Residual Value of Phosphorus Fertilizer on a Calcareous Soil1

Abstract Yield response to applied phosphorus by grain sorghum (Sorghum bicolor L. Moench) was studied for 3 years on a Clareville clay loam, a dark, montmorillonite, calcareous soil typical of the Coastal Bend area in Texas. Field plots to which small amounts of phosphorus had been added over a period of years were compared to those with recent, large applications. Phosphorus applied in small quantities over a period of years was as available for plant growth as recently‐applied phosphorus. Regression analyses showed that in years of drouth stress the amount of applied phosphorus required for maximum yields was larger. This suggested that lack of water severely reduced phosphorus availability in the soil. The total amount of phosphorus applied was a better measure of availability than were extractions of phosphorus by either NH4OAc or NaHCO3. Both extractions gave a poor indication of available phosphorus present.