The Practicalities of Using Recycled Water for Horticulture
The Practicalities of Using Recycled Water for Horticulture 25 August 2006 Recent research commissioned by the National Program for Sustainable Irrigation (NPSI) has been developing the processes and tools that growers need to safely use recycled water, and ensure that fruit and vegetables reaching the market will be safe to eat.
For growers, the main advantage of incorporating recycled water into their irrigation system is gaining an increased supply of high security water. The additional work associated with managing recycled water mostly relates to monitoring and adjusting irrigation and nutrient inputs.
These adjustments are based on the water quality, soil solute levels and nutrient requirements of the crop. To assist growers in planning for the use of recycled water on their farms, and to accurately manage the irrigation of their crops, the research team, led by staff from the Department of Primary Industries Victoria, have produced a 22 page booklet called "Using Recycled Water in Horticulture: A Growers Guide". The booklet uses a 'checklist' format to explore the role of existing quality assurance (QA) schemes, as well as market, regulatory and management requirements. One practical risk of using recycled water, or water that has a higher salinity level, is that crop growth can be reduced.
The researchers have developed two 'measurement wheels' that give the salt tolerance of fruit and vegetable crops for a variety of soil types. The wheels, which come with the publication Growers' Guide mentioned earlier, show the threshold salinity level at which crop damage will start to occur, and the salinity level at which 90 % harvest yield and 75% harvest yield will occur. The wheels also have a conversion between the different units of salinity (deci Siemens per metre dS/m, electrical conductivity ECe, and total dissolved salts).
The crucial long-term risk heightened by using recycled water is the increase of soil salinity in the root zone of the crop. By monitoring this, it is possible to occasionally flush additional water through the soil to decrease the concentration of salt. The use of this leaching water can effectively contain salt build-up under saline water irrigation. However, unless the water table is very deep or the lateral groundwater drainage is sufficiently rapid, the extra irrigation can cause a progressive rise of the water table. Therefore, the amount of water applied must be optimised to allow leaching without a water table rise.
Other work commissioned by NPSI is investigating the amount of leaching water required, and the factors that affect this. Water doesn't travel uniformly through the soil, but tends to follow worm tunnels, root tunnels and areas that may be more sandy. The researchers have found that the concentration of salt leached from the root zone varies, depending on how quickly the water is moving through the root zone. This has major implications when planning to flush salt through the soil, particularly when precision irrigation systems, such as micro-sprinklers, drip and sub-surface drip irrigation systems are being used.
The researchers are focusing on the measurement of water and nutrient movements through the root zone in subsurface irrigation of vines and citrus, including studying the root zone water regime and deep drainage from citrus grown with 'Open Hydroponics' which may have a higher risk of nutrient leaching. The research, across 17 sites in NSW, Victoria and South Australia, is continuing, and will assist in optimising the timing and amount of leaching water to use under the variable conditions found within any given paddock.