With the widescale uptake of minimum tillage, controlled-traffic and precision farming techniques across Western Australia's grainbelt, new sampling guidelines are required to better measure nutrient availability in the soil.

Researchers believe current soil sampling techniques might not be giving growers a clear picture of:

• what nutrients are available to a plant;
• how deep these nutrients might be in the soil profile;
• where in the paddock the nutrients are most concentrated.

A new research project led by CSIRO, with GRDC investment and involvement from the Western Australian Department of Primary Industries and Regional Development (DPIRD), is investigating the availability of nitrogen, phosphorus and potassium in soils farmed with these more modern methods.

Standard sampling procedure

CSIRO senior experimental scientist Dr Yvette Oliver says the project hopes to develop a standard soil sampling procedure to ensure consistent interpretation across the industry, allowing for more accurate and economic fertiliser decisions.

"Many growers in Western Australia rely on soil sampling to guide their fertiliser decisions, and yet there has been no update in sampling techniques for many decades, despite widespread uptake of minimum tillage and precision farming," Dr Oliver says.

The three-year project, which began in 2018, involves investigating different soil sampling strategies including the number, timing, depth and location (both across rows and within paddock) required to provide useful soil test results under various farming conditions.

These farming conditions could include no-till, traditional tillage, on-row sowing (precision sowing), banding of fertilisers and windrows, as well as soil mixing processes, such as ripping, delving, mouldboard or other modified ploughs and spading used for soil amelioration.

The research will also investigate the impact of soil mouldboard ploughing and rotary spading on the distribution of nitrogen, phosphorus and potassium in the soil.

"We now know that we have to be more specific in regard to how we test paddocks that have been farmed in certain ways, or that have undergone amelioration or tillage treatments," Dr Oliver says.

Results

In 2018, sampling occurred on farming properties in Williams, Jingalup and Gnowangerup.

Soil samples were taken from both on-row and the inter-row areas in the paddock.

Dr Oliver says first-year results suggest that on-row and inter-row sampling will be important in situations where potassium is marginal.

"In the first-year trial data, soil potassium was significantly higher in on-row samples compared to off-row, or inter-row, samples at all three trials," she says.

"This means if you are only sampling off the row, then the test will indicate a lower potassium level than what actually might be available to the crop, so it's important to have a mix of soil sample locations."

This year, the sampling will occur in locations right across the wheatbelt and will consider nutrient availability on and off windrows in controlled-traffic paddocks, in an attempt to better understand what soil sampling techniques suit these different farming methods.

Impact of amelioration

The soil tests will also analyse the impact of soil amelioration and timing of soil testing.

"Soil amelioration has become prevalent throughout the wheatbelt to combat water repellent soils, compaction and even weed burdens," Dr Oliver says.

She says understanding how deep the nutrients are after certain amelioration techniques will help scientists develop accurate sampling methods.

"Some growers may be shallow sampling without realising there are nutrients at depth, and may be over-capitalising on fertiliser applications," she says.

Soil sampling timing

The timing of soil sampling is also being examined to ensure more accurate in-season analysis occurs.

"Most soil testing is completed in early autumn before fertiliser decisions are made for the coming season," Dr Oliver says.

"What we want to ascertain is whether these samples in February or March are accurately predicting the availability of nitrogen, phosphorus and potassium to in-season crops."

She says it is hoped that the project will culminate in the release of standard guidelines for soil testing in late 2020.

Decision making tool available for 2019 season

GRDC has produced a useful webinar with crop nutrition expert Rob Norton that provides fertiliser advice about ‘Crop Nutrition Implications for 2019 from the 2018 season.’

More information: Dr Yvette Oliver, yvette.oliver@csiro.au