Farm Table says:
Potential impacts of subsoil constraints on canola productivity in southern NSW
What is the problem?
This study aim was to investigate the impact of subsurface compaction, acidity, and/or subsoil sodicity or salinity on canola yields in replicated on-farm experiments undertaken between 2007 and 2009.
What did the research involve?
At the commencement of the project, farmers’ paddocks in areas of southern NSW which either had been identified as experiencing under-performing canola crops by previous studies.
Information presented here is derived from on-farm studies undertaken between 2007- 2009 on three acid/compacted sites (Greenethorpe, Milvale, and Culcairn), three sodic/compacted sites (Lockhart, Rand, and Corowa), and a site with a variable saline subsoil.
The trials represented a mixture of commercial size replicated strips that were sown using farmer equipment, and smaller scale ‘white-peg’ studies.
What were the key findings?
The effects of compaction, acidity, and sodicity were less pronounced than salinity.
Each of these potential soil constraints will be examined in more detail individually in the following sections.
A canola paddock survey conducted across soil types in southern NSW in 2004/05 indicated that 37 out of 39 paddocks had compacted subsurface layers (soil strength 2MPa or greater), with >60% of the canola crops examined south of Wagga Wagga exhibiting severe root distortion.
On the red and red-brown earth of southern NSW with low buffering capacities, farming practices have resulted in surface pH levels ranging from around pHCa of 6.0 to < 4.5 in the absence of lime.
Soils are generally considered to be sodic if the ESP is >6% in the topsoil or >15% in the subsoil. Sodic soils tend to disperse when wet and result in a hard, dense structure when dry. Applying gypsum can improve the structure of sodic soils by preventing the soils from dispersing.
A soil is generally considered saline if ECE is >2 dS/m. Apart from the direct toxic effects of some salts such as sodium and chloride on the plant, subsoil salinity can decrease plant growth and yield by reducing the capacity of plant roots to extract water from the soil as a result of the osmotic effects of the salts.
Where subsoil salinity is suspected, EM surveys combined with ground-truthing should be used to identify paddocks where canola may not be suitable. A more salt-tolerant species such as barley may be a more appropriate crop choice in such circumstances.