GRAIN growers can readily determine how much deep phosphorus (deep-P) to apply for profitable crop nutrition using a new online calculator designed to guide decision making.
The Deep-P calculator was developed after research trials across the northern grain region found applying deep-P on soil deficient in phosphorus at depth paid dividends for growers with yield increases of five to 10 per cent or more at most sites.
The online decision support tool, hosted by the University of Southern Queensland, is a joint investment by the GRDC with the Queensland Department of Agriculture (DAF), and CSIRO and is available online. (CLICK HERE).
The Deep-P development team (consisting of nutrition researchers, economists, extension officers, modellers and software designers) identified, via industry consultation, the need for a support tool to assist in deep-P decision-making.
While research has found applying phosphorus as a starter fertiliser with seed in the top 10 centimetres of soil was vital for promoting root growth and maximising the early establishment of grain, trials determining the benefits of deep-P application have been conducted for a much shorter time and determining the economic benefits has proved more difficult.
The deep-P calculator incorporates soil water-holding capacity, rainfall, machinery and fertiliser costs, crop value, nitrogen levels and crop rotation.
- Jayne Gentry, DAF
The application of deep-P is quite different to other nutrient management decisions as it is a longer-term decision due to initial cost of application and phosphorus fertiliser having benefits that can last many seasons.
However there are risks due to unknown future season types. The grains industry wanted to be able to answer the question “how much phosphorus and how often?” The Deep-P Calculator provides a range of outputs that confidently answers this question for growers’ individual paddocks.
DAF senior development extension officer Jayne Gentry said the on-ground research led by Associate Professor Mike Bell from the University of Queensland was merged with sophisticated bioeconomical modelling and ground-truthed with growers and agronomists.
The result was an easy-to-use tool that calculates the economics of various application rates of phosphorus in different crop rotations.
“The Deep-P calculator incorporates soil water-holding capacity, rainfall, machinery and fertiliser costs, crop value, nitrogen levels (nitrogen constraints will mask a phosphorous response) and crop rotation to calculate the economic benefits over a range of deep-P application rates and seasonal outcomes,” she said.
Ms Gentry said trials across Queensland had shown soil with phosphorus levels below 10mg/kg Colwell in the 10-30cm layer was likely to produce a yield response to deep-P application.
Ms Gentry said as phosphorus was an immobile nutrient, replacing it in this layer of subsoil meant using machinery to place it at depth, or moving it there mechanically after surface application.
This makes deep-P placement a longer-term decision because it involves a large upfront cost with potential yield benefits lasting years. The soil disturbance from this application may also impact on plant establishment of the subsequent crop. Hence, the application interval needs to be as long as practically and economically possible.
“Typical outputs generated from the calculator show economic benefits from the deep application of 20 to 40kg phosphorus per hectare,” Ms Gentry said.
“One scenario shows that 20kg/ha phosphorus at a cost of about $160/ha, provided average annual benefits of $100/ha for up to four to six years. However, it may take more than a year to pay back the initial investment costs.
“Whilst higher rates didn’t show a significant increase in yield response, they are likely to provide benefits for a longer duration.”
However, Ms Gentry said growers needed to keep in mind that responses to phosphorus varied depending on the season, as in-season rainfall allows plants to access phosphorus in the 0-10cm layer of soil and reduces the crop’s reliance on sub-soil phosphorus and therefore the potential benefits from deep-P placement.
“Where there is minimal in-season rainfall, growers would expect to see greater benefits from deep-P as plants are foraging for water and nutrients deeper in the soil profile,” she said.
“The Deep-P calculator has been designed to give growers an indication of the benefit of applying deep phosphorus, such as return on investment and time for pay-back, while considering seasonal impacts in their paddocks.”