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Considerations in using recycled water on crops and turf

In the driest inhabited continent on Earth (only Antarctica is drier), water is a luxury. As global warming gathers pace and southern Australia dries, it will become precious. So more and more uses will be found for recycled water.

Already, treated domestic effluent is supplied, via a dedicated reticulated pipeline, to new houses in the north-west of Sydney, for toilet flushing and garden watering. The use of recycled water in various industries will eventually become commonplace.

Irrigation of crops and turf is an obvious use for recycled water, particularly where a ready supply of waste water is already available. Intensive animal enterprises such as dairies and piggeries use a lot of water for hosing and cleaning. The water that runs off is replete with nutrients that crops and pastures need. In fact, the careful reuse of dairy, piggery and feedlot effluent can save around half a farm’s fertiliser costs (although this depends on a number of factors).

But the use of recycled water for irrigation requires care. Two factors exert the most control. The first is salt. All recycled water contains salt, picked up from a variety of sources. The other is the nutrient content.

Salt

Too much salt will render a soil unfit for cropping, and can cause soil to disperse and lose all structure. So before effluent can be applied to soil, its salinity must be known. This is easily determined with an EC (electrical conductivity) meter. (For more information, see How salinity is measured.) The salinity of the soil must also be determined. This also can be done with an EC meter, although the answer is not as straightforward, as the EC of a soil depends on the mineral composition as well as the salt content. The relationship of these two salinities has a strong influence on whether or not the effluent can be applied: If the soil is already saline, the addition of saline water will only make matters worse.

Table 1 shows the various classes of salinity as determined by EC.

Table 1. Interpretation of EC values.

Critically, the crop or pasture’s tolerance to salt also determines whether the effluent can be applied. Table 2 lists a range of crops and the salinities they can tolerate. If the salinity of the effluent is too high for some crops, other crops could be substituted.

Table 2. Salt tolerance of some crops.

From K.I. Peverill, L.A. Sparrow & D.J. Reuter, 1999. Soil Analysis – An Interpretation Manual.

Minimising the salinity of the effluent by controlling what goes into it pays better dividends than trying to control the use of the effluent afterwards.

Leaching

Carefully chosen application rates can allow the use of moderately saline water as long as the soil has high permeability. If the water can move down through the soil profile without hindrance at a rate that exceeds the combined evaporation and transpiration, it will flush accumulated salts down out of the rootzone. However, if it is allowed to evaporate from the soil, then its salts will remain behind and so make crop or pasture growth more difficult.

Sodicity

A related factor is sodicity, or the ratio of sodium in the water (or soil) to magnesium and calcium ions, which can partly balance the toxic effects of the sodium. Sodicity can be controlled to some extent by the addition of gypsum (which supplies calcium).

Limiting-nutrient concentration

The nutrients in effluent are beneficial to the crop or pasture (and the bank balance). However, any crop or pasture can take up only so much of each nutrient. So it is necessary to know the nutrient contents of the effluent and the soil and the nutrient requirements of the plant. Provided the quantity of nutrients supplied in the effluent plus the quantity already present in the soil does not exceed the quantity that the crop or pasture can take up, then the nutrients will not build up to harmful levels in the soil or, more critically, run off into streams (poisoning water life) or leach into the groundwater (poisoning the people who drink it).

The most critical factor is the nutrient that comes closest to supplying the crop or pasture’s total requirements for the season – this is the limiting nutrient. For example, if the effluent applied has already supplied all of the crop’s requirements for nitrogen but only half of its requirements for phosphorus and potassium, then extra P and K must be supplied as fertiliser, even if there is enough P and K available in unused effluent. The alternative is to apply too much N, which can kill the crop and leach out and poison waterways.

Therefore, it is essential to test both your effluent and your soil for all critical plant nutrients and determine the limiting nutrient, which ultimately determines how much effluent you can reuse.

Further information

Dairy Australia will shortly publish the Effluent and Manure Management Database for the Australian Dairy Industry, which (among other topics) covers all aspects of nutrient and salt budgeting for the reuse of effluent on crops and pastures. Check http://www.dairyaustralia.com.au/ for news.

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