Delving into water repellence

05 Apr, 2017 02:00 AM

Farm Weekly


STRATEGIC deep tillage was chosen as a tool to ameliorate soil water repellence and subsoil constraints at a trial established by the Mingenew Irwin Group (MIG).

MIG research and development manager Debbie Gillam said severe water repellent soils in the region were typically low in water holding capacity and fertility and as a result had limited productivity.

"Low productivity means that the amelioration approach needs to be as cost effective as possible," Ms Gillam said.

The trial, which was funded through GRDC's water repellence project, looked at the use of the cost effective one-way disc ploughing.

The plough is a robust tool that is simple to modify for partial soil inversion.

Ms Gillam said the research site compared a one-way disc plough to rotary spading (a proven amelioration option) and to other deep ripping approaches, including some of the newer very deep rippers.

"One of the two key messages to come from the research includes the difficulty of achieving productivity benefits if other constraints such as weeds and compaction remain unaddressed," she said.

"The other is that rotary spading and modified one-way ploughing improved water infiltration, the evenness of soil wetting, weed control and crop yield."

Compaction was identified as an issue at the Mingenew trial site.

Subsoil compaction was severe (2.5MPa) from 22 centimetres and extreme (3.5MPa) from 30cm.

Of the rippers used, the Tilco deep ripper effectively loosened the soil to 54cm, the Terraland ripper to 48cm, the Ausplow ripper to 38cm, the one-way plough to 36cm and the spader to 30cm.

In the untreated control moisture infiltration followed the typical pattern for repellent soils with preferential flow paths and large areas of "dry patch" indicating bypass flow.

Water infiltration was significantly improved by rotary spading and one-way ploughing.

Deep ripping treatments did not improve soil water infiltration and soil water content.

DAFWA research officer Steve Davies helped with the trial and said for many deep sands, deep ripping with topsoil slotting could be one of the most economical ways of overcoming subsoil compaction and acidity (to depths of 40-60cm).

"However, on severely repellent sands it could be that deep ripping is inadequate and may require options that overcome repellence and assist with weed control while at the same time addressing subsoil compaction and acidity," he said.

"Addressing water repellence without also addressing the other subsoil constraints will limit the likely productivity benefits and must be addressed."

Trial work is on-going and MIG will release further findings in 2017.

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