Estimation of Relative Value of Phosphate Rock and Superphosphate to Plants on Different Soils

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The relative value of phosphate rock and superphosphate as sources of P for plants was evaluated by a test crop of sorghum on 19 soils in greenhouse cultures. The results were expressed as availability‐coefficient ratios. In the laboratory, three sets of measurements were made of P extractable from untreated soil, phosphate‐rock‐treated soil, and superphosphate‐treated soil: ( 1 ) P extracted by 0.025 N HCl, 0.03 N NH 4 F solution used in a ratio of 1 part of soil to 50 of solution by weight (method of Bray and Kurtz modified by Smith and Grava) after a 30‐day incubation period, ( 2 ) P extracted by an anion‐exchange resin without prior incubation, and ( 3 ) P extracted by an anion‐exchange resin after incubation for 30 days. Methods 2 and 3 were modifications of the method of Amer et al. The results were expressed as extractability‐coefficient ratios. The correlation between availability‐coefficient ratios and extractability‐coefficient ratios was highest with method 2 and lowest with method 1. The correlation between soil pH values and availability‐coefficient ratios was equally as close as that obtained between availability‐coefficient ratios and extractability‐coefficient ratios by method 2.