SOIL-PHOSPHORUS DYNAMICS DURING 17 YEARS OF CONTINUOUS CULTIVATION - FRACTIONATION ANALYSES

Excluding fertilizer P, a finite quantity of soil P exists for plant u ptake. To improve our understanding of sinks and sources of long-term plant-available P, soil P fractions to a depth of 30 cm were determine d 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 lo amy sand (fine-loamy, siliceous, thermic Typic Kandiudult) and Davidso n clay loam (clayey, kaolinitic, thermic Rhodic Paleudult). Superphosp hate 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 soybea n [Glycine mar (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 N orfolk soil with annual P applications that were in excess of crop rem oval and decreased with annual P applications that were less than crop P removal. Besin-extractable soil P decreased to below 3 mg P kg(-1) in the Davidson soil regardless of P application or removal rate. Inor ganic soil P extracted with NaHCO3 and NaOH increased with excess P ad ditions and decreased with deficient P additions for both soils. Organ ic 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 mine ralized by 1992. In both soils, inorganic P extracted in the resin, Na HCO3, and NaOH fractions, and organic P in the NaOH fraction of the No rfolk soil, represented the biologically dynamic P.