Assessing break crop options for enhanced soil phosphorus availability

Any reduction in the need for fertiliser application through strategies that mobilise P from the 'soil P bank', enabling uptake by crops will provide a substantial economic benefit for the grower. Whilst many controlled environment studies report that certain break crop species have the capacity to mobilise P from less labile pools, there is scarce information suggesting mobilisation occurs in the field. This paper reports measurements of available P (Colwell and DGT) for root zone soils under break crops and wheat from five field locations in southern Australia, covering a range of climates and soil types differing in P sorption capacity. The aim was to identify potential for P mobilisation by break crops. Results demonstrate the enormous variability in plant-available P in the root-zone of different break crops across sites, as well as a clear influence of soil type on this soil P pool. More differences were detected between crops for DGT P than Colwell P. Across all sites, available P in the root zone soil of peas was consistently greater than other break crops, but not cereals. At one site, there was more available P in the root zone soil under lupins than the other break crops. However, it is concluded that the P mobilisation potential for any break crop cannot be assessed solely from available P measures in the root zone and a P budget for each crop is required.