Tracing Foliar Phosphorus Fertilizer Uptake in Grain.

It is important to apply some phosphorus (P) to the soil at the beginning of the crop growth cycle to provide essential P for early growth and to replace P exported in previous crops. With low rates of P added at sowing there may be sufficient P reserves to grow crops to tillering, but in seasons of increased yield potential a top-up application of P may be required. Foliar P application can be applied directly to the plant when required and in some cases has been shown to provide benefits for increasing P use efficiency and the P concentration in grain. However, tests of foliar P fertilization to date have had inconsistent results. Our aim was to accurately measure the ability of foliar P products to increase grain yield and contribute to grain P uptake using a 33P tracing technique. Wheat (Triticuum Aestivum cv. Frame) was grown in two marginal P soils with soil moisture maintained at 80% of field capacity. Eight foliar P treatments labeled with 33P as a tracer were applied at Feekes growth stage 9, at 2 kg P ha-1 with 120 L water ha-1 equivalent. Plants were grown to maturity and harvested. Grain, chaff and stems were digested separately to measure dry weight, P concentration and 33P radioactivity. Foliar P applied as phosphoric acid or as a phosphoric acid plus technical grade mono-ammonium phosphate product, with LI700® as an adjuvant gave a significantly greater uptake of the 33P labeled foliar P fertilizer in both soils. The use of the tracing technique has enabled an accurate measure of the direct contribution of the foliar P product to grain P.  There were grain yield responses to some of the foliar P products but the outcome was less consistent than the 33P uptake.