Changes in phosphate availability of ultisols with long-term cropping

Many soils in the southeastern United States have high levels of available P. Long-term field studies were conducted to determine the amount of phosphorus (P) that could be supplied before corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] responded to P fertilization and changes that occurred in soil test P. The soils were a Norfolk loamy sand (fine-loamy, siliceous, thermic Typic Kandiudult), Lynchburg loamy fine sand (fine-loamy, siliceous, semiactive, thermic Aeric Paleaquult), and a Davidson clay loam (clayey, kaolintic, thermic Rhodic Kandiudult). Phosphorus as concentrated superphosphate was broadcast annually at rates of 0, 10, 20, and 40 kg P ha(-1). Response to P was obtained the 13th year of the study on the Davidson and the 14th year on the Norfolk. No response was obtained on the Lynchburg which had the highest level of available P. The Davidson soil supplied 130 kg P ha(-1) and the Norfolk 204 kg P ha(-1) before a response was obtained. The Lynchburg supplied 178 kg P h(a-1) during the 14 year study. It was estimated that the Lynchburg would supply a total of 314 kg P ha(-1) before a response would be obtained. For each kg P ha(-1) removed in the harvested grain the M1P decreased 0.13 mg dm(-3) in the Norfolk, 0.24 mg dm(-3) in the Lynchburg, and 0.05 mg dm-3 in the Davidson. An annual application of 20 kg P ha' maintained the initial MIP levels on the Norfolk and Davidson, but required 40 kg P ha(-1) on the Lynchburg which had a very high level of P.