Phosphorus Supplying Capacities of Previously Heavily Fertilized Soils

Four Coastal Plain soils and one Piedmont soil heavily fertilized in the past were intensively cropped in greenhouse pots to determine changes in extractable soil P as measured by North Carolina, Bray I and Olsen extractants. Slopes relating changes in extractable P with P removed by cropping were very similar for the three sandy soils. Changes in extractable P with the North Carolina and Bray I extractants were correlated with P buffer capacity, % clay, and soil surface area. The principal source of P removed by cropping was NH 4 F‐P in sandy Coastal Plain soils; NH 4 F‐P and NaOH‐P supplied equal amounts in the clayey Piedmont soil. Sandy Coastal Plain soils which originally contained more than 74 ppm North Carolina extractable P and 71 ppm Bray I extractable P supplied 82 to 108 kg P/ha to nine crops of pearl millet ( Pennisetum americanum , cv. Gahi‐1).