ADSORPTION CURVES AND PHOSPHORUS REQUIREMENTS OF ACID SOILS

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Abstract
Four acid, low-phosphate soils from southeastern USA were limed at 4 levels and amended with P at 5 levels. Barley was grown for 60 days on these amended soils in the greenhouse. Plant yields and P uptake were measured. With these same amended soils, 24 h equilibration P adsorption curves and equilibrium phosphate concentrations (the P concentration supported in solution by a soil at near field moisture capacity) were determined. The greenhouse and laboratory data were used to develop a 2-point adsorption-curve method for estimating soil P additions to correct P deficiencies. The method is simple and includes the use of a correction factor that relates the amount of fertilizer P a soil adsorbs during the first 24 h of addition to the amount the soil adsorbs during a growing season. Correction factors were estimated for the 4 soils, each at 4 different pH levels, on the basis of the lowest equilibrium phosphate concentration that supported 0.25% P in barley tissue.

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