Abstract:
Pedological characterization was carried out on soils developed on gneissic granites of Southern Province of
Rwanda. Three representative soil profiles (GSK-PI, GSK-P2 and TMB-PI) were identified, described and
sampled using standard manuals. A total of nineteen soil samples were analyzed in the laboratory for physicochemical
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 ofPedon TMBPI
consisted of loamy and sandy textures. Pedons GSK-PI 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/crrr', from 2.18 to 2.59 g/cm! 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 rug/kg) and decreased with depth, phosphorus retention capacity was < 50% and
increased with depth in Pedons GSK-PI and GSK-P2 but didnot show any definite trend in PedonTMB-PI. All
studied pedons indicated suboptimal nutrient ratios with reference to the basic cations, implying potential nutrient
imbalance and toxicity of these nutrients. SiD" AI,O, and Fe,O, 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-PI>GSK-P2>TMB-PI. According to USDA Soil Taxonomy, the studied pedons
classified as Ultisols (GSK-PI and GSK-PI) and Inceptisols (TMB-PI), 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.
