Clay quality key to successful spreading, delving

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Yield improvements of 30 per cent are common on South Australia’s Eyre Peninsula when clay spreading or delving are performed correctly, and benefits may continue for up to 30 years. Delving (pictured) is effective when clay is less than 65 centimetres below the surface but clay should be spread when delving to a depth of more than 65cm.

PHOTO: Brett Masters

Knowing a soil’s clay content, consistency and chemical composition is the key to successful spreading and delving, if combined with the correct dose and full incorporation into the soil. That is the key message from research into both practices on the Eyre Peninsula (EP) in South Australia.

Clay spreading improves the wettability of sandy topsoils, helping to improve germination of crops and weeds. This helps for timing weed control and improves surface cover to reduce wind erosion.

Clay spreading also increases nutrient availability and microbial activity to the depth of clay incorporation. It is the most appropriate soil modification option where clay-rich subsoils are more than 65 centimetres below the surface.

However, on sand-over-clay soils, where the clay subsoil layer is within 65cm of the surface, the paddock may be clay delved. While clay spreading can improve the soil to the depth of the clay’s incorporation, clay delving has the potential to improve the soil profile to the depth of the delver by depositing clay throughout the profile.

For ease of operation and to facilitate the movement of clay up the delving tyne, clay delving is best undertaken in spring while the soil still contains some moisture.

Rural Solutions soil and land management consultant Brett Masters says recent trials are helping improve knowledge about clay spreading and delving to increase yields on light, sandy soils.

Brett says while they adapted the process from the south-east, there were some lessons to learn in understanding the impact of high lime (calcium carbonate) levels in the EP subsoil clays compared to those of the south-east.

“EP clays often contain high amounts of lime which, when spread at high rates on the soil surface, can induce long-term issues with nutrient availability, particularly trace elements and phosphorus,” he says.

“Delving has been effective where the clay material is less than 65cm below the surface. However, since the delving technique is newer than clay spreading, some of the areas that we were clay spreading might have been appropriate for clay delving.”

Yield improvements on EP of 30 per cent are common when the processes have been performed correctly and benefits can continue for up to 30 years.

“A lot of growers that we’ve talked to say that, when done correctly, the increased production is paying for the cost of the soil modification in the first couple of years,” he says.

The cost of spreading varies with the rate of clay-rich material applied and how far the pit is from the application site. For most sites, this is between $250 and $350 per hectare. Delving can be around half the cost of clay spreading, depending on clay depth and moisture levels.

However, Brett says he has also seen failures in clay spreading and delving in the region. The three main issues are:

  • too much clay – this can result in a hard-setting layer on the soil surface. Given the EP’s rainfall, its subsoils contain about 30 per cent clay. When using traditional incorporation technologies, a claying rate of 150 tonnes/ha has been appropriate for many EP sites;
  • high carbonate levels – spreading at high rates of clay that contain high levels of lime has resulted in long-term nutrient deficiencies; and
  • clay too deep for delving – long-term wind erosion has been caused at some sites where clay material has been deeper than 65cm. This is because delving has not brought up sufficient clay to increase moisture or nutrient-holding capacity at the site.

SA Research and Development Institute farming systems specialist Linden Masters, at Minnipa, SA, says that, in his experience, delving when appropriate produces better results because it is not as hard to incorporate and pull the clay up through the whole profile. However, he says poor results have been observed when the clay subsoil is deeper than 65cm. 

“The process essentially becomes a deep-ripping exercise and has limited lasting value,” he says.

“When clay spreading, it is imperative you use the right material. I’ve seen the wrong clay used and those paddocks are not even yielding 0.2t/ha.”

If clay spreading has been unsuccessful, Linden says there are still options to improve the soils.

“For example, you could run a spader over it, which allows for deeper incorporation. This dilutes the clay from the first 15cm, where people normally get it, down to 30cm. You then won’t have a band that the roots and moisture stay in and they don’t want to go into the sand beneath it.”

More information:

Brett Masters
08 8688 3460

Linden Masters
08 8680 5104

Clay addition resource for growers

The GRDC has published a booklet for growers considering spading, inversion, delving or spreading on their property. The booklet provides expert advice on how to perform the different processes and ensure the claying has the highest chance of success. Also included in the booklet are a number of case studies of clay addition from around the country.

Spread, Delve, Spade, Invert – a best practice guide to the addition of clay to sandy soils is available from Ground Cover Direct, email or free phone 1800 11 00 44, or free download from the GRDC website.


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