Assessment of the phosphorus and sulphur status of subterranean clover pastures. 2. Soil tests

Abstract

Values obtained by a number of established soil test procedures for phosphorus and sulphur were correlated with yield responses to addition of the relevant nutrient, by subterranean clover-based pastures at 21 sites in south-eastern New South Wales. Colwell's bicarbonate-soluble P and Bray's P, phosphorus values showed sufficiently close associations with response to added phosphorus to be useful for predictive purposes ; Bray's P, values generally gave smaller coefficients. In general, the pasture on soils testing less than 25 p.p.m. bicarbonate-extractable P in the surface three inches responded appreciably to applied phosphorus (relative yields were <85 per cent). The corresponding value for the Bray P, procedure was 10 p.p.m. P. Soil samples from 0-1, 0-3, and 3-6 inch depths gave similar correlations with response. The time of soil sampling did not affect the relationships but winter pasture production was not as closely related to soil test values as was spring production. By contrast, soil tests for sulphur were not reliable but some discrimination between soils could be made with a 500 p.p.m. phosphate extraction. Values from soil samples collected in the winter were less closely related to response than were values from samples collected in the autumn.