Calculator crunches numbers for optimal inputs

GroundCover Live and online, stay up to date with daily grains industry news online, click here to read more

A new tool to help calculate the cost of crop nutrition is under development

Australian currency notes

The deep-phosphorus calculator will provide an economic tool to guide crop nutrition decisions.

PHOTO: Nicole Baxter

A new online economics tool to guide decisions about how much phosphorus to apply for profitable crop nutrition strategies, and how often, is being finalised.

Applying starter fertiliser in the top 10 centimetres of soil with the seed has been, and remains, an important technique to ensure the crop has optimum early root vigour and growth, to maximise the grain number established in the immature heads.

Optimum rates

The deep-phosphorus calculator will be able to indicate the possible optimum deep-phosphorus rate and the economic outcomes for a multi-season crop rotation. The critical soil phosphorus level below which a yield response is likely is considered to be 10mg/kg Colwell in the 10 to 30-centimetre layer.

However, as surface soil dries the plant relies on phosphorus reserves deeper in the soil.

Research shows that the subsurface layer (particularly 10 to 30cm) has been depleted in many soils and that, under certain climatic conditions, there can be a 10 to 20 per cent yield increase from replenishing this phosphorus source.

Placement of phosphorus fertiliser at depth is possible with the latest zero-till planters or specialised machinery. However, it is a longer-term decision that typically will entail a single, high application rate of phosphorus fertiliser and investment in fuel, but offers potential benefits that can last for many seasons.

It involves risk due to undetermined future season crop types, as well as to the immediate crop because of potential moisture loss and disturbance of the seedbed.

So, to help growers and advisers manage this risk, a ‘deep-phosphorus calculator’ is being developed. It is funded by the GRDC, the Queensland Department of Agriculture and Fisheries (DAF) and CSIRO, with input from researchers, growers and agronomists from across the northern grains region.

It incorporates the results from current deep-phosphorus research to provide an analysis framework to examine the economics of this longer-term decision. The adequacy of nitrogen rates can also be tested, and results from future nutrition research will be incorporated into the calculator to continually improve the accuracy of the results generated.

Phosphorus is an important nutrient for crop development as it enables cell division, energy storage and growth. Without sufficient phosphorus, young plants are likely to be stunted, exhibiting short erect leaves tending to orange-red discolouration, and the overall plant displaying a dark bluish-green colour. Root growth can also be impaired. When these symptoms are visible, there is little that can be done to rectify the resulting potential yield loss.


The deep-phosphorus calculator will be hosted on a website with several other decision-support tools including CropARM, NitrogenARM, ClimateARM and FallowARM.

Flow diagram showing how the deep-P calculator works

Figure 1 How the deep-P calculator works.

SOURCE: Andrew Zull

The calculator will integrate the effects of soil water-holding capacity, location (rainfall), machinery and fertiliser costs, crop value, nitrogen availability and length of the cropping rotation to calculate the net economic benefit over a range of both deep-phosphorus application rates and seasonal rainfall outcomes.

The optimal deep-phosphorus application rate is driven by both physical and economic components of the cropping system in a paddock (Figure 1). The tool will give an indication of the benefit of applying deep phosphorus, such as return on investment and time for pay-back.

The calculator will be able to indicate the potential longevity of applied deep-phosphorus rates on the basis of a simple soil phosphorus balance, even though it is known that very complex pools and interactions occur in the soil. 

The calculator is still undergoing testing and refinement to increase confidence in its predictions. Improvements in the analysis will occur as the research proceeds, with additional nutrients or ameliorants being included within the calculator.

The calculator is available as a spreadsheet for research purposes and once further validation is undertaken a website will be available, with case studies that can be modified to a grower’s individual situation, or new scenarios can be created.

Initial findings from the deep-P calculator

  • Soil with a very low level of phosphorus (P) will require at least the total crop removal rate to maintain the current level of productivity. For example, a three-tonne-per-hectare wheat crop removes eight kilograms of P. If this was the average yield each year, 40kg P/ha would be removed over five years.
  • Double-cropping and planting sorghum and pulse crops will increase demand for nutrients.
    ¢ It may not be economic to apply deep P to soils with low water-holding capacity, as seasonal moisture availability will limit any productivity benefits from improved P status.
  • The critical soil P level below which a yield response is likely is considered to be 10 milligrams/kg Colwell in the 10 to 30-centimetre layer. Initial results from this calculator indicate there may be a financial benefit in some situations from applying P prior to reaching this critical level.
  • Soil nitrogen (N) and P interact strongly and this is accounted for in the calculator. However, it is recommended a specific N fertiliser calculator is used to inform N rate decisions.
  • The optimum P rate is derived across the full range of season types that may be encountered for a site.

More information:

Howard Cox, Queensland DAF,

07 4688 1381;

Andrew Zull, Queensland DAF,
07 4688 1407


eXtensionAUS connects paddock to lab


Research supports dollar-saving decisions

GRDC Project Code CSA00036, UQ00063, UQ00078, DAN00166, DAQ00183

Region North