Topdressing requirements of established perennial pastures with a substantial history of superphosphate use

Abstract

Topdressing requirements of grazed perennial pastures with histories of substantial super phosphate use were assessed from 25 field trials in the high rainfall area of the Southern Slopes of New South Wales during 1972 and 1973. Yield responses from an application of superphosphate were related to a number of site variables including yield of unsupered pasture, total past superphosphate application, 'time since last application', available soil phosphate determined by three extractant methods, the phosphate sorption capacity of the soil, soil buffering capacity, and plant phosphate. Significant reponses were recorded on 16. of the 25 sites but no single site variable adequately predicted these responses. Analysis also revealed no significant change in the proportion of responsive and nonresponsive sites when these were classified as low, medium or high on the basis of extractable soil phosphate. Multiple regression analysis using the best combination of site variables to predict yield of unsupered pasture, yield increase from superphosphate and yield increase as a percentage of unsupered yield, was similarly unsuccessfuI ; only 3ZI4 and 44 per cent of variation respectively, was accounted for. These results suggest that the direct approach of applying a superphosphate strip and assessing the pasture's response is the best practical method currently available for deciding if a paddock with a substantial superphosphate history requires topdressing.