Comparison of Phosphorus Soil Test Extractants for Plant Availability and Environmental Assessment

Abstract

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.