Concentrations of major and trace elements in soil and grass at Shimba Hills National Reserve, Kenya

Abstract

Concentrations of major and trace elements in soils and grass are determined at Shimba Hills National Reserve in Kenya using geochemical mapping techniques. The study investigates the influence of soil and vegetation type on the concentrations of Na, K, Mg, Ca, Mn, P, Co, Cu, Zn, Mo, Ni and Se in soils and grass. The implications are assessed for the nutrition of the sable antelope, of which the Reserve supports the last remaining viable population in Kenya. Low concentrations in surface soils of a number of major and minor elements are attributed to the geochemical nature of the underlying parent materials of sands, sandstone and grits. Within the Reserve, variations in the element status of surface soils are related to the vegetation and soil types. Elevated element concentrations in surface soils in natural forest areas are attributed to the influence of litterfall whilst in grassland areas, soil element status is controlled by soil type and decreases in the order ferralsols > acrisols > arenosols. The general depletion of major and minor elements in soils at Shimba Hills is not reflected as fully in grasses in which nutrient concentrations were of similar magnitude to those reported from other Kenyan conservation areas. Burning of grassland areas leads to elevated concentrations of K, P, Co, Cu and Mo in grasses, elevated soil-plant uptake ratios for P and K and elevated soil pH. It is suggested that increased availability of P in soils at elevated soil pH levels contributes to its enhanced uptake into grass. A tentative assessment of the mineral status of grass at the Reserve using guidelines developed for domestic ruminants indicates deficiencies of Na, K, P and Zn and that the Ca:P ratio exceeds the tolerable range for animals. In addition, the low concentrations of Cu, Co and Zn in surface soils in the Reserve indicate that the potential supply of these elements to plants is limited.