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.

Table 1. Non-responsive site 0 kg P/ha Current farmer rate – 88 kg P/ha Recommended rate – 32 kg P/ha
Yield t/ha 56 57 57
Income $/ha @ $280/t (Fresh) $ 15,680 $ 15,960 $ 15,960
Additional income $/ha $ – $ 280 $ 280
MAP required kg/ha (22% P) 0 kg 400 kg 145 kg
Cost of P @ $700/t $ – $ 280 $ 102
$ return for every $1 invested $ – $ 1.00 $ 2.76
36 Ha Pivot – MAP $ – 14.4 t 5.2 t
Transport $20/t $ – $ 288 $ 104
Fertiliser + Transport cost – ROI $ – $ 0.49 $1.36

 

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.

 

Table 2. P fertiliser responsive site 0 kg P/ha Current farmer rate – 110 kg P/ha Recommended rate – 60 kg P/ha
Yield t/ha 67 79 82
Income $/ha @ $270/t (Processing) $ 18,090 $ 21,330 $ 22,140
Additional income $/ha $ – $ 3,240 $ 4,050
MAP required kg/ha (22% P) 0 kg 500 kg 270 kg
Cost of P @ $700/t $ – $ 350 $ 189
$ return for every $1 invested $ – $ 9.26 $ 21.43
36 Ha Pivot – MAP $ – 18 t 9.7 t
Transport $20/t $ – $ 360 $ 194
Fertiliser + Transport cost – ROI $ – $ 4.56 $ 10.57

Project Partners

Potatoes South Australia PIRSA RSSA National Landcare Programme The University of Adelaide premiumgreen-logo