Key Messages for growers

  • The level of yield response has a greater impact on economic returns than the cost of MAP.
  • A wide range of P rates will achieve the same net return at most of the case studies here.

With global phosphorus (P) supply chain issues continuing and prices for fertiliser expected to stay high in 2022, decisions around P fertiliser application rate will have greater influence on the budgets of grain-producing businesses than usual next season.

Higher prices for P fertilisers have prompted growers to question whether they should reduce rates to maximise economic returns from applying P. The answer depends on the level of yield response to P per hectare that is expected.

A current Grains Research and Development Corporation (GRDC) investment is assessing how soil properties and climate can influence yield response to P fertiliser in wheat. This research is in progress, however, completed field trials provide an opportunity to examine some economic case studies.

As part of this research, researchers are investigating what precision in P rates is required to maximise economic returns from P fertiliser. The calculations show the range of rates that maximise net return, referred to as the optimal range, shown below in Table 1

The impact of an increase in price for P fertiliser was assessed using three price scenarios for MAP: $700 per tonne (close to the five-year average), $1050 per tonne (1.5 x average) and $1400 per tonne (2 x average). The economic analysis was done for five sites that represent the range of responses observed, from very low to very high yield response to P. The price for wheat (allowing for delivery and levies) was set at its five-year average of $273/t.

The results show that the level of yield response to P has a greater influence on the optimal range than fertiliser price. For example, doubling the MAP price from $700/t to $1400/t shifted the optimal range from 20-55kg P/ha, to 11-56kg P/ha at Boyup Brook where a yield response of 2.27t/ha occurred. At Wandering, where a yield response to P of 0.65 t/ha occurred, the same increase in price shifted the optimal rate from 13-23kg P/ha to 6-22kg P/ha. The greatest shift in optimal range occurred at the Northam site, where a response to P of 0.38t/ha occurred; it decreased from 7-27kg P/ha to 2-11kg P/ha. At Tammin, where there was no significant yield response to P, the optimum rate was 0kg P/ha under all price scenarios.

Table 1: Summary of economic analysis for the case study trials.  n.s. = yield response not statistically significant.

Economically-optimum rate (kg P/ha)
Boyup Brook

 

 

Brookton

 

Wandering

 

Northam

 

Tammin

 

Response  percentile 100th 75th 50th 25th 0
Yield response (t/ha) 2.27 0.97 0.65 0.38 n.s.
MAP price ($/Ha) Rate Range Rate Range Rate Range Rate Range - -
700 37 20-55 27 22-32 18 13-23 17 7-27 - -
1050 36 11-60 23 13-33 16 5-27 12 5-19 - -
1400 34 11-56 20 13-28 14 6-22 6 2-11 - -

 

The optimal range is quite wide in these cases studies as the rate-return relationship is flat across a wide range (see link for full details below). In fact, there is overlap in the optimal range for the three price scenarios. For example, at the Wandering site a P rate between 13 and 22 would fall within the optimal range for each price scenario.

The wide range of rates within the optimal range demonstrates that growers have flexibility to reduce financial risk without reducing profit. The optimal range at Boyup Brook and Brookton are above rates typically used in those areas – we’ll know more about how current practice compares to economic optimum when this research is complete. The optimal range at Tammin is unlikely to be used by growers; it’s more likely that growers will apply P at replacement rates in this scenario to maintain soil P fertility.

For more detail on the reduction in net return by applying P rates below the optimum range click here.

The field trials used as case studies here are from a series of trials where nitrogen, potassium and sulphur are applied to ensure only P is limiting. Adequate supply of nutrients other than P is important to capture economic benefit of applying P in your crops.

While these case studies provide an assessment of the sensitivity of the optimal range of rates to increases in MAP prices, they are not intended to provide recommendations. Soil testing for nutrients, in combination with assessing different price and yield scenarios are the best approach for determining the rate required for individual paddocks.

To see the full detail of the economic analysis for the case studies, click here.

This article was produced by the GRDC project ‘Increasing profit from N, P and K fertiliser inputs into the evolving cropping sequences in the Western Region’ (GRDC Project code: UWA1801-002RTX).

Field trials used for this work were established, funded and maintained by Summit Fertilizers and CSBP.

What do I lose by applying P rates below the optimum range?

The economic impact of applying P rates below the optimum range is dependent on the yield response to P (Figure 1). The potential for lost income is greatest at the Boyup Brook site, followed by Brookton, Wandering and Northam.

At Boyup Brook, the average reduction in net return between 10 and 19 kg P ha is $19/ha per kg of P. At Brookton, the average reduction between 10 and 21 kg P/ha is $5/ha per kg of P. At Wandering, the average reduction between 0 and 12 kg P / ha is approximately $8/ha per kg of P. At Northam, the average reduction between 0 and 7 kg P / ha is approximately $4/ha per kg of P.

Relationship between net return and P rate at levels below the optimum range

Figure 1: Relationship between net return and P rate at levels below the optimum range. Values within or above the optimum range not shown here.

Case Studies

Case Study 1 - Boyup Brook

Year 2020 Soil test data 0-10 cm
Crop Sceptre Wheat Colwell P (mg/kg) 74
Sowing date 22nd May PBI 395
OC (%) 4.9
pH (CaCl2) 5.5
Economically-optimum rate (kg P/ha)
MAP price Rate Range.
700 37 20 to 55
1050 35 11 to 60
1400 33 11 to 56

Optimal P rates for Case Study1 Boyup Brook

Case Study 2 - Brookton

Year 2020 Soil test data 0-10 cm
Crop Sceptre Wheat Colwell P (mg/kg) 19
Sowing date 26nd May PBI 11
OC (%) 0.6
pH (CaCl2) 5.7
Economically-optimum rate (kg P/ha)
MAP price Rate Range
700 27 22 to 32
1050 23 13 to 33
1400 20 13 to 28

Case study 2 Brookton

 

Case Study 3 - Wandering

Year 2020 Soil test data 0-10 cm
Crop Sceptre Wheat Colwell P (mg/kg) 35
Sowing date 18th May PBI 81
OC (%) 2.7
pH (CaCl2) 5.9
Economically-optimum rate  (kg P/ha)
MAP price Rate Range
700 18 13 to 23
1050 16 5 to 27
1400 14 6 to 22

Case Study 4 - Northam

Year 2020 Soil test data 0-10 cm
Crop Sceptre Wheat Colwell P (mg/kg) 37
Sowing date 28nd May PBI 37
OC (%) 1.3
pH (CaCl2) 4.9
Economically-optimum rate (kg P/ha)
MAP price Rate Range.
700 17 7 to 27
1050 12 5 to 19
1400 6 2 to 11

Case study 4 Northam

Case Study 5 - Tammin

Year 2020 Soil test data 0-10 cm
Crop Sceptre Wheat Colwell P (mg/kg) 22
Sowing date 16th May PBI 29
OC (%) 0.8
pH (CaCl2) 5.6
Economically-optimum rate (kg P/ha)
MAP price Lower Upper
700 0 4
1050 0 -
1400 0 -

Case Study 5 Tammin