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 TMBP1.
All pedons had suboptimal nutrient ratios implying potential nutrient imbalance and
toxicity. On the basis of silt/clay ratios, degree of weathering decreased from GSKP1>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.