UNE 1501 – ‘ Phosphorus availability in raingrown cotton’, was conducted at ‘Colonsay’, via Brookstead, on the Darling Downs in Queensland, at a long term nutrition site, owned and maintained by Incitec Pivot for 30 years. The experiment was conducted in raingrown cotton cv SICOT 74 BRF.

At this experimental site, phosphorus had been applied to the same plots at the rates of 0, 10, and 20kg/ha P each year since the commencement of the experiment. For the 2015 experiment, the existing 50m plots were then split into three treatments to include the addition of deep applied P, applied at 20kg/ha to 20cm depth on 50cm spacing, and a tillage treatment to disperse any bands of enriched nutrient. Work by Bell et.al. 2015, and Dorahy 2004, has shown little response in cotton, in terms of yield or plant nutrient uptake, to the application of fertiliser P in the season that it is applied. This experiment provided the researchers with a unique set of circumstances where the soil had been enriched with P such that a gradient of background soil test values was achieved. In this particular experiment very good results with respects to both plant phosphorus uptake, and yield were achieved. Colwell P values in the 20kg/ha P treatments had been enriched to 64 mg/kg in the surface, whilst the 0 kg/ha P treatments were at around 8.6 mg/kg (Colwell), and had remained relatively unmoved for the duration of the Incitec Pivot experiment (30 years). The 20 kg/ha P treatments showed maximum plant P uptake of approximately 32 kg/ha, the yield being 7.92 ba/ha. The uptake in the 0 kg/ha P treatments was around 18 kg/ha in the plant tissue, with a yield of 5.62 ba/ha. It is not perfectly clear as to the mechanism driving these responses although it is postulated that the favourable early season growing conditions resulted in excellent surface root activity, or that soil enriched with P behaves differently than that with fertiliser P applied for that growing season, or a combination of both.

When looking at the response curve comparing yield and soil test Colwell P values, it appears as if the critical level, for cotton, using Colwell P ( 0-10cm) may be around 25 mg/kg, significantly higher than the accepted 6 mg/kg.

Very little response was observed when both deep applied P was added or when tillage was included to disperse bands of enhanced P. This may concur with other work by Wang et.al. 2007, and McLaren et.al. 2013, inferring that deeper in the profile the plant is likely to be drawing on native, and other residual reserves of P held in a range of P pools, and again that applied fertiliser P ( as the deep applied P ) is unlikely to provide a short term response in terms of either plant P uptake, or yield. Because of the favourable early season root activity in this particular experiment, it was difficult to distinguish differences in nutrient taken up from soil containing dispersed P, or soil containing P held in a band.

Plant tissue testing using the youngest mature leaf, still provides a valid basis for assessing in-crop P status in cotton. The reliability decreases after peak flowering, as plants partition plant P in to maturing fruiting bodies, and seed.