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This article in SSSAJ

  1. Vol. 29 No. 5, p. 511-514
     
    Received: Nov 12, 1964


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doi:10.2136/sssaj1965.03615995002900050013x

Change of Phosphate Potentials of Calcareous Soils on Adding Phosphorus1

  1. L. V. Withee and
  2. Roscoe Ellis2

Abstract

Abstract

In two calcareous soils studied the phosphate potentials (0.5 pCa + pH2PO4) and lime potentials (pH − 0.5 pCa) indicated that the forms of mineral phosphates that determine the amount of phosphorus in solution were basic calcium phosphates similar in solubility to octocalcium phosphate, Ca4H(PO4)3·3H2O. Phosphorus as H3PO4 added to the soils at the rate of 200 ppm P saturated them with respect to octocalcium phosphate, and 500 ppm P saturated them with respect to dicalcium phosphate. Wetting and drying the soils for 5, 10, or 20 cycles after adding phosphorus increased the phosphate potentials. No tendency for calcium phosphates to convert to hydroxyapatite was observed after 20 wetting and drying cycles. Repeated extraction of soil samples—some in their original condition and some to which phosphorus was added—with 0.01m CaCl2 solution increased their phosphate potentials. The dominant form of calcium phosphate in the soil to which phosphorus had been added changed with extraction from dicalcium phosphate to a form or mixture of forms intermediate between dicalcium and octocalcium phosphate.

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