Soil sodicity, acidity and salinity throughout Queensland and NSW grains regions are estimated to adversely impact on cropping by about $440 million annually. While research has identified some solutions to these problems, further research continues to be required to improve yield and profit for many of these problem situations.
For example, research is needed to improve knowledge of how to manage these issues when they occur in the subsoil, particularly in vertosols (high clay content minerals that shrink and swell) and how best to use ameliorants to treat multiple constraints concurrently. Further identication of crop species or cultivars that yield best on constrained sites is required, especially where multiple constraints occur together.
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These are among conclusions from a study "Management of the major chemical soil constraints affecting yields in the grain growing region of Queensland and NSW, a review" published in CSIRO journal Soil Research (volume 56(8), 2018). Author Kathryn Page, of the University of Queensland's School of Agriculture and Food Sciences, along with other leading scientists, reviewed past and present research for the analysis as part of a GRDC-funded study (UQ00084).
Many vertosols are characterised by surface soil sodicity, often combined with sodicity in the subsoil. Subsoils can also be saline, and either highly alkaline or acidic acidity often occurs in subsoils dominated by Brigalow and Belah. Subsoil constraints are particularly challenging because of difculties and expense in applying ameliorants to them. Also subsoil constraints tend to occur variably across landscapes.
Sodicity, defined as elevated levels of sodium on soil exchange sites, often above six per cent, leads to soil dispersion or a collapsed soil structure when wet, and can occur in the top and subsoil at various or multiple layers. Salinity is an excess of dissolved salts in the soil, which has a negative impact on plant water uptake, and can also occur at various layers. Sub-soil salinity is increasingly recognised as being a major constraint on crop yield.
Selecting varieties and plants more tolerant to soil constraints is a key research objective. For example, species able to tolerate higher chloride levels such as barley, canola and bread wheat perform better than sensitive species such as durum or chickpeas. For acidic sites acid tolerant species, such as or aluminium tolerant cultivars of cereals, can improve yields.
Deep-rooted perennials (for example, grasses) on compacted or dense sodic subsoils can loosen soil layers and lead to improved water availability and growth of sensitive crops grown in rotation. This can increase gypsum efficacy by increasing leaching, as well as soil sodium removal. Perennials adding organic matter to problem sodic topsoils can also be helpful.
Applying gypsum to treat sodicity is a well-recognised management practice. Cultivation or deep ripping to break compacted sodic layers and surface seals can be a partial remedy, but without added ameliorants sodic soils quickly revert to previous conditions in most cases. Adding gypsum to subsoils is logistically difficult. However, slotting ameliorants in narrow bands is being examined in field experiments. Adding organic matter to subsoils is costly, but benefits may be significant, and this is also under investigation.
Not all soils diagnosed as sodic respond to gypsum applications. More accurate soil test guidelines (e.g. percentage of sodium) are required, for example in soils with high exchangeable magnesium. Adding acidic/acid generating products like elemental sulphur, which produced sulfuric acid, can lower pH and dissolve calcium carbonate in sodic soils with calcium carbonate nodules. However, large amounts are required to decrease harmful alkalinity levels and release calcium ions to help improve soil structure. The use of these substances to treat sodicity is largely untested in Australia.
The value of organic matter
Increasing organic matter with amendments like manure, 'green manuring' (plants ploughed into the soil), or 'brown manuring' (plants sprayed out) require further study. Adding organic matter leads to increases in soil binding agents, improved soil structure, and this in turn can lead to increased stability, which offsets many of the symptoms of sodic soils.
Some studies have also shown that when organic material and gypsum are added together, gypsum solubility can increase and improve the effectiveness of gypsum application.
Where soils are highly saline, management practices that promote water movement into the prole, such as no-till with residue retention, also can increase leaching rates and the movement salts out of rooting zones. Zero-till with residue retention can also benet sodic sites because reduced soil disturbance and greater surface organic matter encourages soil aggregate stability.
As an outcome of the report GRDC have established a significant investment in "The economics of ameliorating soil constraints in the Northern Region" starting in 2018. The five-year, $5 million research investment is working with Queensland and NSW farmers in collaboration with the University of Southern Queensland, the University of Queensland, the University of New England and the Queensland Department of Agriculture and Fisheries.
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