Rating land qualities in grazing areas: a case study of greyzems in south West Kenya

The criteria for rating soil fertility in grazing areas is not well elaborated. This is because, the levels of pasture nutrients are wholly dependent on the; minerals to weather, organic matter to decompose, and on these two processes to release the nutrients and the soil colloids to retain those nutrients. Hence a soil fertility capability evaluation, based on the ability of the soil colloids to retain the nutrients was therefore considered to be more appropriate. Soil fertility capability is dependent on how soils retain plant nutrients. For this reason therefore, two major plant nutrients (PK) ~ere correlated with the soil colloids; organic matter, oxides and clay minerals. The two plant nutrients were found to have moderate to high correlations with the oxalate extractable oxides (Mn%, Fe%), kaolinite and illite. That is, r=O.62, 0.56, 0.67 and 0.68 with K and r=O.84, 0.39, 0.81 and 0.80 with P respectively. A rating system was then developed combining; CEC, base saturation and % C as recommended by FAO guidelines together with oxalate extractable oxides (Mn%, Fe%) and illite to kaolinite ratios as identified in this study. For the oxides an easier criteria of using colour hues was also tested. The Greyzem classification units were finally classified into three soil fertility capability groups as follows; moderate (Gleyic), low (Vertic, Haplic) and very low (Lithic). This capability classification also reflects the stage of weathering whereby the Gleyic Greyzems are the least weathered while the Lithic Greyzems are the shallowest and relatively the highly weathered. The Greyzems are therefore, generally low in terms of soil fertility capability. Soil management in the study area should therefore aim at raising this soil property. Of the five rating criteria used, only the organic matter can be managed with reasonable ease in order to elevate the soil plant nutrients retention capability. Therefore measures aimed at conserving the organic matter should be taken, for example, by avoiding overgrazing and grass burning.