Dig deep for nitrogen picture

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Deep soil testing to 40 centimetres or below across Western Australia’s agricultural regions has revealed there is more nitrogen in the subsoil that can be accessed by crops than previously realised.

Testing to these depths provides detailed mineral nitrogen (ammonium plus nitrate) profiles through subsoil layers and can help diagnose any potential crop production constraints, such as an acid band, compaction or hardpan.

Growers, agronomists and consultants who use Yield Prophet® are already realising the fertiliser use efficiency benefits from soil testing below 40cm for N.

But Yield Prophet® does not adjust root growth to account for constraints in the subsoil that can seriously reduce access to nitrogen at depth.

Collaborative research by Murdoch University, CSIRO and the Department of Agriculture and Food, WA, with funding support from the GRDC, aims to help rectify this and improve the accuracy of nitrogen decision support tools.

Key points

  • Deep soil testing aids efficient nitrogen fertiliser use
  • Maximum benefit is on clay and loam soils
  • Testing to 120 centimetres at sites across Western Australia found more subsoil nitrogen than expected
  • Crop roots can access nitrogen at depth
  • Subsoil nitrogen should be factored into fertiliser decisions

To date, researchers have analysed soil samples collected between 2000 and 2012 from 474 sites in WA’s northern, eastern and south-coast agricultural regions just prior to the start of the growing season.

This has included Agricultural Production Systems sIMulator (APSIM) modelling to determine the effectiveness of nitrogen deeper in the profile for crop production.

CSIRO agricultural research scientist Dr Yvette Oliver presented the preliminary results from this ‘More Profit from Crop Nutrition’ initiative at the GRDC–DAFWA 2013 Agribusiness Crop Updates in Perth in February.

Nitrogen profile

Although highly variable across soils, seasons and locations, more nitrogen was found in the subsoil at depth than researchers had initially thought. This could be accessed by crops – depending on subsoil constraints and location.

Image of a deep soil profile sample

A deep soil profile sample taken as part of the deep soil testing project carried out across WA.

Image of Dr Yvette Oliver in a field

CSIRO agricultural research scientist Dr Yvette Oliver inspecting a deep soil test sample.

The average of all soil sample profiles was 98 kilograms of nitrogen per hectare as mineral nitrogen measured to a depth of 120cm.

Only 30 per cent of this was in the top 10cm, 40 per cent was in the 10cm to 40cm layer and 30 per cent was below 40cm. 

Researchers found there can be as much as 2.35 times more nitrogen in the profile than in the top 10cm, but this is highly variable.

The weak correlation between topsoil and deep nitrogen (at 120cm) reinforces the need to soil test to depth to determine more accurate subsoil N levels.

Access to nitrogen at depth

Effectiveness of nitrogen found at depth compared with effectiveness of nitrogen in the top 0 to 20cm layer was found to be highly variable and seasonally dependent across all soil types and locations.

Where there were no root constraints in the subsoil, deep mineral nitrogen found at 100cm to 120cm was up to 60 per cent as effective as the same amount of nitrogen in 0 to 20cm and Dr Oliver says it should be accounted for in fertiliser decisions in these situations.

She says when roots are constrained at a layer, such as an acid band or hardpan, nitrogen becomes inaccessible to crops.
However, if constraints are present on heavier clays and loams it may still be worth accounting for deep nitrogen in fertiliser decisions – but with a lower relative effectiveness.

Soil testing

Image of a soil core drilling rig

The soil core rig in use for deep soil testing at the Liebe Group’s Dalwallinu site trials into deep soil testing during 2012.

Preliminary data indicates the most value to be gained from deep soil testing and accounting for mineral nitrogen in fertiliser recommendations is on heavier soils with no subsoil constraints in all rainfall zones.

Dr Oliver says it is likely an extra 40kg/ha of mineral nitrogen could be available in the 20cm to 60cm layer in these soils compared with what is indicated in 0 to 20cm soil test measurements.

She says about 70 per cent of this could be included in estimates of total available soil nitrogen when calculating fertiliser recommendations.

This would not apply to soils with root constraints, where inclusion of deep soil nitrogen in fertiliser recommendations would be highly dependent on location, soil type and crop rotation.

Cost of sampling to depth and analysis compared with likely benefit would need to be considered. It is recommended that growers check with their agronomist or farm consultant about the best depth to soil sample on individual paddocks.

More information:

Dr Yvette Oliver, agriculture research scientist, CSIRO
08 9333 6469

See 2013 Agribusiness Crop Updates Paper at www.grdc.com.au/UpdatePapers

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GRDC Project Code UMU00035

Region West