Pedogenic Development of Soils at High Altitude in Burundi.

Abstract

To evaluate the effects of long-term cultivation and pasture on deforested soils at high altitude in Burundi, comparisons of morphological, physical, chemical and mineralogical properties of soils under natural forest with their counterparts under cultivation and pasture were performed. A typical soil under natural forest classifies as fine, oxidic, isomesic, Typic Haplorthods. Cultivated soils classify as fine, oxidic, isothermic Humic Hapludoxs. Pastured soils classify as fine, oxidic, isothermic, allic Humic Hapludoxs. When the sombric horizon is prominent, they classify as Sombriudoxs. We proposed a Spodic subgroup for the Hapludoxs. The studied area was highly weathered prior to faulting and uplifting and the spodic properties formed during the cool and humid parts of the Pleistocene Epoch. On 32 selected parameters, comparisons showed treatment effects on 18 of them, namely: OC, Bd, water retention, water dispersible clay, N, Bray I P, Ca, Mg, K, BaCl$\sb2$ acidity, extractable acidity, extractable Al, extractable H, NH$\sb4$OAc CEC, Sum CEC, NH$\sb4$OAc BS, Sum BS, and Al saturation. Seven of these parameters (OC, Bd, water retention, water dispersible clay, BaCl$\sb2$ acidity, extractable H and Sum CEC) were significantly different on both surface and subsurface horizons. Substantial improvements of pastured soils are expected when the latter are cultivated with continuous addition of O.M. (30 tons/ha/yr). The studied soils are rich in organic matter, gibbsite and iron oxides. All three components are well known for their binding effects. The soils exhibit steep slopes and rainfall erosion constitutes a serious limitation to land use. Removal of iron oxides and organic matter resulted in increased clay dispersion. This increase was far from total dispersion. Aggregation at the study area involves Al, besides Fe oxides and organic matter. Addition of organic matter to pastured soils tremendously improves their productivity though they are already rich in organic matter. The organic matter present in pastured soils does not contribute to the nutrient supply for plants. Characterization of the humic substances should elucidate the mechanisms involved in organo-mineral associations.