Factors explaining variable lime responses in pastures: Calcium X zinc X phosphorus interactions

Abstract

Perennial ryegrass was grown on a wide range of unlimed and limed soils in an attempt to identify plant‐soil factors which could explain the variable nature of lime responses in pastures. On most soils, lime responses appeared to be due either to enhanced soil nitrogen mineralization or to the alleviation of aluminium toxicity. On a small number of soils, however, the responses were not explicable by any known mechanism. On the basis of certain plant mineral compositional features it was proposed that these responses may have been due to improvements in the supply of calcium to shoot tissue as a result of liming. It was suggested that relatively low phosphorus/zinc ratios in grass shoots had stimulated the production of higher than normal concentrations of auxin, which in turn had increased the calcium requirement of growing tissue. Using selected data it was possible to show that relationships existed between the critical concentration of calcium in grass and the concentrations of phosphorus and zinc, and between the critical concentration of phosphorus in grass and the concentrations of calcium and zinc. The results suggested that the reputed phosphorus‐sparing effect of lime on grassland, may be due in part to the lower internal phosphorus requirements of grass grown on limed soils.