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This article in SSSAJ

  1. Vol. 63 No. 4, p. 999-1006
    Received: May 13, 1997

    * Corresponding author(s): fmagdoff@zoo.uvm.edu
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Comparison of Phosphorus Soil Test Extractants for Plant Availability and Environmental Assessment

  1. F. R. Magdoff *a,
  2. C. Hryshkoa,
  3. W. E. Jokelaa,
  4. R. P. Durieuxa and
  5. Y. Bua
  1.  aDep. of Plant & Soil Science, Hills Building, University of Vermont, Burlington, VT 05405 USA


A number of soil tests have been proposed to predict crop response to added P or to assess potential for soil P loss to runoff waters. A series of four separate experiments were conducted over a 10-yr period to evaluate soil test methods on a total of 163 Vermont and New York field soils. The experiments included the following: (i) a pot study with alfalfa grown in the greenhouse with 31 soils either unfertilized or fertilized with 18 mg P kg−1; (ii) routine chemical analysis on 54 soils; (iii) a 360-d incubation study with 24 soils receiving either 0, 20, or 40 mg P kg−1 as CaH2PO4, in which soils were analyzed for desorption and adsorption and the equilibrium P concentration (EPC0); and (iv) another set of 54 agricultural soils incubated with 0 or 40 mg P kg−1 and analyzed for CaCl2, distilled water, and ammonium acetate (Vermont 1)–extractable P (VT1P) and EPC0 Although P extracted by VT1 was significantly correlated with P removed by F extractants, it was better correlated with the ratio of F-extractable P/Al extracted by either acetate or F. Phosphorus additions increased VT1P, as well as P extracted by acetate + F (Vermont 2 [VT2]), and they decreased reactive soil Al (VT1Al) and P adsorption. The amount of P needed to increase VT1P by a certain amount was directly related to the amount of Al in the VT1 extract. Phosphorus availability to plants, CaCl2-extractable P, and the EPC0 were all more closely related to VT1P than P extracted by solutions containing F, such as Mehlich 3 (M3), Bray and Kurtz 1 (BK1), and VT2. In a number of instances the ratio VT2P/VT1Al had a better relationship with CaCl2P and EPC0 than did VT1P. Thus, the fraction of reactive Al that has reacted with P (as estimated by VT1P or the ratio of VT2P/VT1Al) appears to be a better indicator of P availability and potential P desorption to runoff water than is P extracted with F.

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