Many research workers have assumed that phosphorus extracted by Bray I solution (0.03 N NH4F + 0.025 N HC1) from soils treated with phosphate rocks is the available P derived from the reaction products rather than from the unreacted phosphate rocks in the soils. The objective of this study was to examine 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. Incubation increased the amounts of phosphorus extracted with Bray I from the soil treated with phosphate rock. At the rate of 800 ppm P added, the amounts of Bray Iextractable P derived from the unreacted phosphate rocks varied with the source of the phosphate rock, ranging from 9.6 to 93.7 ppm. Although these represent only from 1.1 percent to 11.6 percent of the P added, they may contribute significantly to the total Bray I-extractable P in the samples incubated at 50°C for 3 weeks. 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 phosphorus 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 weeks 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 results obtained in this study strongly suggest that the phosphorus extracted by Bray I from acid soils treated with phosphate rock is partially derived from the unreacted phosphate rocks, as well as from the reaction products, and both sources can provide available P to the plant. © 1978 The Williams & Wilkins Co.
