INTERPRETATION OF BRAY I-EXTRACTABLE PHOSPHORUS FROM ACID SOIL TREATED WITH PHOSPHATE ROCKS

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Abstract
Many research workers have assumed that P extracted by Bray I solution (0.03 N NH4F + 0.025 N HCl) from soils treated with phosphate rocks is the available P derived from the reaction products rather than from unreacted phosphate rocks in the soils. The role of Bray I in evaluating the available P from an acid soil treated with phosphate rock as affected by source of phosphate rock, rate of application and time of reaction was examined. Incubation increased the amounts of P extracted with Bray I from the soil treated with phosphate rock. At the rate of 800 ppm P added, the amounts of Bray I-extractable P derived from the unreacted phosphate rocks varied with the source of the phosphate rock, ranging 9.6-93.7 ppm. Although these represent only 1.1-11.6% of the P added, they may contribute significantly to the total Bray I-extractable P in the samples incubated at 50.degree. C for 3 wk. The amounts of Bray I-extractable P in the soil treated with phosphate rocks, before and after incubation, correlated very well (r = 0.97) with the citrate-soluble P of the phosphate rocks. In terms of the P added to the soil before incubation, the amounts of P extracted from the unreacted phosphate rocks decreased as the application rate increased. Conversely, the contribution from the unreacted phosphate rocks to the total Bray I-extractable P in the samples incubated for 3 wk increased as the application rate increased. In soil treated with 400 ppm P of phosphate rocks, the contribution of the unreacted phosphate rocks to the total Bray I-extractable P decreased as incubation time increased. The P extracted by Bray I from acid soils treated with phosphate rock is apparently partially derived from the unreacted phosphate rocks, as well as from the reaction products, and both sources can provide available P to the plant.

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