Foliar fertilisation of wheat plants with phosphorus

Abstract

Phosphorus (P) is an important macronutrient essential for plant growth. Broadacre cropping often requires additional inputs of mineral P fertiliser to grow profitable crops. Current management practice is to apply all fertiliser P at sowing. If the conditions under which foliar applied P could reliably increase grain yield are met, foliar application of P could be used as an in-season management strategy to top up P supply of wheat. This could be of significant benefit to farmers to reduce risk in regions with variable climate. Through a series of plant experiments under controlled environmental conditions, this thesis investigated plant physiological (leaf wettability and growth stage) and foliar formulation (form of P, P concentration, adjuvant choice and pH) factors affecting the efficacy of foliar P uptake and translocation. The first experiment investigated the influence of leaf side and its corresponding wettability on the uptake and translocation of foliar applied P. The second and third experiments examined the effect of adjuvants on the wettability of wheat leaves and the associated uptake and translocation of foliar applied P (from phosphoric acid) after a few days and when harvested at maturity. The last experiment investigated the effect of foliar formulations differing in pH, P source and adjuvant, on wheat growth and uptake and translocation of P. A number of methods and techniques were used throughout the thesis. Investigations on the effect of leaf morphology on uptake and wettability involved the use scanning electron microscopy. Wettability of leaves by both water and fertilisers was characterised using contact angle measurements with a combination of static, advancing, receding and spreading contact angles over time measured. Uptake and translocation of the foliar applied fertilisers was quantified through the use of dual or single labelling isotopic tracer techniques. Absorption and subsequent translocation of foliar applied P was higher for the adaxial (upper) leaf side despite it being more difficult to wet than the abaxial (lower) side. When the foliar P concentration was increased the contribution of foliar P to plant P uptake increased but was translocated away from the site of application at a lower efficiency, likely due to the higher scorch experienced by the leaves at higher concentrations. Importantly, the morphology of the wheat leaf influenced both the retention and contact angle of the fertiliser on the leaf surface and the uptake and subsequent translocation of the foliar applied P. Foliar application of P at ear emergence had higher absorption and subsequent translocation of P than when applied at anthesis. The inclusion of a surfactant in the foliar P formulation is essential because wheat leaves are difficult to wet. Application of foliar P without a surfactant resulted in lower levels of fertiliser retention on leaves. When applied with phosphoric acid the choice of adjuvant affected the spreading dynamics and leaf wetting area but did not affect the foliar uptake of P. The yield response to foliar applied phosphoric acid was inconsistent despite the uptake and translocation being the same for all formulations that included a surfactant. The timing of application was more important than surfactant choice with higher translocation of foliar applied P when it was applied at flag leaf emergence compared to tillering. While increases in P uptake by wheat plants with foliar application of phosphoric acid were consistent, increases in plant growth and yield were not. Although foliar P from phosphoric acid was absorbed, only a small proportion was translocated. Specific combinations of adjuvant and P sources other than phosphoric acid were able to increase both plant P uptake and peak biomass. These foliar fertilisers ranged in associated cations (potassium, sodium and ammonium phosphates) and pH (2.2, 4.3, 6.5 and 8.7). Increases in plant P uptake did not always translate to biomass increases with translocation of foliar applied P playing a more crucial role than foliar uptake of P. This thesis has made important progress in our understanding of the effects of wheat leaf morphology, leaf wettability and crop phenology on the recovery of foliar applied P fertilisers in wheat plants. The processes of retention, absorption and translocation of foliar-applied P have proven important for inducing positive biomass and grain yield responses and this has been achieved using several foliar P formulations. However, a single characteristic of the formulation that optimises these processes has not been identified and as a result prediction of the exact scenarios when positive responses of wheat to foliar-applied P should occur has not been achieved. It appears that there is some plasticity in the response by wheat plants to additional P supplied via the leaves and some remaining uncertainty about the effects of scorch that are influencing the predictability of the response. Field validation is required to ascertain whether the positive response found in controlled experiments can be replicated when environmental conditions are more varied and unpredictable.