Factor in soil tests, gypsum

Comparison between sites where groundwater has been used for 5 years and for more than 20 years
Left: Groundwater has been used at this site for more than 20 years but an active soil monitoring program and timed gypsum applications have ensured there has been no decline in productivity.
Right: Continuous groundwater has been used for five years at this site but no gypsum has been applied and the soil has degraded to the point where production is minimal.

Water quality is as important as quantity for long-term farm viability when irrigating crops and pastures with groundwater.

Most irrigators understand the importance of knowing the salinity of their groundwater but, in a survey of Murray Valley groundwater users, only 41 per cent of respondents knew the sodicity of their groundwater.

High sodium levels affect soil behaviour by increasing dispersability and this makes cultivation and good seed beds more difficult to attain, as well as reducing water entry and decreasing the amount of water in the soil profile that is available to plants.

Research conducted by Dr Iain Hume at Deniliquin in the 1990s showed that even a short period of two to five years of irrigation with saline/sodic deep groundwater led to soil structural decline.

Complementary research by Dr Peter Slavich showed that accumulated salts leached from soils previously irrigated with groundwater when rice was grown on the site using fresh channel water, but this did not leach accumulated sodium.

In order to leach sodium from the root zone and reclaim sodic soil, it is necessary to apply gypsum.

The application rate is usually based on the exchangeable sodium percentage over the root-zone depth but this can result in uneconomically high rates of gypsum being recommended.

By knowing the levels of sodium at which structure is affected and applying the right amount of gypsum at the right time, it is possible to economically maintain soil structure when irrigating with groundwater.

This requires regular soil testing but the expense of these tests can be a disincentive for many farmers.

A collaborative project currently being conducted by Dr Pichu Rengasamy of Adelaide University and Sam North and Lindsay Evans of NSW Department of Primary Industries at Deniliquin has been developing guidelines for the management of sodic soils and groundwater in the Murray irrigation districts of southern NSW.

In particular, the project will determine threshold sodicity levels and minimum gypsum application rates for local soils and develop cheaper and easier methods of testing soils that can be adopted by farmers, to help them better manage their groundwater use.

Physical and chemical tests are being conducted at 46 sites on nine properties, covering a range of district soil types and the two major land uses: rice cropping and dairying.

A number of the project participants have observed that their soils have become more difficult to manage; being untrafficable and “sticky” when wet and then drying quickly to a hardness that leaves cultivating implements “scratching across the top”.

Two of the study sites exemplify the difference that proper management can make when irrigating with groundwater.

Perennial pasture is grown on a similar soil type at both these sites and the quality of the groundwater is also similar.

The key message from the owner at this site is that groundwater irrigation can be sustainable, but soil testing and gypsum applications need to be factored into the cost of pumping groundwater when doing farm budgets.