Critical soil test values for predicting the phosphorus and sulphur status of subhumid temperature pastures

Abstract

Field experiments in which five levels of phosphorus application were combined factorially with five levels of sulfur application were carried out on a range of improved pastures on the Southern Tablelands and South-West Slopes of New South Wales. Dry matter responses by the pastures in the winter-spring period were correlated with soil test values obtained early in the growing season. When a best-fit curve of the Mitscherlich form was fitted to the relative yield-bicarbonate extractable phosphorus relationship, critical values for the surface 7.5 cm of soil varied between 25 and 35 ppm phosphorus, depending on the method of choosing the optimum yield. The parallel relation with phosphate-extractable sulfur was so ill defined that no critical value could be identified. Deeper sampling was of no advantage with either nutrient.