Many South-West soils are no longer deficient in phosphorus and farmers should examine other limiting factors when making fertiliser decisions, according to latest research from the Department of Agriculture and Food.

Department development officer Peta Richards presented the findings of a six-year soil sampling program at the 2015 WA Soils Conference in Mandurah today.

The results highlight the importance of soil testing before applying fertiliser and identify opportunities to improve crop or pasture yields.

“The findings highlighted potassium, sulphur and pH levels as limiting factors that should be addressed first to make efficient use of the nutrients in the soil,” Ms Richards said. “The days of a bag an acre are now over.”

DAFWA undertook a paddock-based soil sampling program from 2009 to 2015 in a rainfall zone of more than 600mm, as part of the Whole Farm Nutrient Mapping project.

“The data assessed soil phosphorus, potassium, sulphur status and soil acidity, and was presented to the farmer as whole farm nutrient maps,” Ms Richards said.

“We collected 12,000 samples from more than 500 farms,” Ms Richards said.

“Seventy per cent of paddocks contained sufficient phosphorus, whilst 90 per cent were considered acidic.

“Of the high phosphorus status soils, 80 per cent were acidic and about one third were deficient in potassium or sulphur.

“These imbalances can lead to underutilisation of applied and soil stored phosphorus, reduced production outcomes and unnecessary phosphorus losses to the environment.”

The project also used the Australian standard Accuspread method to assess the spread pattern of farm-based spreading equipment from 2012-2015.

“Data from 200 tests indicate that around half of the spreaders would need to spread at less than 10 metre widths (less than half the target width) to achieve an even distribution of fertiliser meeting the Australian standard,” Ms Richards said.

“Many farmers aim to spread superphosphate or urea at around 25 or 20 metre widths respectively. Some farmers had noted striping in paddocks when nitrogen fertilisers were applied, but not when superphosphate was applied, adding anecdotal evidence of adequate phosphate levels in many soils in South West WA.”

Ms Richards said the results indicate that testing is required to guide decision making for improved farm economic performance through improved fertiliser management practices, with a longer term outcome being a reduction in unnecessary phosphorus use and inflows to coastal waterways.

The work has had funding support from State NRM.

The 2015 WA Soils Conference will run from 6 to 9 September, coordinated by Soil Science Australia.

Sampling sites as part of the Department of Agriculture and Food’s whole Farm Nutrient Mapping project.

Media contacts: Jodie Thomson/Lisa Bertram, media liaison +61 (0)8 9368 3937