With decreasing soil pH, selenium is less available, but the uptake of some cationic metals such as copper is increased (Burk, 1978 Williams, 1977). Alkaline soils lead to an increased biological availability of some trace elements such as selenium and molybdenum (Williams, 1977). This concentration is influenced by pH, moisture, organic matter, leaching, and the presence of other elements and the microbial activity of the soil (Burk, 1978 Williams, 1977). The availability of minerals in soil depends upon their effective concentration in soil solution. Soil factors, such as acidity, moisture or drainage conditions, temperatures and seasonal effects influence mineral uptake by crops and pastures (Horvath and Reid, 1980).
Factors which influence the mineral composition of the soils are: the origin of the parent rock (Fletcher and Doyle, 1978), glaciation, leaching, surface erosion, evaporation, salinization, the application to the soil of pesticides, fertilizers, trace elements, manures and sludges, and the aerial fallout from industry and transportation (Horvath and Reid, 1980 Williams, 1977).
The trace mineral content of our natural feeds is determined primarily by the mineral availability from the soil and secondly by the actual mineral composition of soil (Smart et al, 1981 Williams, 1977).
