EFFECTS OF SOIL ORIGIN AND MINERAL-COMPOSITION AND HERBAGE SPECIES ONTHE MINERAL-COMPOSITION OF FORAGES IN THE MOUNT ELGON REGION OF KENYA.1. CALCIUM, PHOSPHORUS, MAGNESIUM AND SULFUR

Samples of topsoil (0-30 cm) and dry season herbage from 135 sites in the Mt Elgon region of Kenya were classified according to farm (n = 84 ), site altitude, underlying soil bedrock (6 types) and botanical comp osition (6 classes). Effects on pasture concentrations of Ca, P, Mg an d S were determined using a mixed model for unbalanced data sets and t he Wald (W) statistic to assess the significance of fixed effects. Ass ociated effects on pH, plus extractable Ca and P concentrations in the topsoils were also evaluated. Soil bedrock influenced herbage concent rations of S (P < 0.001) but not those of Ca, P or Mg. Mean herbage S concentrations were lowest on volcanic and metamorphic gneiss associat ions (1.2 g/kg DM) but only extreme values would be inadequate for gra zing livestock. Altitude appeared to affect the concentration of P (P < 0.01) and not those of Ca, Mg and S in herbage but the effect on P w as dependent on soil P. Geological and topographical maps cannot be us ed to predict macro-mineral disorders in livestock in the Mt Elgon reg ion. Herbage species differed markedly in their concentrations of S (P < 0.001), Ca (P < 0.001) and Mg (P < 0.05) but not P. Ca deficiency m ay arise on setaria, S deficiency on some napier grass pastures and P deficiency on some dry season pastures irrespective of botanical compo sition. Low herbage P concentrations may reflect advanced maturity rat her than low soil P status (mean value 20 mgP/kg DM). The correlation between soil P and herbage P was significant (r = 0.595), and similar in slope and intercept for all herbage classes but not strong enough t o predict deficient herbages. Herbage Ca was not correlated with soil Ca.