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Closed Watering Systems

Whether it is because of the ever-rising cost of purchased water, an absolute shortage of water caused by drought, or the need to prevent pollution of nearby waterways with nutrients, many nurseries now recycle much of their runoff water.  In this article, we address the benefits and potential problems you may encounter from utilising runoff water.

The main issues to consider are: concentrations of nutrients in the runoff water, the alkalinity of the water, salinity, pathogens and chemical residues.

Nutrients

The nutrients in runoff water can come from water draining from containers or from sprinkled fertigation solution that missed containers. All drainage water contains dissolved nutrients, even when the main source of nutrients for plants is controlled-release fertilisers. The concentrations of nutrients are highest in the first few drainage events after potting and thereafter decline as proliferating roots intercept increasing proportions of released nutrients. Also, concentrations in summer can be several times those in winter. For example, Cresswell and Huett (1996) reported that monthly winter/summer leaching losses (in g/m3 growing medium), for nitrogen were 31 versus 155, for phosphorus 1 versus 6, for potassium 29 versus 113, sulphur 38 versus 158, calcium 38 versus 180 and magnesium 16 versus 58. Higher temperatures and more frequent irrigation both contributed to the greater losses in summer.

Concentrations of nutrients in runoff water will depend in part on the proportions of the water that comes from irrigation water that bypasses containers or from drainage through growing medium, on the amounts of nutrients leached from leaves by irrigation water and on the concentration of nutrients in drainage water.

The efficiency of an overhead irrigation system has a major influence on the concentration of nutrients in runoff water. A system with a low coefficient of uniformity (CU) will tend to produce runoff with lower nutrient concentrations than will a system with a high CU, but the total amount of nutrients leached will be higher. Typical concentrations of nutrients can be (in mg/L) N: 15, P 2, K 12, Ca 30, Mg 8. The greater the acidifying ability of a fertiliser (ie. the higher its proportion of ammonium-N) the greater will be the losses of calcium and magnesium (and some of the trace elements).

Use these numbers as a rough guide only. You must test your water from time to time, maybe every 3 months, or in relation to rainfall events. You can use test strips for nitrate, a Hanna Checker for phosphorus, a swimming pool test kit for total alkalinity and an EC meter for salinity. That is enough to know the chemistry of your water. Doing your own testing is significantly cheaper than sending samples to laboratories, and you get instant results.

Fertiliser programs need to take into account the amounts of nutrients in runoff water being used for irrigation. Such adjustments to fertiliser programs may be especially needed for those nurseries who grow phosphorus-sensitive plants amongst non-sensitive species.

Total alkalinity of the runoff water

Runoff waters typically have a higher total alkalinity (ie. higher bicarbonate content) than the irrigation water used to create them. Acidification of the growing medium by fertiliser ammonium produces drainage water that is less acidic than the irrigation water used. Often, extra bicarbonate comes into the runoff water as it flows over concrete paths and drains, or is held in concrete tanks. This effect is particularly large in nurseries where crushed concrete or crushed dolomite is used for paths and standing grounds. This extra alkalinity can be beneficial if it is low enough to counter the acidification effect of fertilisers. However, if total alkalinity is above about 120 mg/L calcium carbonate equivalent, you may need to acidify the water before re-use.

Salinity

Dissolved nutrients will always cause the salinity of runoff water to be higher than that of the irrigation water that created it. If the irrigation water itself had a low salinity, the extra nutrients will not raise salinity enough to cause problems. But the higher the salinity of the irrigation water, and especially the higher its content of sodium and chloride, the greater will be the potential for the salinity to be increased to a plant growth-limiting extent. This potential for damage can be minimized through the use irrigation systems such as drippers and sub-irrigation that prevent the recycled irrigation water from contacting leaves. In extreme situations, some of the water may need to be run through a reverse osmosis system.

Plant pathogens

A basic assumption regarding runoff water is that it will be contaminated with plant pathogens, no matter how clean your nursery or how excellent your hygiene practices. These must be killed before the water is re-used. Fundamental to all treatment systems that might be used is that particulate matter must be filtered out of the water before it is treated. If they are left in the water, they will clog systems (eg. Slow sand filtration), partially or completely negate the treatment (eg. UV light), or cause a need for much higher rates of chemical use.

Here is a very brief overview of the systems that can be used.

  • Slow sand filtration: involves slowly running the water down through a bed of sand in a wide tank. Microbial activity in the uppermost part of the sand kills plant pathogens. It is very effective, but requires a considerable amount of space for the filter itself and for storing the treated water.
  • UV light is effective so long as all suspended and coloured matter in the water has been removed by filtration and flocculation. Flow rate must match the UV intensity.

There are many chemicals that can be used.

  • Chlorination, by hypochlorite usually, but also chlorine gas, has long been the mainstay of water disinfection. Water pH should be lower than 6.5.
  • Chlorine dioxide is preferred for alkaline waters.
  • Chlorobromination uses a combination of sodium hypochlorite and sodium bromide. It is very effective, especially for neutral to alkaline waters, but it is somewhat more costly than chlorination.

If you ever have any work done on the plumbing down-stream from the pathogen-removal system, be aware that this has the potential to introduce pathogens into the system. You need to insert strong hypochlorite solution into that part of the system that is immediately downstream of the work site and let it remain for some hours to kill any pathogens introduced. Flush to waste.

Chemical residues

Any herbicides and pesticides used in the nursery have the potential to end up at low concentrations in the runoff water. The possibility of problems is usually slight from pesticides, but some herbicide residues may cause problems. The atrazine/simazine group of herbicides probably have the highest potential to cause problems. Be aware of what is being used in and around the nursery; read labels carefully.

More information 

For more information, please contact one of our grower services representatives.

Reference

Cresswell, GC and Huett, DO. Managing nursery runoff: techniques to reduce nutrient leaching from pots. 1996. pp. 40. NSW Agriculture publication.

Disclaimer

Please note, any advice displayed is of a general nature only, and an intending user of a product should obtain and only rely on professional advice particular to their intended purpose, climate, soil conditions and other factors.