How critical is the critical level?

Critical soil nutrient levels indicate whether a yield response to fertiliser is likely – or not. Their role is to help avoid unnecessary or uneconomic applications of fertiliser. Critical levels are used in interpreting soil test results to make fertiliser recommendations.

The critical level is the value required to achieve 90% of maximum production. If a soil test result for a nutrient is above the critical level, the probability of a yield response to fertiliser is less than 10%. As you get further above the critical level the likely response to fertiliser eventually gets down to zero.

Recently, focus has shifted from critical levels to a critical range. Both vary between soil types for each nutrient, and crop type. For example, the critical level for 90% of maximum wheat yield on a Kandosol is 25 mg/kg Colwell P and the critical range is 21 to 30 mg/kg. A critical range exists because, aside from soil test results, other factors influence the grain yield response to fertiliser. The timing and amount of rainfall, and the adequacy of soil sampling are important.

Recommendation systems and tools based on the yield response from fertiliser are widely used to guide decisions about fertiliser rates. The best of these are underpinned by the BFDC Interrogator database. The soil test-crop response trial calibration relationships in this database are recognised by Fertcare as the ‘best available data’ to underpin soil test interpretation in Australia.

Critical levels or ranges are one of a few approaches that can be used in recommending fertiliser rates. For example, applying enough P to replace what was removed in last years’ crop is a sensible approach to maintain soil P levels at above critical levels.

The term critical level is sometimes used interchangeably with sufficiency, but they are not the same. Sufficiency is the level required to meet 100% of production potential. The sufficient range starts above the critical range. When a nutrient is in the sufficient range it should not be a yield-limiting factor and the expected response to fertilizer is zero.

Further Information

GRDC: Crop Nutrition – soil testing fact sheets: Northern region, Southern region, Western region

CSBP: Is extension moving away from science? Reviewing phosphorus and lime responses in Western Australia

Acknowledgements

Craig Scanlan, DAFWA, Chris Dowling, Back Paddock Co., Luke Gaynor, NSW DPI, Rob Norton, IPNI.

This article was first published in 2015 but the information is still relevant in 2018.

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