Mopping up groundwater: try deep rooted pastures

A lucerne crop turned a waterlogged paddock (top) to this (above).

There's a staggering annual cost to Australian agriculture through drainage-related soil degradation: conservative estimates hover around $200 million.

Waterlogging and salinity have already led to an average yield reduction of 10 per cent within the Murray-Darling Basin, and currently cost growers anywhere up to $65 per hectare.

In the search for answers, researchers are looking with greater interest at the hydrology under pre-cropping conditions. Native vegetation uses water throughout the year. As cropping areas expanded, native plants were replaced by annual crops, such as wheat, which use water for a much shorter period.

"Under current cropping systems, there are long periods during the year in which water is not taken up by plants and drains into the watertables, which rise as a result," said Mark Peoples of CSIRO Plant Industry.

Dr Peoples and colleague Frank Dunin are analysing data from a suite of research projects, supported by growers through the GRDC and involving the CSIRO and the Victorian and New South Wales Departments of Agriculture.

The projects are investigating how a rotation of perennial pasture can 'mop up' excess water in cropping systems.

Long-season, deep roots

Perennial pastures — especially lucerne — have a much longer period of water-use than annual crops, growing right into summer, and they are vigorous and deep-rooted. This means they are very effective at removing water from the soil.

On red earth soils at Wagga Wagga and Junee, and the grey and red clays at Temora and Morangarell in New South Wales, the CSIRO researchers showed that lucerne roots are capable of growing to a depth of 2-2.5 m in the first year following sowing, whereas annual plants, such as subclovers and crop plants, generally don't grow below the top metre.

The CSIRO trials in New South Wales have shown that lucerne in the first year has the ability to use up to 90 mm of rainwater which escapes from the root zone of an annual crop, while in Western Australia lucerne caused the watertable at a research site to fall by three-quarters of a metre relative to subclover.

Results repeated in NSW, Victoria

Researchers such as Brian Dear, Jim Virgona and Graeme Sandral at the Wagga Agricultural Research Institute, and Anna Ridley from Agriculture Victoria have found very similar results under different environments from those studied by Dr Peoples and Mr Dunin.

Dr Ridley, working at Rutherglen, is assessing lucerne's ability to modify groundwater recharge in soils subject to waterlogging compared to annual pasture or continuous cropping.

Dr Dear's team is comparing nutrient cycling and water use under pastures containing either one of a number of introduced and native perennial grass species, lucerne or subclover, at a number of district sites near Junee and Ardlethan.

Extracting water

At all sites in NSW and Victoria perennials extracted water below 1.5-2 m. Differences in subsoil moisture between perennial pastures and annuals ranged from 40 mm in the dry environment at Ardlethan to 150 mm at Rutherglen.

Reduced sowing rate in drier country

The NSW Agriculture research team is working with very much reduced seeding rates to get the pastures established at Ardlethan.

Instead of the 30-40 perennial plants/m2 usual in eastern pastures around Junee and Wagga, they're recommending 5-6 plants/m2 for this drier country, and a six-week rotational grazing cycle to get the best use out of the pasture.

Brian Dear also admits that timing the return to a crop in a rotation with perennial pastures is likely to be critical.

"These pastures are using water that could be used by the following crops and this could cause problems for crops in a dry year and in dry environments like Ardlethan," he said.

"It's important to spray out the phalaris or lucerne pasture at the right time to give the following crop the best chance.

"All our trial sites return to the cropping phase this year and we will be measuring the impact of removing the pasture phase at different times on moisture recharge and nitrogen availability," he said.

John Passioura, program leader at CSIRO Plant Industry and the vice-chairman of the GRDC's southern panel, noted that the spread of dryland salinity and rising watertables give urgency to the lucerne research in the interests of cropping profitability.

Dr Passioura said a number of new projects are being coordinated under the umbrella of the National Dryland Salinity Program.

Programs 3.5.2., 3.4.2., 3.4.2 Contact: Mr Frank Dunin 08 9333 6683 Dr Mark Peoples 02 6246 5244 Dr Brian Dear 02 6938 1856 Dr Anna Ridley 03 6030 4500