A five-year trial program has given farmers answers on how to nearly double crop yields by addressing soil constraints on sandy soils, such as water repellence,compaction and acidity.
And Rural Solutions SA senior soils consultant Mel Fraser expects the benefits of the spading and organic matter treatments, applied in 2014 to three sites across the state, will be long-term.
The trial is finished now but in many plots, the spaded clay has moved well into the soil profile and pH has become more neutral.
In 2014, sites were established at Cadgee in the South East, Brimpton Lake on the Eyre Peninsula and Karoonda in the Mallee under the state government's New Horizons project.
The aim was to help SA farmers find the most effective methods to overcome non-wetting and compacted sands or heavier soils with poorly-structured subsoils.
It was touted an additional $800 million a year could be generated from overcoming constraints in about 4.1m hectares of SA's farmland.
After two years, this New Horizons project became part of the GRDC Sandy Soils Project.
We doubled yields with no additional fertiliser or other inputs because of the ability of the amended soil to retain and supply water and nutrients to the plants
- Rural Solutions SA senior soils consultant Mel Fraser
Dr Fraser says spading alone produced a lift in yield, but treatments that also included organic matter were generally the highest yielding.
The largest yield differences were seen in dry finishes, where crops had deeper root systems and increased access to stored soil water.
In the fifth and final year of monitoring at Brimpton Lake, the plots which had 500 tonnes/ha of clay applied and 10t/ha of lucerne hay, which was spaded to a depth of 30 centimetres, yielded 4.76t/ha of wheat.
This was a 100 per cent increase over the control plots at 2.35t/ha.
"We doubled yields with no additional fertiliser or other inputs because of the ability of the amended soil to retain and supply water and nutrients to the plants," she said.
Cadgee produced a similar response adding 1.58t/ha above the control yield of 1.56t/ha for the same treatment.
"The only year we didn't get a response was 2016 when it was a wet year, long-term we are looking at about 40pc yield increase through spading alone," she said.
The very dry season at Karoonda impacted grain yields on the whole site in 2018, but Dr Fraser said there was still 60-100pc increases in biomass in spring.
Research will continue at Cadgee for another three years looking at fodder production.
Dr Fraser says the three sites have raised the awareness among farmers of the benefits of deep ripping and spading to overcome compaction, but in many cases there were other constraints such as acidity and water repellance that also needed to be addressed.
More research is being undertaken on different rates of organic matter with the economics of applying 10t of lucerne hay - the rate used at the PIRSA sites - more the gold standard than a cost-effective option.
In the first year of a trial at Murlong, yields jumped from 0.5t/ha to 3t/ha in areas where 5t/ha of lucerne hay had been spaded in.
"We think if we can keep the addition of organic matter under $500/ha, it can have a relatively quick pay back," she said.
This sandy soils research will continue until 2020, with 18 new replicated trials across the southern region through the Sandy Soils Impact Program, including 12 in SA, which will focus on targeting the identified constraints at each site.
Soil sampling to show change
More than 900 soil samples were taken at three New Horizon sites earlier this year to quantify the long-term changes in the soil after organic matter and spading treatments.
The water-holding capacity and organic carbon levels of the soil will be analysed, as well as chemical, biological and physical characteristics such as bulk density.
Rural Solutions SA's Mel Fraser expects the sampling will show the yield increases are related to increased soil biological activity and nitrogen supply.
"What we will hopefully see is an increase in soil carbon and general fertility with the incorporation of clay and nutrients," she said. "These treatments produced so much more biomass above the ground and root mass underneath as well, which in turn feeds the biological cycle and nutrient supplies."
