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.