The following articles are drawn from papers presented by Dr Anna Ridley and Dr Chris Smith at the workshop. The papers were overview papers that summarised work by a large number of scientists present.
THE WORKSHOP on Managing Water to Improve Productivity and Control Dryland Salinity summed up the 'state of our knowledge' of plant-based salinity solutions.
- Regardless of pasture or crop type, the amount of water draining past roots (deep drainage) is higher for wetter, cooler climates than for sites where May-September rainfall was lower and potential for evaporation was high. Deep drainage can range from less than 5 mm/year under native vegetation, to 30±20 mm/year for summer rainfall areas and to 40±30 mm/ year for winter rainfall areas.
- Deep-rooted perennials use more water than annuals and dry the soil in proportion to their rooting depth. As a result, deep drainage during really wet periods occurs less often under perennials. Other factors affecting plant growth such as climate and soil are secondary. This applies to trees, lucerne or any other deep-rooted perennial.
- The timing of summer cropping is as important as increasing cropping frequency. Reducing deep drainage requires both reducing the length of the fallow and marching crop water-use to periods of greatest rainfall. In general, it appears that production systems based entirely on winter-active annual species may not be environmentally acceptable.
- It is clear that a greater area of perennials is needed if farming is to become sustainable — some growers are aiming to cover 30-40 per cent of their properties with a combination of 'fixed' (native vegetation) and 'rotating' (e.g. lucerne) perennials.
- Perennials (particularly lucerne) can change the soil — they can increase cracking, they can extract so much water (called 'overdrying', and lucerne has been found to be the main culprit) that the soil has trouble wetting up again, and they can create new macropores along old root channels. Some changes could add to deep drainage if water bypasses the roots to go directly to the watertable.
- Other changes, particularly drying of the topsoil, significantly reduces deep drainage, as roots of subsequent crops have been known to follow existing macropores to tap into the water in them, as occurs in deep-ripped soils. This was one area flagged as needing more research.
- Overdry soils can cause crop yield reductions in a dry year, particularly for canola. An increase in crop yield is likely in wet years after perennials. Strategies such as early removal of lucerne can help manage the risks to yield, as can developments in flexible cropping rotations.
- The yield penalties in dry years with intercropping (direct-drilling a crop into a dormant perennial species) suggests that this practice may have promise for combating rising watertables in waterlogged environments or highly reliable winter rainfall environments.
- Economic models suggest that, in general, for the majority of land at risk of salinisation, there are no profitable perennial plant options currently available that are as profitable as annual cereals and that could be implemented on a scale sufficient to address catchment level problems. There is a need to identify the profitability of new perennial farming systems in a range of climates and landscapes.