Clay tests vital in setting up soil productivity this summer

Author: | Date: 12 Dec 2014

Key Points:

  • Up to five million hectares of water repellent, sandy textured soils in WA and SA could benefit from clay application.
  • Clay spreading has been shown to provide a long term solution to water repellence.
  • Pre-application tests of both soil and clay are important to determine clay rates and manage post-clay crop nutrition.

Long term WA research indicates crop yields can increase up to 50 per cent by boosting topsoil clay content to 5-6 per cent on water repellent deep sands and sandy duplex soils.

GRDC-funded Department of Agriculture and Food WA (DAFWA) clay trials at Dalyup, near Esperance, found wheat, canola and lupin yields lifted by 50 per cent over a 15-year period after clay incorporation in 1999 at a rate of 300 tonnes/ha (6 per cent topsoil clay content).

At a clay rate of 200t/ha (3 per cent topsoil clay content) in 1999, crop yields had increased on average by 40 per cent over 15 years.

The yield increases due to claying were consistent over the 15 years of the trial (1999-2014), as shown in the graph below:

DAFWA senior research officer at Esperance David Hall says claying increased crop yields in all years of the Dalyup trial, even in those seasons when there were no differences in crop establishment between clay and non-clay treatment.

One reason for this was improved soil nutrition associated with the clays, in particular potassium (K).

Clay treatment was also found to increase nutrient retention, with higher phosphorus (P) levels found in the clayed layer.

Costs and benefits of applying clay

At a cost of $700-900 per hectare (or more in some areas), the expense of applying clay can be prohibitive and often results in only small areas being treated annually.

Delving of sub-soil clay to the surface layer is a cheaper option, but is only suitable if the sub-soil clay layer is within 50cm of the surface.

Mr Hall says there was a positive economic response from incorporating clay at all rates used in the long term Dalyup trials, but it took six or seven years to generate a return from the lowest rates of 50t/ha and 100t/ha.

He says if clay can increase yields by 1t/ha, at a cost of $700-800/ha it will pay for itself after three years.

Generally, clay spreading in WA is economical if the clay source is within half a kilometre of the area where it is to be applied.

Researchers estimate about 160,000ha of sands with water-repellent topsoil in WA and SA have already been treated with clay-rich subsoil.

Productivity benefits arising from clay incorporation on these water repellent sands are being driven by overcoming constraints that include:

  • Uneven distribution of soil moisture leading to poor or staggered emergence of crops and weeds
  • Low nutrient holding capacity
  • K deficiencies
  • Wind erosion.

The importance of testing clay sources

Pre-application tests of both soil and clay are important to determine clay rates and to manage post-clay crop nutrition, according to Mr Hall.

He says improved nutrition after clay application is the next frontier of research for DAFWA and Murdoch University researchers, who are being funded by GRDC through its More Profit from Crop Nutrition (MPCN) initiative.

Testing clay sources (using any major soil testing laboratory) will provide a comprehensive analysis that includes Phosphorus Retention Index (PRI), standard nutrients, percentage of clay, boron (B) and particle size.

PRI is a measure of soil P-sorption and involves mixing a quantity of soil in solution with a single amount of P for a set period of time.

The amount of P remaining in solution measures the soil’s ability to fix P.

Through the MPCN project, initial testing of 43 WA paddocks treated with clay in 2013-14 showed 15 per cent had a high PRI (greater than 100mL/g), which affects P availability and can make P less available to crops in the short term.

Nutrition after clay

Professor Richard Bell, of Murdoch University, says these results highlight the importance of monitoring and potentially adjusting P rates in the first few years after claying.

He says other clays sampled through the MPCN project were found to be able to supply enough K and/or sulphur (S) to change treated soil from deficient to adequate for these nutrients.

No major toxic – salt and B – properties were found in the clays tested, but Mr Hall warned that higher levels can often be found in low rainfall mallee soils.

Based on these preliminary findings, it is recommended that the claying material be tested prior to spreading and growers/agronomists monitor closely the need for extra nutrients required for the higher yield potential of the clayed soil - especially nitrogen (N).

The recommended tests should include clay percentage, PRI, EC (salt), K, S and B.

MPCN field and laboratory research in the next two years will further investigate the relationships between clay rates, types and incorporation methods with nutrient availability and crop nutrient uptake on a range of sands across WA.

Professor Bell says researchers will also revisit long-term clay trial sites and study the paddocks of growers who have been using clay for long periods to assess how crop nutrition management has changed post-treatment.

The aim is to develop soil fertility trends and produce more robust recommendations for growers about how to adjust nutrient strategies after clay enhancement and cultivation of sandy soils.


More information:

David Hall, DAFWA
08 9083 1111

Dr Richard Bell, MPCN Murdoch University
08 9360 2370

Useful resources:
GRDC Hot Topic Claying to ameliorate non-wetting soils:

GRDC’s Delivering solutions for water repellent soils: 

GRDC Grow Notes:

DAFWA’s Claying to ameliorate soil water repellence:

GRDC Project Code UMU00042

Region West

GRDC Project code: UMU00042