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

Abstract

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 SrCl 2 ) 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 SrCl 2 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.