Phosphate Sorption Isotherms for Evaluating the Phosphate Requirements of Soils

Abstract

Phosphorus sorption isotherms were constructed for soils which varied in mineralogy, texture, and past history of phosphate fertilization. Equilibrating conditions were: 25C, 0.01M CaCl₂, and 6 days. Effects of heavy phosphate fertilization of soils with high phosphate sorption capacities were evident in phosphate sorption isotherms 10 years later. The sorption curves were used as a basis for fertilizing soils in pots so that phosphate in soil solutions varied from about 0.01 to 1.8 ppm P. Millet (Pennesitum typhoides, var. Gahi‐1), was grown for 4 weeks. Yields approached 95% maximum when phosphate in soil solutions was adjusted to 0.2 ppm. The phosphate concentrations at which near maximum yields were attained were little influenced by soil texture or past fertilizer treatments, but absolute yields of millet growing in the sandy Norfolk soil (low phosphate buffering capacity) was greatest if the soil had been previously phosphate fertilized.