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New deep P tool calculates the benefits and payback for you

Subsoil phosphorus (P) deficiency costs growers in Queensland and Northern NSW around $80–100 million each year in reduced yield. Deep P fertiliser applications into the subsoil can improve yield but are a long-term decision. They are moderately expensive and returns are not immediate. The Deep-P calculator helps Northern region growers decide if deep P will provide an economic benefit.

How it works

Growers input:

  • paddock properties such as crop rotation and P levels, and
  • economic data such as grain and fertiliser price.

The Deep-P calculator estimates:

  • optimal deep P application rate
  • expected $/ha net benefit
  • internal rate of return
  • payback probability.

Users can compare the financial benefits of the P-rate through average, poor, and excellent seasons. Growers can run different scenarios that might affect the return on investment.

                      Optimum P rate

How season might affect the optimum P rate net benefit.


The tool is based on a number of assumptions relevant to Queensland and Northern NSW:

  • Starter P is applied
  • 50% P is used from the topsoil, and 50% from subsoil
  • P in stubble is not returned to the subsoil P pool.

There are other factors used in the model that will improve as research continues. These include the critical soil P values that affect yield, the degree of yield reduction caused by deep P deficiency, and how P cycles between soil pools.

The deep P calculator should be used as a decision support tool, rather than a strict cost-benefit analysis. Used properly, the tool can help growers quantify the potential economic benefits and payback period for deep P applications.
The online Deep-P calculator tool is available here. A video on how to use it is included with the calculator.  

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