Relative Availabilities of Native, Residual, and Fertilizer Phosphorus to Winter Wheat

The uptake of fertilizer P by winter wheat ( Triticum aestivum L.) was investigated for four Southern Plains soils in 900‐cm 2 field plots over 4 yr, to quantify the relative availability and uptake of native, fertilizer, and residual fertilizer P. Statistically significant differences between duplicated treatments were determined by analysis of variance. Winter wheat was harvested 70 d after the September planting. At this time the proportion of soil Bray P from native and residual P increased (3.7‐ to 4.6‐fold) with successive annual fertilizer P applications (30 kg P ha −1 yr −1 ), while that from fertilizer P remained fairly constant. The uptake of winter wheat P from native and residual P increased and from fertilizer P decreased with successive fertilizer applications, due to an increasing amount of residual P uptake. Residual fertilizer P uptake by winter wheat was distinguished from native soil P and quantified as the difference in uptake determined by nonisotopic and isotopic methods. The nonisotopic method represents the difference in uptake between check and fertilizer P treatments, which will include fertilizer and residual P uptake, whereas the isotopic method represents only fertilizer P uptake. It was thus calculated that uptake of residual fertilizer P increased annually, representing from 4 to 40% of P uptake by winter wheat at harvest. By comparing the amount of wheat P from soil and fertilizer, it was found that the relative availability of native and fertilizer P did not change with continued fertilizer application to the four soils.