SOIL-PHOSPHORUS DYNAMICS DURING 17 YEARS OF CONTINUOUS CULTIVATION - A METHOD TO ESTIMATE LONG-TERM P AVAILABILITY

The ability to predict long-term plant-availability of soil P provides an additional management tool for sustainable agriculture. Our object ive is to present a methodology using P fractionation data for predict ing long-term plant-availability of soil P. Soil samples were collecte d (0-30 cm) in 1975, 1985, and 1992 from two continually cropped field trials. Soils were a Norfolk loamy sand and a Davidson clay loam, two Ultisols from North Carolina, USA. Four rates of P were applied from 1975 to 1986, and subsequently discontinued. The relationships between the resin and inorganic NaHCO3 fractions, and between the inorganic N aHCO3 and NaOH fractions, indicated that some level of equilibrium app eared to exist between these three fractions of soil P. Given this equ ilibrium condition, removal of resin P, as the most plant-available fr action, would subsequently reduce the levels of P in the inorganic NaH CO3 and NaOH fractions. Conversely, adding P as fertilizer would incre ase P in the resin fraction with a subsequent increase in the inorgani c NaHCO, and NaOH fractions. Although P applied as fertilizer was not completely accounted for in crop removal or net change in soil P, esti mated numbers of crops based on our model for predicting available P r eflected trends in yields at these two field sites. Sixteen and five c rops were estimated for the Norfolk and Davidson soils, respectively. Both corn and soybean yields continued to be high on the Norfolk soil through 1992, while corn yields had declined after 1985 on the Davidso n soil. Quantifying the long-term availability of soil P provides some measure of potential return on a capital investment of P fertilizatio n in low-input agriculture. (C) 1997 Elsevier Science B.V.