Influence of pH, time and rate of application on phosphate rock dissolution and availability to pastures

Abstract

The agronomic effectiveness of an unground reactive phosphate rock from Sechura, Peru, was compared with that of monocalcium phosphate in a severely P deficient and highly P retentive soil (vitrandept) over a period of three years. Soil pHs were adjusted to pH 5.1, 5.3, 5.6 and 6.4. The sward consisted mostly of ryegrass (Lolium perenne) and white clover (Trifolium repens). Fertilisers were applied at six rates at pH 5.3 and three rates at other pHs in the first year. For two of the rates fertilisers were reapplied in the second year. Dry matter yields, P uptake and ground cover of clover were determined during the experimental period. In phosphate rock treated plots a negative linear relationship was obtained between soil pH and the logarithm of yield. The agronomic effectiveness of phosphate rock relative to monocalcium phosphate increased with time at all pHs. Calculated at fertiliser rates which produced near maximum yields, relative agronomic effectiveness at soil pHs 5.1, 5.3, 5.6 and 6.4 were respectively 58, 60, 18, and 5 in year one; 118, 125, 77 and 38 in year three. At pH 5.3, as the rate of application increased the relative agronomic effectiveness of the phosphate rock generally decreased in year one but was enhanced in the intermediate rates in years two and three. The data for ground cover of clover gave a similar trend to that for herbage yield and P uptake.