More and more, farmers, advisers and researchers are looking below the topsoil as they find productivity limited by subsoil constraints to a much greater degree than previously appreciated.
Subsoil constraints include soil salinity and acidity, soil compaction, sodicity and various toxicities including boron.
Says the chairman of the southern panel of the Grains Research and Development Corporation, Ian MacKinnon: "we consider hostile subsoils to be a major impediment to plants' ability to access moisture and nutrients and, hence, to yield to their potential".
"The water and nutrients that plant roots fail to use from such subsoils lead to leakage to groundwater, so increasing the risk of dryland salinity developing."
There is already significant investment this field by growers and the Federal Government through the GRDC. A recent GRDC-convened workshop helped focus more urgency on the problems of subsoils.
Following are three updates on subsoil problems and current research.
$$ boost for soils
The yield-depleting effects of 'hostile' subsoils are being tackled as part of the grains industry research effort in SA, Victoria, southern NSW and Tasmania.
The chairman of the southern panel of the Grains Research and Development Corporation, Ian MacKinnon, said these problem soils were widespread and highly variable.
"There are sodic hard-setting soils throughout the region; soil acidity at depth is becoming more widespread as is salinity; and boron and other toxic elements are also cutting yields."
New research investments include:
- $109,500 for CSIRO research aimed at identifying and overcoming. soil constraints in Mallee soils
- $102,332 for NSW Agriculture to develop more efficient rotations in acid soils
- $82,874 for the University of Adelaide to breed wheat varieties more tolerant to a range of toxicities
- $41,000 for Agriculture Victoria to research the impact of hostile subsoils on beneficial microorganisms.
Contact: Mr Ian MacKinnon 03 6391 5558
Leaking water, missed opportunity
Buronga (NSW) farmer Gary Doyle chairs the Mallee Sustainable Farming Project, a GRDC-backed initiative aimed at boosting sustainable production in the Mallee country of NSW, Victoria and SA. (See also our feature story p18.)
Mr Doyle says the traditional wheat-fallow rotation in the Mallee increases the risk of groundwater recharge leading to salinity, but it also represents a missed opportunity for farmers.
"When water passes through the root zone of cereal crops, not only do we lose valuable moisture but we lose expensive nutrients as well," he says.
Lucerne effective in low rainfall, light soils
"There is a cure. Lucerne in the low-rainfall Mallee region has been seen to send roots to 6 metres, effectively drying out the soil profile, unlike many high-rainfall areas where trees are possibly the only form of control.
"Crops such as canola, vetch and other legumes that can access moisture below the root zone of cereals will also allow us to turn lost water and nutrients into yield."
Mr Doyle sees soil-moisture monitoring as a key to deciding whether lucerne and other deep-rooted perennials can be included in a rotation. In a National Heritage Trust-funded program, 60 bores have been established on 30 Mallee farms in NSW with the program being extended to Victoria this year.
"The bores are lined with PVP pipe and a neutron probe is used to monitor moisture in the soil profile," he said. "The information from these bores allows us to manage our rotations to combat recharge."
Contact: Mr Gary Doyle or Ms Marion Murphy 03 5021 9413
Sodic subsoils: live with them, or try agro-forest
A widespread subsoil constraint is sodicity. About one-third of Australia's land mass is occupied by sodic soils which have exchangeable sodium on their clay surfaces. Their inherent instability is compounded when they are cultivated, overgrazed and then exposed to rain.
Sodium ions build up in sodic soils making it difficult for roots to extract water from the soil.
In Victoria's southern Mallee and Wimmera, Roger Armstrong and his Agriculture Victoria colleagues are identifying subsoil limitations to high water-use efficiency in cereals and also determining whether these constraints are affecting nitrogen nutrition of cereals.
Looking at cropping soils in western Victoria, Dr Armstrong said growers have long recognised the potential impact of sodicity in the surface soil. However, exchangeable sodium percentage levels increase significantly with depth. "And there are only very limited options for overcoming subsoil sodicity.
While gypsum applications can ameliorate sodicity in the surface soil, "you need huge amounts to treat subsoil sodicity; at about $23/t applied, plus the fact that it takes a long time to work through to the subsoil, gypsum becomes a very costly treatment option for subsoils".
Organic matter promising but limited
"Promising results have been obtained recently with the use of high rates of organic matter such as pig bedding litter, but transport costs and lack of availability mean there never will be enough pig litter to treat all sodic soils in the region.
"Large soil pores can improve structure in sodic soils but my feeling is that no annual crop will do this. Even traditional perennials such as lucerne have problems sending roots into highly sodic and saline subsoils which are widespread in north-western Victoria."
He said agro-forestry linked to greenhouse credits might be a future option.
"Meanwhile perhaps we have to accept that sodic subsoils are there, that it is difficult to change them, and that we should concentrate on getting the management absolutely right in the topsoils if we want to maximise profitability."
Contact: Dr Roger Armstrong 03 5362 2111