Soil limitations put to the test

Soil limitations put to the test


Cropping
University of QLD school of agriculture and food sciences' Dr Kathryn Page, was the lead author of a report indicating soil sodicity, acidity and salinity throughout QLD and NSW grain regions, cost cropping about $440m annually.

University of QLD school of agriculture and food sciences' Dr Kathryn Page, was the lead author of a report indicating soil sodicity, acidity and salinity throughout QLD and NSW grain regions, cost cropping about $440m annually.

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While research into soil sodicity, acidity and salinity has identified some solutions to these problems, further research is required to improve yield and profit.

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Soil sodicity, acidity and salinity throughout Queensland and NSW grain regions are estimated to adversely impact on cropping by about $440 million annually.

While research has identified some solutions to these problems, further research is required to improve yield and profit.

For example, research 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 identification of crop species or cultivars that yield best on constrained sites is required, especially where multiple constraints occur together.

These are among conclusions from a study titled ‘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 and which the Grains Research and Development Corporation helped fund. 

Author Dr Kathryn Page, from the University of Queensland’s school of agriculture and food sciences, together with other leading scientists, reviewed past and present research.

Many vertosols are characterised by surface soil sodicity, often combined with subsoil sodicity, or can be saline and either highly alkaline or acidic (subsoil acidity often occurs in soils dominated by brigalow and belah).

Subsoil constraints are particularly challenging because of difficulties and expense in applying ameliorants to them. Also subsoil constraints tend to occur variably across landscapes.

This wheat trial assessed some of the traits plant breeders have identified to improve tolerance to some soil constraints, such as acidity and subsoil salinity.

This wheat trial assessed some of the traits plant breeders have identified to improve tolerance to some soil constraints, such as acidity and subsoil salinity.

Sodicity, defined as elevated levels of sodium (Na) on soil exchange sites, often above 6 per cent, leads to soil dispersion and collapsing soil structure, and occurs in the top and subsoil at various or multiple layers.

Salinity, which refers to excess dissolved salts in soil, can occur at various layers and has negative impacts on plant water use. Chloride toxicity is increasingly recognised as having a major impact on crop yield.

Selecting plants more tolerant to soil constraints is a key research aspect. 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 chickpea. For acidic sites, acid tolerant species such as or aluminium tolerant cultivars of cereals, can improve yields.

Deep-rooted perennials, such as grasses, on compacted or dense sodic subsoil can loosen soil layers and lead to improved water movement and crop growth. This can increase gypsum efficacy by increasing leaching, as well as soil Na removal. Perennials adding to organic matter can also be helpful to sodic topsoil.

Gypsum to treat sodicity is well recognised. Cultivation or deep ripping to break compacted sodic layers and surface seals can be a partial remedy, but commonly without added ameliorants, soils quickly revert to previous conditions. Adding gypsum to subsoil is difficult. Slotting (mix of ameliorants in 15cm parallel bands) is an option. Adding organic matter is also under investigation.

Not all sodic soils respond to gypsum. More accurate soil test guidelines are required for applying ameliorates like gypsum, for example in soils with high exchangeable magnesium.

Adding acidic/acid generating products like sulfuric acid, can lower pH and dissolve Ca carbonate in sodic soils with calcium carbonate nodules. This helps decrease harmful alkalinity levels and releases Ca ions to improve soil structure. 

Further identification of crop species or cultivars that yield best on constrained sites is required, especially where multiple constraints occur together. - Bob Freebairn

Increasing organic matter by amendments like ‘green manuring’ (plants ploughed into the soil), or ‘brown manuring’ (plants sprayed out) require further study.  

Increasing organic matter leads to increases in soil binding agents. This in turn can lead to increased stability, which offsets many of the symptoms of sodic soils.

Some studies reveal 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 to promote water movement into the profile, such as zero-till with residue retention, can increase leaching rates and help move salts out of root zones.

Zero-till with residue retention can also benefit sodic sites because of greater surface organic matter and aggregate stability.

Next week: Tropical grass plus light soil; drought rescuer.

  • Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or call 0428 752 149.
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