Economic assessment of the results shows that there are considerable savings to be made by better targeting phosphorous application rates to meet crop needs and site conditions.

Fertiliser P rates in use at the eight non-responsive sites that fell within the >310 µg/L zone were in the range 66 to 114 kg P/ha. Results show that replacement P rates would be sufficient at these sites, equating to P application rates in the order of 11 to 40 kg P/ha.

An example from one of the sites in the Riverland that had a high starting soil DGT-P of 717 µg/L (Colwell-92, Olsen-48 and Bray-417 mg/kg) showed that the application rate in use by the grower was 88 kg P/ha (400 kg MAP/ha). At a targeted replacement rate of 32 kg/ha, the grower can save 56 kg/ha of P fertiliser, equating to 255 kg of MAP at a cost of $178.50 per hectare (Table 1). Even still, the results show that at a site with such a high starting soil P level, the return on investment (ROI) for every dollar spent on MAP would only be $2.76 (Table 1); once the costs for transport, handling and spreading are taken into consideration, the returns would be minimal.

Similarly, savings can also be made at P responsive sites with better targeted P application rates. An example from a site in the South East shows savings of $161/ha (230 kg MAP) and a substantial increase in ROI from $9.26 to $21.43 for every dollar invested (Table 2), based on trial results. In the case of a 36 ha pivot, this equates to a total saving of $5,796 that otherwise would have been invested prior to the crop being planted.