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 objective is to present a methodology using P fractionation data for predicting long-term plant-availability of soil P. Soil samples were collected (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 NaHCO3 and NaOH fractions, indicated that some level of equilibrium appeared to exist between these three fractions of soil P. Given this equilibrium condition, removal of resin P, as the most plant-available fraction, would subsequently reduce the levels of P in the inorganic NaHCO3 and NaOH fractions. Conversely, adding P as fertilizer would increase P in the resin fraction with a subsequent increase in the inorganic NaHCO, and NaOH fractions. Although P applied as fertilizer was not completely accounted for in crop removal or net change in soil P, estimated numbers of crops based on our model for predicting available P reflected trends in yields at these two field sites. Sixteen and five crops 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 Davidson soil. Quantifying the long-term availability of soil P provides some measure of potential return on a capital investment of P fertilization in low-input agriculture. (C) 1997 Elsevier Science B.V.