Differences in the efficacy of various soil phosphate tests for white clover between very acid and more alkaline soils

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Abstract
Fifty-four soils, ranging in pH from 4.0 to 8.1, were analysed for exchangeable phosphate, sorptivity, and extractable phosphate by five commonly used soil tests. Phosphate uptake and yield data for white clover were obtained from two experiments conducted under identical glasshouse conditions. Marked differences were observed between the very acid soils (pH <5.6), and the moderately acid to alkaline soils in the effects of phosphate sorptivity on extraction by soil tests and plant uptake of exchangeable phosphate. On very acid soils the soil tests containing ammonium fluoride and sodium bicarbonate were unaffected by sorptivity and were less effective as indices of available phosphate than on the other soils, while the opposite situation applied to the double-acid Mehlich test. Phosphate sorptivity had no effect on the critical levels of any soil tests on the very acid soils, contrasting with the previously demonstrated effects on sodium bicarbonate tests when used on moderately acid to alkaline soils. The sodium bicarbonate tests were the most effective, and the Mehlich test was the least effective on all soils regardless of sorptivity or pH. The results suggested that the major source of plant-available phosphate in moderately acid to alkaline soils is exchangeable phosphate, whereas in very acid soils it is non-exchangeable mineral phosphates associated with iron and aluminium.