Current nutrient management strategies are based primarily on the concept of cost effective nutrient management (i.e. deriving an economic return from fertilizer investment), unless managers have consciously embarked on a nutrient replacement approach to balance crop nutrient removal. The consequence of cost effective strategies is that soil fertility reserves of (originally) non-limiting nutrients will decline until fertilizer applications become warranted. Soil testing has shown that reserves of P, K and S have been gradually declining but there is little definitive evidence of the threshold soil test values which indicate when fertilizer application becomes warranted. This is particularly so for the alkaline cracking clay soils that support the Australian cotton industry. In addition to the lack of clear guidelines to identify fertilizer responsive field sites, there is also uncertainty surrounding the most effective fertilizer application strategies (rates, placement and timing) to allow efficient crop recovery and use. These issues are particularly important for immobile nutrients which don’t redistribute down the soil profile as moisture profiles refill.

This project undertook an extensive field research program to improve the soil testing guidelines for defining P and K responsiveness in irrigated and dryland cotton systems and to evaluate fertilizer application strategies (soil or foliar applications, fertilizer banding or incorporation) in terms of crop recovery and crop response. Both these nutrients already figure prominently in cotton fertilizer programs.

The key research findings have been that the efficiency of use of applied P and K fertilizers in most cotton farms is extremely poor, due to a combination of a crop root system that exploits fertilizer bands poorly and a phenomenon observed in flood irrigated systems where root activity in the fertilized hill declines rapidly once irrigation commences. If P and K are dispersed through a soil volume where adequate cotton roots are active during peak uptake periods (from first flower to first open boll), the crop can accumulate substantial quantities of both nutrients – even to the extent of clear luxury uptake. However the predominance of banded applications in dryland systems (to facilitate rainfall capture and storage) and the phenomenon of poor utilization of nutrients from the bed after flowering under flood irrigation are combining to seriously restrict nutrient use efficiency. Foliar applications of P and K were not an effective strategy to address these constraints.

The implications for productivity in the broader industry are uncertain, as the lack of recovery of applied fertilizer in most field trials has prevented meaningful interpretations of the degree of P or K constraint evident at each site. We have derived indices (kg nutrient uptake/100 kg lint yield) from long term experiments where P and K responses are substantial to attempt to benchmark crop nutrient status in the regions where research was undertaken. While preliminary, these indices suggest relatively widespread evidence of low-marginal crop P status in NNSW and southern Queensland, but less frequent occurrence of low crop K. While these benchmarks may not easy to adopt more broadly, we have also explored seed nutrient concentration as a possible indicator of crop P and K status. Considerable variation in seed P concentration is evident, but little for seed K. While important for nutrient budgeting calculations, seed P analyses may ultimately offer a more practical way of benchmarking crop P status.

While an improved understanding of the apparent impaired shallow root activity in flood systems is being developed, the irrigated industry should clearly move from banded to dispersed (broadcast and incorporated with tillage) P and K applications, with the depth and thoroughness of the incorporation a key to improving nutrient recovery. In dryland situations the solutions are less clear, but it is apparent that greater efficiencies of P and K recovery can be achieved by the grains crops grown in rotation with cotton than by cotton itself. The most effective interim strategy may be to allow the cotton crop to exploit residual (and potentially more dispersed) fertilizer residues and utilize the post-cotton pupae busting tillage operations as the opportunity to apply most P and K fertilizer in the crop rotation.