The Irony: Australia is the driest vegetated continent, yet 'too much' water underlies our biggest soil degradation problems
A research is showing that for cropping systems to integrate trees profitably, the trees must have extensive lateral roots capable of intercepting drainage below crops. They also should have economic value similar to or greater than that of the crop, to compensate for the area of crop displaced and competition.
Sandplain cropping systems in WA leak water and nutrients which contribute to on-site acidity and off-site salinity, waterlogging, nutrient pollution and eutrophication.
Recent research supported by growers through the GRDC shows that on west-midland sandplain soils there is a 50-50 chance of drainage under a lupin-cereal rotation being 140 mm/year. The research site is near Moora, 150 km north of Perth.
Follow-up work at the University of WA, led by soil scientist Ted Lefroy, has been testing alley farming with wide-row spacing (30 m apart) of the fodder shrub tagasaste, a novel farming system developed by WA sandplain farmers.
Tagasaste can't easily remove water under crops
The results indicate that 25 per cent of the landscape would need to be planted to tagasaste to take up the deep drainage, and only if all trees have access to a watertable.
This is because the trees do not have extensive lateral roots which reach under the crop, and because of the high rate of water movement through sandy sands. Tagasaste has little ability to intercept drainage below the neighbouring crop in winter when rainfall intensity is high and transpiration rates are low.
Effect on yield?
The influence of the trees on crop yield varied from a 20 per cent increase in lupins in 1996, no significant difference for oats in 1997, to a 16 per cent decrease in lupin yield in 1998 when compared to the yield of conventionally grown crops in each year.
Mr Lefroy said the reason for the variance over three years is likely to be an increase in the land occupied by trees from 7 to 20 per cent, and the effect of seasonal conditions.
As a preliminary rule of thumb, increasing the area of trees to 25 per cent could result in a 3 per cent drop in lupin yield and a 22 per cent drop in the yield of the more profitable oats over the whole site. The research team has concluded that the likely drop in cereal yields under these conditions would be unacceptable to growers.
Program 3.4.3 Contact: Mr Edward Lefroy 08 9380 2561