Abstract:
Soils formed from volcanic materials have high potential for agricultural production and support high human
population densities. This study was carried out on soils developed from volcanic parent materials of Northern Province of
Rwanda aiming largely on pedological characterization but to a certain extent on assessment of potentials of soils for
production of major crops in the area. Three representative pedons namely Kinigi Pedon 1 (KNG-P1), Kinigi Pedon 2 (KNGP2)
and Gahunga Pedon 1 (GHNG-P1) were identified and described. Sixteen soil samples were collected from different
pedogenic horizons and analyzed in the laboratory for physico-chemical properties. Pedons were classified using USDA Soil
Taxonomy and FAO-WRB for Soil Resources. Potentials and limitations of the soils were also identified. Results show that
soils were shallow to very deep and well drained. Topsoils were very dark coloured with colour values of ≤ 2 in all pedons.
Texture was generally loamy with bulk densities of < 0.47 g/cm3 in Pedons KNG-P1 and KNG-P2 while they ranged from 0.94
to 1.34 g/cm3 in Pedon GHNG-P1. Topsoils were medium acid (KNG-P1, KNG-P2) and mildly alkaline (GHNG-P1), with
high to very high OC ranging from 3.97 to 13.03%. CECsoil was high (> 32 cmol (+)/kg) in Pedons KNG-P1 and KNG-P2.
Base saturation was low (< 30%) in Pedons KNG-P1 and KNG-P2 while it was high (> 50%) in Pedon GHNG-P1. pHNaF
was > 9.5 in Pedons KNG-P1 and KNG-P2 reflecting exchange complex dominated by amorphous materials and/or humus
complexes. Phosphorus retention capacity ranged from 6.25% to 99.58% and only Pedons KNG-P1 and KNG-P2 met the
“andic properties” requirement of PRC ≥ 85%. Melanic index values indicated that these two pedons were characterized more
by fulvic than humic acids. Nutrient imbalance with reference to basic cations was common in all studied soils, implying
suboptimal nutrient uptake and toxicity. Fe2O3, SiO2 and Al2O3 were the dominant oxides in the studied soils. Degree of
weathering of studied soils was low as indicated by their weathering indices. Using field and laboratory data, Pedons KNG-P1
and KNG-P2 classified as Andisols/Andosols and GHNG-P1 as Mollisols/Phaeozems. Land units represented by Pedons KNGP1
and KNG-P2 were rated as marginally suitable while land unit represented by Pedon GHNG-P1 was rated as moderately
suitable for the major crops of the area. Application of P fertilizers coupled with efficient placement was recommended to
enhance P, and soil conservation should be underscored in study area.
