimal nutrient ratios with reference to the basic cations, implying potential nutrient imbalance and toxicity of these nutrients. SiO2, Al2O3 and Fe2O3 ranged from 45.4 to 58.4%, 17.0 to 27.0% and 16.7 to 24.3%, respectively. On the basis of silt/clay ratios, the degree of weathering of the studied pedons followed the trend GSK-P1>GSK-P2>TMB-P1. According to USDA Soil Taxonomy, the studied pedons classified as Ultisols (GSK-P1 and GSK-P1) and Inceptisols (TMB-P1), respectively, correlating to Alisols and Cambisols of WRB for Soil Resources. The studied soils were generally rated as having low fertility and only marginal suitability for the major crops of the area. Specific land management and cropping systems were recommended for sustainable utilization of soils.