Effects of Nitrogen and Phosphorus Application Method and Nitrogen Source on Winter Wheat Grain Yield and Leaf Tissue Phosphorus

The positive effect of placing N (especially NH⁺₄-N) with P on crop growth has been demonstrated many times in short-term greenhouse studies. Under field conditions, however, the effect has not been well defined. The present objective was to evaluate N and P placement methods on the P nutrition and final grain yield of wheat (Triticum aestivum L.) grown in the field and greenhouse. Anhydrous ammonia (NH₃), urea-ammonium nitrate solution (UAN), and sodium nitrate (SN) were N sources while liquid ammonium polyphosphate (APP) was the P source. Methods of N-P application included: knife N-knife P (‘dual knife’), knife N-hroadcast P, broadcast N-knife P, and broadcast N-broadcast P. Two nitrification inhibitors were included in some studies. In the greenhouse, APP was labelled with ³³P orthophosphate and ³²P pyrophosphate to examine P uptake of its ortho- and polyphosphate fractions with different N-P-K application methods each with either a NH⁺₄-N or NO⁻₃-N source. Dual knife N-P applications gave higher leaf P concentrations and grain yields than other N-P application methods in many of the field studies when ammoniacal N sources were used. Sodium nitrate dual knife applications were not as effective as dual knife applications of UAN or ammonia. The inclusion of the nitrification inhibitor nitrapyrin to dual knife treatments increased grain yield and/or leaf tissue P level in two 1978 studies, but had no effect in 1979. In the greenhouse the highest leaf P levels resulted from the banded NH₃-APP application. The banded SN-APP application gave the second highest P uptake. This study also demonstrated that the N and APP need to be placed together, not separately in the soil. The amounts of the initial ortho- and polyphosphate portions of the APP absorbed by the plants seemed to be a function of fertilizer P demand and not related to fertilizer N in the same band. Including KCl in a band of APP did not affect leaf tissue P concentrations.