Digging in to make sodic clay soils better

Shannon Beattie
By Shannon Beattie
December 20 2021 - 10:00pm
The DPIRD sodic soils research team of Wayne Parker (left), Glen Riethmuller, David Hall, Rushna Munir and Ed Barrett-Lennard.

AFTER several years of low rainfall and low yields, many Western Australian growers with sodic clay soils are celebrating a bumper harvest.

Department of Primary Industries and Regional Development (DPIRD) senior research scientist David Hall said the 2021 growing season, with above average rainfall, had shown what some of WA's more challenged soils, such as sodic clays, could produce.

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"For the past few years where rainfall has been low these soils have yielded poorly," Mr Hall said.

"Yet in a high rainfall season such as this year, they can be the most productive soil type on your farm."

Mr Hall and the rest of his team at DPIRD are researching ways to make these soils more productive across all seasons.

"Sodic clays are often dispersive which means that the clay separates from the sand matrix and when carried with water fill and block pores within the soil," he said.

"This causes poor water infiltration, increases in soil density and strength and a buildup of salts accumulated from rainfall due to poor drainage, called transient salinity.

"We also find high levels of toxic levels of sodium, chloride and boron at depth."

According to Mr Hall, the key to increasing the productivity of sodic clays was to increase the proportion of rainfall infiltrating into the soil profile.

While that may sound simple, finding solutions to that problem has challenged farmers and soil scientists for decades.

A new DPIRD project, funded by the Grains Research and Development Corporation, is developing and testing to identify how growers can get more rainfall into the soil and store it so that it can be used by crops.

Gypsum is the most widely used amendment in sodic soils - it reduces dispersion and improves water infiltration in many sodic soils.

Research at Merredin has shown benefits by combining gypsum with water harvesting mounds.

"The mounds, when made impervious to rainfall, using plastic sheeting or polymers, shed water into the furrows," Mr Hall said.

"When combined with in-furrow gypsum applications, barley yield increases of more than 0.5 tonnes per hectare.

"Further research is continuing to see how low the rates of gypsum need to be to result in yield improvements and how effective spray on polymers are in terms of water harvesting."

Reducing water lost from evaporation can also be achieved through mulches and farmers have noticed crops grow better on sodic clays where chaff is concentrated.

However, mineral mulches can do the same thing.

Trials are underway at Ravensthorpe where gravel or sand has been applied to sodic clay soils.

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"The results so far have shown that just two centimetres of gravel (400t/ha) can reduce the rate of evaporation from these soils five fold," Mr Hall said.

"Where the soils have not been waterlogged, crop yields have increased by more than 1t/ha in a dry season.

"However, it should be noted that mulches will exacerbate waterlogging in wet years."

Deep tillage in sodic clays is rarely recommended as it often brings up large clods that are hard to break down and may also contain toxic properties, such as salty or boron subsoils.

However new research at Geraldton is using slant legged ploughs which uncompacts the soil without digging up any of those hostile subsoils

"The paraplow opens and shatters the soil without delving any of that material to the surface," Mr Hall said.

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"So far we haven't seen too many significant yield increases from that, but the research is still in its early stages."

All of the research being conducted is novel and there are aspects which are blue sky and will require further development before adoption.

However, unless the boundaries in understanding the processes that restrict crop growth on these soils are pushed and solutions based on good science developed, then growers will never see their sodic clay solid reach yield potentials in all years - wet and dry.

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