Managing Sodic Subsoils
WESTERN FARMING Systems trials on grey and brown cracking clay soils in northern NSW have shown the extent of yield losses that result from constraints on plant available water associated with sodic and saline subsoils.
According to Research Agronomist Shane Norrish of the University of Western Sydney, the trials aimed to investigate how plant water availability affected yield when the crops were sown at different dates and plant populations. The trials also compared the effect of varietal characteristics.
"The greatest factor affecting yield was. in fact, the amount of plant-available water in the subsoil," says Mr Norrish. "This can be seen in the strong influence of soil type and sowing date on the yields at all the sites in the trials."
For example. at grower Phil Harris' property 'Marlow', 60 km north-west of Wee Waa, an average of 3.8 tlha was harvested off non-sadic soils with a plant-available water content of 220- 230 mm. A relatively low 2.1 t/ha was harvested off sodic soils, which had only 90 mm of plant-available water (see table for yields).
"The thing to remember is that these soils are not always sodic or saline at the surface," says Mr Norrish.
The problem soils also don't appear to be too dry to growers using a push probe to test soil moisture. The problem is that the amount of sodium and other salts in the subsoil limits root growth and the plants' ability to extract water below depths as shallow as 80 cm.
The associated moisture stress leaves crops more susceptible to crown rot, with the result that growers need to pay attention to potential crown rot carry-over in susceptible areas.
"After talking to growers it became clear that they all knew about the problem areas in their paddocks, they just wanted to understand the problem in more detail and how to better manage it." he says.
Grower: some severely sodic soils. but transitions OK
Mr Harris agrees, saying that on his 2,000- hectare property, at least 100 ha would be severely sodic in the subsoil and they don't lend themselves to being taken out of production.
"Like everyone else around here I have always treated the worst areas differently, but we can actually grow a decent crop in the transitional areas between the good and bad soils," says Mr Harris.
Yields were also strongly affccted by differences between wheat varieties (see table). This is poss ibly because lower tiller production may conserve soil water for grain-filling when water becomes limiting.
"On this basis we suspect that the wheat variety H45 showed the greatest yield in the trials because it tended to have lower average tillers per plant than Sunvale and Sunco," says Mr Norri sh. Although, he points out that, with that one year of data, this type of conclusion is pure speculation.
The clearest result is, however, that season can also override eve rything.
" In a good year we can grow an average crop on the worst soils," says Mr Harris. "In a dry year like this one, we won't even sow some areas. After these trial results we certainly won't be adding extra inputs like nitrogen in future."
Both Mr Norrish and Mr Harris agree that there is currently not much that they can try on the worst areas with the most sodic subsoils, so the best they can do is keep costs, and potential losses, down to a minimum.
|Sowing date||18 May||7 June||2 July|
|Soil type brown||Grey-sodic||Grey-brown brown||Grey-sodic||Grey-brown brown||Grey-sodic||Grey-brown|
Contact: Mr Shane Norrish 02 45701923 email firstname.lastname@example.org