Residual Available Phosphorus in Near‐Neutral and Alkaline Soils: II. Persistence and Quantitative Estimation

A continuous cropping greenhouse study of 28 Colorado soils with phosphorus (P) rates of 0 and 30 ppm was conducted. The objectives were to (i) evaluate the persistence of residual available P in near‐neutral and calcareous soils and (ii) to predict its quantity. Residual available P was defined as the amount of P removed fron soil by plant uptake until the NaHCO 3 ‐P level dropped to 10 ppm. A residual effect of the fertilizer treatment persisted beyond six greenhouse crops for nearly one‐half of the 28 soils. Recovery of applied fertilizer ranged from 47 to 119% and averaged 74%. The following equation was developed to predict the quantity of residual available P in soils: Residual P, mg/2 kg soil = −17.02 + 2.062 (NaHCO 3 ‐P, ppm) + 0.058 (NaHCO 3 ‐P × clay, %) − 0.187 (lime, % × clay). R 2 = 0.90; 55 soil treatment combinations.