Greenhouse Evaluation of Residual Phosphate by Four Phosphorus Methods in Neutral and Calcareous Soils

Four determinations of phosphorus—Olsen‐P, Colwell‐P, total exchangeable P, and resin‐extractable P—were evaluated in terms of total plant P uptake in a 3‐year continuous greenhouse study of 23 high P calcareous and neutral soils. All methods were highly correlated with the total P taken up from the soils by 5–8 successive greenhouse crops. The Olsen‐P procedure extracted an average of nearly 50% of the total plant P while the Colwell procedure extracted nearly 80%. Resin‐extractable P and total exchangeable P values approximated the total plant P uptake, and served as good biological measures of the total plant‐available P in the soil. No significant differences were observed between the carrier and carrier‐free 32 P methods on the 23 test soils.