Evaluation of available phosphorus in highly phosphorus-deficient soils

In a greenhouse experiment single superphosphate was supplied in varying rates to lettuce test plants grown in soils of very low phosphorus availability. A constant amount of radioactive standard was used in order to differentiate between the amount of phosphorus absorbed from the soil and that derived from the superphosphate. A new measure for available phosphorus, namely, the "superphosphate equivalent" was formulated as the ratio of P/b where P=amount of phosphorus extracted by the Bray and Kurtz No. 1 extraction method, b=the slope of the regression line of extractable phosphorus versus added superphosphate after one week incubation. The new value proved to be a good index of phosphorus availability in soils of severe phosphorus deficiency.