Soil Phosphorus Dynamics during Seventeen Years of Continuous Cultivation: Fractionation Analyses

Abstract

AbstractExcluding fertilizer P, a finite quantity of soil P exists for plant uptake. To improve our understanding of sinks and sources of long‐term plant‐available P, soil P fractions to a depth of 30 cm were determined in soils under a continuous cropping system prior to fertilization (1975), after 10 yr of P fertilization (1985), and 6 yr after the last P fertilization (1992). Soil types for two study sites were Norfolk loamy sand (fine‐loamy, siliceous, thermic Typic Kandiudult) and Davidson clay loam (clayey, kaolinitic, thermic Rhodic Paleudult). Superphosphate was applied from 1975 to 1986 at rates of 0, 10, 20, and 40 kg P ha−1 yr−1. Average removal of P, via corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] cropping, was between 10 and 20 kg P ha−1 yr−1 from 1975 to 1992. Resin‐extractable soil P increased in the Norfolk soil with annual P applications that were in excess of crop removal and decreased with annual P applications that were less than crop P removal. Resin‐extractable soil P decreased to below 3 mg P kg−1 in the Davidson soil regardless of P application or removal rate. Inorganic soil P extracted with NaHCO3 and NaOH increased with excess P additions and decreased with deficient P additions for both soils. Organic soil P in the Norfolk soil extracted with NaOH represented a P sink at the 40 kg P ha−1 yr−1 treatment in 1985, but subsequently mineralized by 1992. In both soils, inorganic P extracted in the resin, NaHCO3, and NaOH fractions, and organic P in the NaOH fraction of the Norfolk soil, represented the biologically dynamic P.