How much fertiliser is optimal

01 October 18

Inadequate nutrition has been found to be a major cause of yield gaps in cropping enterprises of the southern region’s high-rainfall zone (HRZ).

Agriculture Victoria soil research scientist Dr Malcolm McCaskill says wheat and canola yields in the southern region HRZ are well below their water-limited potential, with nutrient limitations one of the most common causes of yield gaps.

To help growers in the HRZ achieve the most profitable yields, three Excel based decision support tools have been produced thanks to a GRDC investment, Optimising the yield and economic potential of high input cropping systems in the HRZ. Part of the project is helping grain growers to determine the economic optimum application rate of nitrogen, phosphorous, potassium and sulfur under a range of conditions.

Through a series of nutrient response experiments led by Agriculture Victoria across the HRZ, researchers established that providing sufficient nutrients to soils with a low or marginal nutrient status can lead to wheat and canola yield equalling or exceeding their water-limited yield potential, except in cases of severe drought or waterlogging.  Dr McCaskill says the most common yield responses in these nutrient deficient trials were to nitrogen, followed by phosphorous, sulfur and potassium (Table 1).

Among the 12 experiments, there were no responses to copper or zinc. All trial sites were on land that was regularly cropped, apart from the long-term phosphate experiment at Hamilton, where soils of low phosphorus fertility were sought to develop response curves for applied fertiliser.

Table 1: Summary of nutrient response experiments conducted under the GRDC project, including the rainfall decile (April to November inclusive), grain yield of the ‘all-nutrients’ treatment (tonnes/ha), and the yield gap if a nutrient is either completely omitted or applied at a lower application rate. For example, a yield gap of 29 per cent indicates that the low-fertility treatment achieved a yield 29% below a treatment supplied with non-limiting nutrients.

* yields constrained by drought (2015) or waterlogging (2016, 2017)

Dr McCaskill says four of these experiments were affected by prolonged waterlogging — Bool Lagoon in 2016 and 2017 and Rutherglen in 2016.

“Wheat at Bool Lagoon in 2017 was inundated continuously from mid-July until mid-November, but still yielded 2.6 tonnes per hectare,” he says

Many of the experiments had statistically significant responses to nitrogen, phosphorous, potassium and sulfur, but not the micronutrients copper or zinc.

“Economic analysis has showed that the 90 per cent critical value in soil tests such as Colwell P – the soil phosphorous level that produces 90 per cent of maximum yield – underestimated the economic optimum because of the higher yield potential in the HRZ,” Dr McCaskill says.

“The 90 per cent critical value for a soil test is used typically used to estimate the economic optimum, above which the cost of extra fertiliser applications is not paid by the extra yield produced.  “However, crop yield potential is much greater in the HRZ than drier areas, and a critical figure of 95 per cent may be more appropriate.”

Above: Canola grown on low-phosphorous soils with a starting P of 14mg/kg Colwell and no added P yielded only 300kg/ha (left), whereas with 100kgP/ha (right) it yielded 5.3 t/ha.  Fertiliser resonse relationships from the experiement can be used to determine the most economic quantity to apply

Decision support tools

To help growers in the HRZ fulfil their crops’ yield potential, three different Excel-based decision support tools have been developed.

These tools use conventional marginal investment and return economics to calculate the optimum application rates of nitrogen, phosphorous, potassium and sulfur for a given set of input conditions, grain and fertiliser prices, and the user’s preferred benefit/cost ratio or rate of return on the marginal dollar invested in fertiliser.

The spreadsheets address three different questions important for nutrient management:

  • Awareness: what is the likely response to in-crop nitrogen application based on the initial phosphorous, potassium and sulfur availability?
  • Pre-sowing planning: what pre-sowing and in-crop applications of nitrogen, phosphorous, potassium and sulfur are required based on seasonal forecasts?
  • In season evaluation: Is the crop under-fertilised or over-fertilised given the season so far?

According to Dr McCaskill, the three decision support tools have been prepared to help calculate the optimum nutrient supply under a wide range of conditions, since the economic optimum fertiliser application rate is also dependent on input prices, product price and seasonal outlook.

“While yield potential in the HRZ is generally high, it can vary enormously (Table 1), and this makes estimating nutrient demand and fertiliser decisions particularly difficult,” he says.

“The spreadsheets are populated with yield and nutrient response data from a physiological model but allows modification to suit individual circumstances.

“At present the tools are only populated with seven sites in southern Victoria and the South East SA, but we will be progressively adding further sites from other representative stations within the high rainfall cropping zone.

“This is a prototype version for which we are seeking feedback as part of longer term development of decision support for nutrients.”

Useful Resources

Source: GRDC research code: DA

Author: Alistair Lawson | Date: 29 Jun 2018