Phosphate extractability and availability to plants in phosphate-rock treated soils

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Abstract
Two soils with contrasting phosphate sorption capacities were treated with a range of phosphate additions, applied as mono-calcium phosphate or phosphate-rock. They were then exhaustively cropped with white clover or ryegrass, and the availability of the residual phosphate was assessed by soil extraction procedures and a short-term uptake test with ryegrass. The phosphate present in saturation extracts or removed by electro-ultra-filtration was not well correlated with plant-available phosphate. The Olsen NaHCO3, test currently used throughout New Zealand correlated well with phosphate availability in soils treated with mono-calcium phosphate or phosphate rock, but the calibration of the test differed markedly with phosphate source. The Bray 1 acid fluoride extractable phosphate also correlated well with plant available phosphate and had the advantage of not being influenced by phosphate source.