A few years ago, a Victorian trial that looked into whether subsoil manuring could improve hostile subsoils and crop productivity showed increases in crop yield of as much as 70 per cent.
These results have been motivation for further work says NSW Department of Primary Industries research scientist soil and water unit, Dr Ehsan Tavakkoli.
Dr Tavakkoli, Wagga Wagga, who was also a guest speaker for the Grains Research and Development Corporation at last year's Henty Machinery Field Days, said adding manure to the subsoil, the Victorian trial showed it could transform the poorly structured soil into a much better environment to grow a crop.
"So the message here is that it is possible, but what is the best and most cost effective way of doing it?" said Dr Tavakkoli.
Dr Tavakkoli said poor structure in a lot of southern subsoils was due, in part, to sodicity, high pH and low organic matter. Soils which have stable aggregates are more productive than soils that have unstable aggregates. Stable aggregates allow movement of air and water to plant roots, can usually store more water, and retain their shape when subjected to rainfall and traffic. Soils that lose aggregate stability through slaking or dispersion have reduced productivity.
The relative proportions of major cations associated with the clay minerals affects the stability and structure of the soil, if clay is a major contributor to soil texture and structure.
"What happens is you get too much sodium, and sodium, which is a much smaller ion compared to calcium can not hold those aggregates together, and when rain comes into contact with the clay, it disperses," Dr Tavakkoli said.
This dispersion limits water infiltration, root penetration and contributes to waterlogging and salinity, as well as certain toxicities due to extreme pH.
Along with the abundance in sodium, he said as agriculture had intensified, our soils had also lost carbon.
"If we lose carbon over a decade by one per cent, you are losing a lot of binding agent that keeps those microaggregates together.
"So we were thinking, can we actually come back to this soil that has been degraded over the past couple hundred years in agriculture and put some innovative amendments in to build up that poorly structured subsoil?"
Techniques used across these trials include the application of organic and inorganic amendments, as well as combinations of each.
Organic amendments include manure, or a cheaper option would be biomass left after harvest (such as pelletised stubble placed into the subsoil), he said.
In the trial this was done using pelletised pea hay.
The inorganic amendments included gypsum, but the trial also looked at the use of inorganic nutrients, not only to separate the nutritional and structural improvements but also to test the value of deep placed nutrients to improve productivity, Dr Tavakkoli said.
The trial also included a couple of combination options, one of which was the pelletised pea hay, plus gypsum, plus NPK all in one pellet into the subsoil.
Dr Tavakkoli said the NSW DPI designed and made a ripper that allowed the placement of amendments as deep as 40 centimetres.
What the trials have so far found, Dr Tavakkoli said, was some of the treatments on their own did not achieve much in yield gains.
"Some of those amendments that we added into the soil, including surface application of gypsum, manure or pea hay (by themselves), or even deep application of nutrient, didn't do anything better than the control," Dr Tavakkoli said.
However, some of the treatments that performed well were the combinations of organic and inorganic, so the crop excelled with the mix of the pea hay, gypsum and NPK.
"We had a 53 per cent increase in yield compared to the control last year (2018). It was a dry year. From about 1.5 tonnes a hectare (in the previous year of the trials) we achieved about 2.4t/ha (in 2019)," he said.
Deep pea hay with gypsum also performed well and the top performing treatments appeared to show a consistent response across multiple seasons.
"So what we achieved in the first year, it's also showing a massive response in the second year and the canola crop this year is also showing similar results," he said.
In comparing the top performing treatments, deep pea hay with gypsum; or deep pea hay with gypsum and NPK, gave a consistent increase relative to the control of about 30pc in yield.
It was also important the crops were able to extract and use more water and could convert this to increased yield.
Dr Tavakkoli said if the crop root was more active it would use more water, which would result in more biomass and therefore more grain production.
"So there wasn't a lot of difference between control and nutrients (NPK) alone in that subsoil. (But) as we put in some of these organic and inorganic amendments (in combination) we actually increase water use by 10-15pc across the profile at an important time of the year - flowering," he said.
He also said the crop's ability to access extra water meant it was better able to tolerate heat or frost.
The 2019 canola crop water use from sowing to September 13, showed the deep pea hay, plus gypsum, plus NPK treatment was taking 80pc more water from the subsoil compared to the control.
Dr Tavakkoli further explained the mechanism by which gypsum and organic amendments contributed towards amelioration of subsoil.
"When we added gypsum into that subsoil, the pH at 9 comes back to about 8.4, and this reduction was significantly reducing carbonate and bicarbonate toxicity as well as improving soil dispersion," he said. On the other hand, the application of organic amendment alone didn't affect soil pH or dispersion, however, the breakdown process of organic amendments will result in improving soil aggregation.
"What I'm trying to get here is, if you combine these (organic amendments and gypsum), then we can start getting large improvements in soil structure due to additive and simultaneous amelioration effects on biological and chemical processes."
This project is a multi-party agreement between Victoria Agriculture, GRDC, NSW DPI, South Australia Research and Development Institute and Tasmania DPI.