A review of phosphorus nutrition in irrigated cotton farming systems of Australia

N Australian cotton production predominantly occurs on Vertisols. The average lint yield of cotton grown in Australia is 2 260-2 700 kg.hm(-2), which is 2.5 to 3 times the world average. This high productivity per unit of land area requires efficient use of resources such as water and nutrients. However, high yields accelerate the export of nutrients such as phosphorus (P) in seed, depleting the soil reserves of P more than in other countries with lower cotton yields. Recent surveys of cotton industry indicate that P application rates should match seed P export (30 similar to 40 kg.hm(-2)), but historical depletion within subsoil is still evident and is continuing. Depletion of soil P is typically more pronounced in the subsoil than in the topsoil (0 similar to 20 cm) where P fertiliser is applied, as cotton roots rely on these layers as important sources of plant available water and available P. This mismatch between zones of P uptake and resupply may increase stratification of available P in the soil profile. Recent studies showed that cotton responded poorly to banded applications of fertiliser P, while dispersal of fertiliser throughout the plant beds was more successful. Researchers have also observed sporadic cotton responses to applied P fertiliser in soils where available P concentrations were well above the previously determined critical concentrations indicative of fertiliser P responses in Australia. To sustain high-yielding cotton production in Australia, a greater understanding of cotton root acquisition of applied P, as well as a re-examination of critical soil P concentrations for each production region are required.