Changes in phosphate activities and availability indexes with depth after 40 years of fertilization1
1989
SCHWAB, A. P. | KULYINGYONG, S.
The concentrations of available phosphorus and H2PO4 activities were determined with depth in a silty clay soil receiving 40 kg P ha yr (surface broadcast with no incorporation on brome sod) for 20 or 40 yr. The soil was sampled in increments to a depth of 3.0 m. Available P, by the Bray and Kurtz P-1 method, and H2PO4 activities, from total solubilities in 0.01 M CaCl2 extracts, were determined. Available P was found to be greater in fertilized soils than in the unfertilized control soil to a depth of 45 cm. Similarly, Bray-P concentrations were greater to a depth of 30 cm in soils receiving continuous fertilization than soils in which fertilization was stopped after the first 20 yr. The activity of H2PO4 was strongly dependent on soil depth, P fertilization, and soil pH. Measured H2PO4 activities showed only weak correspondence to solid phase solubilities, but a short equilibration time and a lack of Al activity data made the comparison incomplete. Available P and H2PO4 activities were correlated when both were high, but there was no correlation when Bray-P concentrations were less than 13 mg P kg. The high activities in the soil extracts indicate that leaching of inorganic P may have a major role in the translocation of P, but the role of highly mobile organic P compounds could not be discerned from the present data.
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