AgMemo Northern Agricultural Region

Deep ripping no guarantee to fix water repellent soils

potos of soil pits
Soil profile images following 50ml of rain. The wet areas are darker.  

Grain growers who have taken advantage of summer rainfall and deep ripped paddocks have been warned the practice does not necessarily overcome soil water repellence.

While deep ripping was an effective way to reduce soil compaction, it did not guarantee improved soil water infiltration.

In some cases, deep ripping could exacerbate topsoil water repellence.

Deep ripping does not alter the soil surface condition, only the subsoil condition.

In fact, ripping when the repellent topsoil is quite dry can make expression of the repellence worse.

Dry sown crops can sometimes have worse establishment on strongly water repellent soils that have been deep ripped, with poorer water infiltration when the dry topsoil has been loosened.

An on-farm demonstration trial with the Mingenew-Irwin Group in 2016 found there were far greater gains to be made by soil inversion treatments, than deep ripping, when it came to overcoming soil repellence.

The soil moisture content on 25 May after soil inversion or rotary spading treatments was 100% higher than the control at 10 centimetres, 50-80% higher at 20cm and 10-40% higher at 30cm.

By comparison, three deep rippers were also tested but did not have a significant benefit on infiltration and soil moisture content.

Growers who had deep ripped strongly water repellent soils may want to implement cropping strategies to aid crop establishment.

Options include avoiding dry seeding and seeding cereals later on these soils, once there has been good rainfall.

Other options include increasing the seeding rate, using a banded soil wetting agent – either applied on the surface of the furrow or banded near the seed – or using other soil amelioration measures to address severely impacted areas.

Research by the Department of Agriculture and Food, Western Australia (DAFWA) has found the use of a rotary spader or mouldboard plough can generate significant benefits to soil quality and subsequent yields on strongly repellent soils.

Trials at Badgingarra and Esperance over the past five years on strongly water repellent soils revealed that inverting the soil through these processes have decreased repellency to zero, which has increased cereal grain yields by 0.4 to 1.2 tonnes per hectare.

Department research on soil water repellent soils will continue this year, including the continuation of three trials at Irwin, Marchagee and York.

These trials will examine the impacts of deep ripping on water repellent soils and whether there is a positive interaction with soil wetters to help ensure good crop establishment.

It was difficult to draw a conclusion from the trials in 2016, as it was a wet year, which produced reasonable establishment and variable results in response to the deep ripping and soil wetters.

At the Irwin site, deep ripping with topsoil slotting increased wheat grain yield by 0.6t/ha, while at a Watheroo site, soil wetters increased yield by 0.1-0.2t/ha but ripping resulted in a small yield reduction.

More information about soil water repellence is available on the Soil water repellence webpage of the DAFWA website.

For more information contact Stephen Davies, Research Officer, Geraldton on +61 (0)8 9956 8515.