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Soil Phosphorus Supplying Capacity Evaluated by Plant Removal and Available Phosphorus Extraction¹

ABSTRACT

Many Missouri soils test high in extractable P after long periods of rock phosphate and processed P fertilization. This study was undertaken to determine the P-supplying capacities of five Missouri soils possessing high levels of extractable P. Initial P levels were evaluated with selected extractants (Bray I, Bray II, Mehlich II, double acid, dilute SrCl₂) and through evaluation of the P buffering index. Each soil was cropped under greenhouse conditions for 7 consecutive harvests of grain sorghum (Sorghum bicolor L. cv. Warner 744R). After each harvest, plant P uptake was calculated and extractable P measured as before. For each soil test, regression analyses of plant P uptake were performed. With only one exception, the decline in extractable P in all soils, as measured by all five extractants, was significantly correlated with cumulative P uptake. The highest correlations were obtained with Bray I for Sharkey clay (r = 0.95), Bray I and Bray II for Kennebec silty clay loam (r = 0.97), Bray II for Mexico silt loam and Broseley loamy fine sand (r = 0.95 and r = 0.90, respectively), and Mehlich II for Tiptonville silt loam (r = 0.96). Slopes of the decline in extractable P following intensive cropping show similarities between Bray I and Mehlich II, Bray II and double acid. The quantity of plant available P extracted by Bray II and dilute SrCl₂ was found to be inversely related to the P buffer index (PBI). When extrapolating to critical P levels for Bray I (11 mg kg^–1) and Bray II (24 mg kg^–1) only the Bray II extracted predicted number of harvests in order of soil PBI.

¹ Contribution from the Missouri Agric. Exp. Stn. Series no. 9366. Univ. of Missouri, Columbia, MO 65211. This work was supported by Missouri Agric. Exp. Stn. Proj. 387 (Maximizing Plant Nutrient Utilization).

² Formerly CAPES (Coordenacao do Aperfeicomento de Pessoal de Nivel Superior) Graduate Student, presently Escola Superior de Agricultura de Mossoro, Caixa Postal 137, Mossoro, RN, Brazil; and Associate Professor of Agronomy, Univ. of Missouri, Columbia, MO 65211.

Received for publication May 4, 1983. Accepted for publication May 7, 1984.