Wheat grain yield response to and translocation of foliar-applied phosphorus T. M. McBeath A,C , M. J. McLaughlin A,B , and S. R. Noack A A School of Agriculture, Food and Wine, The University of Adelaide, PMB 1, Waite Campus, Glen Osmond, SA 5064, Australia. B CSIRO Sustainable Agriculture Flagship, CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia. C Corresponding author. Email: therese.mcbeath@adelaide.edu.au Abstract. It is important to apply 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 supply to grow crops to tillering, but in seasons of increased yield potential a top-up application of P may be required. Foliar P can be applied directly to the plant when required and in some cases have been shown to provide benefits for increasing P-use efficiency and the P concentration in grain. Wheat (Triticuum aestivum cv. Frame) was grown in two soils of marginal P status with soil moisture maintained at 80% of field capacity. Seven foliar P treatments labelled with 33 P as a tracer were applied at Zadoks growth stage 39, at 1.65 kg P/ha with 120 L water/ha equivalent. Grain, chaff and shoots were harvested to measure yield and then digested to measure P concentrations and 33 P activities. There was no crop response to top-up soil or foliar P on one soil, but on the other soil, foliar phosphoric acid plus adjuvant gave a 25% wheat grain yield response. The use of the tracing technique enabled measurement of the portioning of foliar P fertiliser between stem, chaff and grain. The most responsive treatment had the greatest amount of grain P uptake and the greatest partitioning of the foliar P fertiliser to grain. Additional keywords: dryland cereals, fertilisers, isotopic tracing, nutrient management. Introduction The low efficiency of phosphorus (P) fertilisers in Australian soils is a major limitation to achievement of full crop yield potential (Cornish 2009). In 2006–07 $A1.7 billion was spent on fertilisers in Australia (ABARE 2008), with P fertilisers accounting for approximately one-third of nutrients used (FIFA 2009). Currently in Australian dryland cropping systems, almost all P fertiliser is supplied at sowing and this application strategy is not responsive to the climatic conditions of the season. The provision of all P fertiliser at sowing can result in low fertiliser efficiency if a crop is fertilised for a yield that the prevailing seasonal conditions do not support. It is important to apply starter P at sowing to provide essential P for early growth and to replace P exported in the previous grain crop (Batten et al. 1986; Grant et al. 2001). Using low rates of starter P fertiliser may provide sufficient P to grow crops to tillering, but we hypothesise that in seasons with increased yield potential due to good late-season rainfall, a strategic top- up application of P may increase yield (Noack et al. 2010). Furthermore, there are cases where supplemental P in the foliar form could help to overcome P access issues, whereby P stratified in the drying topsoil is not readily available to roots. This strategy is only useful if the plant has access to adequate water to fulfil the yield potential created through improved P nutrition. This approach is similar to tactical management strategies currently used for N fertilisers (Gooding and Davies 1992; Angus 2001). While there have been several studies of the effectiveness of foliar P for cereal nutrition, this work has predominantly been based in the United States (Barel and Black 1979a, 1979b; Sawyer and Barker 1994; Mosali et al. 2006; Girma et al. 2007) and only a small body of work has been published for plants grown in Australian dryland cropping soils (Bouma 1969; Alston 1979), and very little work has examined nutrient-use efficiency of foliar-applied P using isotopic tracers (Koontz and Biddulph 1957; Bouma 1969). Most tracer-based studies of foliar uptake have used single droplets with uptake measured over a short timeframe (Koontz and Biddulph 1957; Bouma 1969). These studies have provided valuable information about the mechanisms for foliar fertiliser uptake and the rate and amount of nutrient absorbed and translocated in the plant. We have extended this approach by using multiple drops of tracer-labelled fertiliser at dilutions equivalent to field applications, with plants grown to, and harvested at, maturity, to measure growth response and fertiliser uptake. The aim of this study was to measure the response of wheat grown in two Australian cropping soils with marginal P status to an in-season top-up addition of fertiliser P applied in the foliar form. Materials and methods Soil collection and chemical properties Two surface soils (0–10 cm depth) were collected from agricultural sites in the southern cropping region of Australia: Ó CSIRO 2011 10.1071/CP10237 1836-0947/11/010058 CSIRO PUBLISHING www.publish.csiro.au/journals/cp Crop & Pasture Science, 2011, 62, 58–65