Residual Value of Fertilizer Phosphorus in Selected Kansas Soils

Quantification of the factors affecting availability of applied P in soils would improve fertilizer‐P recommendations for maximum P efficiency. A greenhouse study was conducted to determine the soil‐test P drawdown with cropping, apparent fertilizer‐P recovery, and plant‐available P in selected Kansas soils. The 55 soils (0–15 cm layer), which ranged widely in soil‐test P concentrations and other properties, received either no P or 100 mg P kg −1 soil. Soils were cropped sequentially with perennial ryegrass ( Lolium perenne L.) and ‘Honey Chow’ hybrid sorghum‐sudan ( Sorghum bicolor [L.] Moench). Linear P drawdown slopes (Bray and Kurtz no. 1 P/Cumulative P removal) for 20 unfertilized soils (initial Bray and Kurtz no. 1 P, >28.0 mg P kg −1 ) were less than those for the same fertilized soils. The P drawdown rate for soils already high in indigenous available P was 1.4 to 1.8 times greater with P fertilization than without. Apparent fertilizer‐P recovery (fertilized — unfertilized P uptake/fertilizer P added) for 48 noncalcareous soils was from 8 to 67%. Initial Bray and Kurtz no. 1 P concentration alone accurately predicted the plant‐available P until the soil test dropped to 15 mg kg −1 for 23 unfertilized soils ( R 2 = 0.83). The R 2 increased to 0.91 when interacting variables of organic matter and silt were included.