Effects on botanical composition on mineral concentrations in dry pastures in "Western Kenya

Abstract

The influence of botanical (pasture species), geographical (altitude) and pedological (bedrock type, soil pH and extractable mineral concentration) factors on mineral concentrations in dry season pasture was studied in samples of topsoil and herbage from 135 sites on 84 farms in the Mt Elgon region of western Kenya. Of the four major elements measured in herbage, only S was affected by geology, low values being found above Tertiary volcanic (TV) and metamorphic gneiss (MG) bedrocks. By contrast, only P was not affected by species, Setaria being low and Kikuyu grass usually high in macro minerals. Of the seven trace elements analysed in herbage, geology influenced only one: Cu; low values were again found above TV and MG but Cu availability to grazing ruminants would be relatively high because of the associated low S values. By contrast, only Se was unaffected by species, Kikuyu grass being high in all but Mn. Soil bedrock had a greater influence on soil composition but correlations between extractable soil and herbage mineral concentrations were poor for all elements, even within botanical species after correction for soil pH and soil contamination. Concentrations of Ca, P, Mg, S, Cu and Zn were often less than tabulated requirements for grazing livestock. However, risks of deficiency could not be predicted from the pedological factors measured.