Get the right advice on soil tests

Author: Rebecca Barr | Date: 16 Feb 2015

Why is soil testing important?

  • Knowing the potential nutrient limitations to crops ensures the optimal nutrient balance can be obtained to maximise yield.
  • In most soils, phosphorus and potassium are best added at seeding, so without soil testing it’s not possible to identify if the soil is deficient until it is too late.
  • Soil tests can be used to determine the nitrogen ‘budget’ for the season, ensuring N is prioritised to the soils that will be most responsive.
  • If soil tests show nutrient levels are increasing, it means that too much fertiliser is being applied, and cost-savings that can be made.

Obtaining professional advice to interpret soil test results is a critical to maximise yield potential by ensuring the optimum fertiliser program is put in place for your soils.

International Plant Nutrition Institute’s Rob Norton says that using accredited laboratories and advisers is fundamental for growers to get the best advice.

“Laboratories accredited by ASPAC – the Australasian Soil and Plant Analysis Council – can be trusted to use approved methods and ensure the results are appropriate for Australian soils. When taking this results from test values to fertilizer recommendations, a Fertcare® accredited adviser has the training to assess those results, in conjunction with a grower’s situation including rainfall, crop, and soil type, to prepare a fertiliser plan,” he said.

The GRDC-funded project Better Fertiliser Decisions for Crop Nutrition collated nearly 6000 Australian field experiments that matched soil test values to crop yield response to develop critical soil test values for nitrogen, potassium, phosphorus and sulphur.

“These results give advisers confidence in the ability of particular soil tests to predict is there is a yield response to be had, and are one of the reasons it is best to ensure your advisor is Fertcare ® accredited, so you know they have access to this information,” Dr Norton said.

When sampling the soil for P and K or soil properties, Dr Norton recommends combining 100 topsoil cores per area, where an area is a section of a paddock that will be managed as a single block. If variable fertiliser strategies will be used within a paddock, such as a sand-hill, this should be kept separate.

Rob Northon using a corer to take a soil sample in a paddock

International Plant Nutrition Institute’s Rob Norton (pictured collecting soil samples) recommends using accredited laboratories and advisors when analysing soils.

Depth of sampling is critical to get the right result. Tests of immobile nutrients like P and K rely on samples taken from the surface to 10 centimetres deep. Because most of these nutrients are in the topsoil, taking a deeper sample would mean the result would be falsely low because of dilution by deeper soil which has less nutrient. If the sample is taken too shallow (less than 10cm), then the result would come back with a higher soil test value. Deeper sampling, to 50 or 60 cm, is a good strategy for mobile nutrients like N and S which can move in the soil. An accredited advisor can interpret the results based on the different sampling depths to define an appropriate nutrient strategy.

“Each area doesn't need to be tested each year. For P and K, sampling around one in three years would be a good balance, particularly if the paddock is going into a wheat or canola rotation. These crops are some of the most responsive to nutrient application, and are important for most growers in terms of financial return,” he said.

“By sampling correctly, using a certified laboratory and then consulting with an accredited adviser, growers can effectively use soil tests to ensure they are maximising their yield potential while minimising cost.”

Rob Norton's tips for growers

Dr Norton says there are five key aspects to soil sampling:

  • Nutrient management is about getting the Right nutrient, applied at the Right rate, at the Right time and put in the Right place (these are known as the 4Rs of nutrient management, see the GRDC fact sheet for more information on the 4Rs).
  • Soil testing can help identify the Right nutrient and assist with getting the Right rate.
  • Make sure samples represent the area to be managed.
  • Take samples to the required depth – usually 0-10 cm for P and K, or deeper to 50-60 cm for N and S.
  • Always use certified laboratories and accredited advisers.

More information

Rob Norton
03 5381 2673

Useful Resources

GRDC Project Code DAN00132

Region South, North