Nutrient guidelines refined by MPCN initiative

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The bank of knowledge about crop nutrition in the southern region has been dramatically expanded as key gaps have been filled by MPCN projects

Fast fact

In no-till farming systems, Colwell P may overestimate phosphorus availability when compared with cultivated systems.

Historically, nitrogen and phosphorus nutrition in wheat has been well understood in the southern region, but little was known about other crops and other nutrients. A More Profit from Crop Nutrition (MPCN) project led by the NSW Department of Primary Industries (DPI) has aimed to fill the knowledge gaps that have existed for potassium and sulfur responses for all grain crops, and nitrogen and phosphorus response for non-wheat crops.

The project found that potassium and sulfur data used in the Making Better Fertiliser Decisions for Cropping Systems in Australia (BFDC) Interrogator is a good guide for the southern cropping region, despite the potassium and sulfur responses being mostly based on Western Australian data.

Another key finding is that while the trend to precision farming assists in managing variation across a paddock, it is just as important to be aware of changes down a soil profile by testing the zero to 10 centimetre, 10 to 30cm and 30 to 60cm depths to improve nutrient management.

Albert Oates, NSW DPI, taking trial soil samples to 1.2 metres.

PHOTO: NSW DPI

Phosphorus

In no-till situations, the standard zero to 10cm soil test appears to overestimate what the crop ‘sees’, explaining claims of crop response at higher phosphorus soil-test values. These values do not necessarily represent conditions experienced by growing plants. Most soil phosphorus test calibration was established under a traditional conventional tillage farming system, but with the change to minimum or no-till systems stratification of phosphorus is typical.

If soils are cultivated for the incorporation of lime, or as part of an integrated weed, pest or disease management program, the old guidelines (for example, 30 to 35 milligrams of phosphorus per kilogram for wheat on red kandosols) remain reliable for wheat, but other crops, such as canola and chickpeas, are still being assessed. In no-till systems there is a tendency for the critical range of Colwell P to rise.

Nitrogen

The biggest gap in knowledge with nitrogen budgets is the quantity of nitrogen that mineralises between the time of the deep nitrogen test and crop maturity. The work of PhD student Katherine Dunsford has identified some promising options for improved soil testing (see page 16).

In the southern region, the loss of nitrogen to the atmosphere due to denitrification is minor compared with the warmer northern region and has minimal impact on nitrogen budgets in dryland situations.

The large quantities of nitrogen required by crops compared with other nutrients, and the large variation in both demand and supply due to seasonal conditions, make soil testing important, but not necessarily the dominant factor, in making nitrogen fertiliser decisions. However, for those using a nitrogen budget approach, deep nitrogen testing to at least 60cm remains a good standard protocol.

Potassium

Potassium deficiency seems to be most pronounced in acidic soils. This project has enabled more confident interpretation of soil potassium test results and the use of results from soil surface tests is a reasonable protocol for most cases. For example, in gross terms, cereals are likely to respond to applied potassium when the soil test concentration is less than 50 to 55mg/kg (or 0.10 to 0.13 centimoles of positive charge per kilogram in the surface 10cm of soil). Subsoil reserves of potassium can negate this response, but factors such as soil acidity, soil compaction and waterlogging modify root growth and can limit access to subsoil potassium.

Sulfur

Despite finding cropping sites with low sulfur in the surface soil (zero to 10cm) there were no grain yield responses to applied sulfur, indicating that low sulfur is not yet a widespread issue for the grains industry in this region. Therefore, the critical soil test value (zero to 10cm) remains poorly defined. It appears that cereals are tolerant to low sulfur because their roots quickly penetrate deeper into the soil where reserves of sulfur seem to be adequate. Occasional deep sampling for sulfur to the depth of any root barriers may help to quantify subsoil nutrient reserves.

GRDC Research Code DAN00168

More information:

Dr Ehsan Tavakkoli
02 6938 1992
ehsan.tavakkoli@dpi.nsw.gov.au

GRDC Fact Sheet: Soil testing for crop nutrition (southern region)