Response to sulphur and comparison of soil tests for predicting sulphur status of soils for grazed clover — grass pastures in New Zealand

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Abstract
Seventy-seven field experiments which measured response to both phosphorus (P) and sulphur (S) fertilisers were run for 3 or 4 years on nine soil types representing five main soil groups of New Zealand. Because single superphosphate had been the only P fertiliser used, S and P fertiliser histories at each site were similar. However when fertiliser was withheld, the incidence of response to S was much less than to P. For example, in the first year there was response to S on 27 sites, but response to P on 55 sites. On some soil types response to S was rare over 4 years irrespective of fertiliser history. The lack of response to S fertiliser was associated with proximity to the coast and high sulphate retention but, more generally, with sites where improved clover — grass pastures were long established. It is suggested that on these sites the organic matter content of the soil is at or near equilibrium level between immobilisation and mineralisation of S and the latter is a major source of available S. Three S soil test methods, all based on extraction with phosphate solutions, were compared. Prediction of response to S using regression between response and soil tests was very poor because of the inconsistent response to S on individual sites. An approach using soil tests to predict probable minimum yield instead of mean yield was used and gave a very useful prediction of the probability of achieving a target relative yield when no S fertiliser is used. The most satisfactory S soil test was sodium phosphate solution in acetic acid at pH 3. Subsequently, it was found that, for New Zealand soils, there was a close 1: 1 relationship between S extracted by the sodium phosphate solution and S extracted by the more commonly used extractant, 0.01M calcium phosphate. The latter extractant has been adopted by the Ministry of Agriculture and Fisheries for its soil testing service because of its suitability for autoanalysis. It is calibrated to predict probable minimum yield in spring (the fast growth period) and in autumn — winter (the slow growth period). As with response to P, percent response to S is less in the fast growth period than in the slow growth period.