Tips from natural systems: how acacias open up soils by Guy Cotsell

Down and dirty for science: (left to right) Ben Gready, Cas'sie Schefe and Isa Yunusa, all of Agriculture Victoria, Rutherglen, measure water infiltration in a soil pit. Acacia roots form large pores that conduct water

Acacias could boost crop yields by improving soil structure while minimising waterlogging, but are not necessarily the best plants for cropping systems.

These are some conclusions from Agriculture Victoria research that took advantage of the removal of a six-year-old shelter belt of acacias (A. dealbata and A. melanoxylori) at a Rutherglen research farm. This provided the opportunity to study the effect of acacias on soil and following crops.

Team leader Isa Yunusa believes that positive results from this work may prompt future large area sowings of what is being called 'primer plants'. That is, they prime the soil for following crops.

How the natives do it

The acacias/wattles are deep-rooted and thrive on the inhospitable acidic, sodic soil in the area. As their roots die and decay, they form pores, which act as channels conducting and storing water, and so overcome waterlogging.

Only nine months after the wattles were removed, the number of large pores in the subsoil had increased by 13 per cent compared to the cropping soil. Less than a year later, the difference between the two soils had increased to 55 per cent, and some roots were still decaying.

Recharge eliminated... too much so

Studies predicted that the potential for groundwater recharge was eliminated under wheat on the soil 'primed' by the wattle, when in adjacent cropping belts up to 24 per cent of annual rainfall drained beyond the root zone.

However, the downside is that the wattles also dry out the soil. Lack of soil water and heavy weed infestation in the wattle belt reduced the grain yield from a following oat crop by 38 per cent.

Still, the acacias produced other benefits — a 200 per cent increase in mineral nitrogen from fixation of atmospheric N, and a 40 per cent increase in total carbon due to the absence of tillage while the wattles were growing.

"This study demonstrates the feasibility of the primer plant concept in ameliorating subsoil constraints in high-rainfall environments," Dr Yunusa said. "It does not, however, mean that acacia or any similar shrub is the most appropriate for the cropping industry."

The next task for Dr Yunusa and his team is to find other plants that can do a similar job to acacia, and to develop guidelines to manage them within the cropping system. They will be on the watch for plants that improve the soil over a wider area in a shorter time, and which are easily manageable in terms of weed control and simple elimination.

Program 3.4.2 Contact: Dr Isa Yunusa 02 6030 4500