Following on from last month’s question on water quality, here are some more guidelines. These list the Dutch guidelines for the maximum levels of sodium that are acceptable in recirculating solutions for a range of crops.
I have also included more recommendations as to different water sources, especially the collection of rainwater.
Answer by RICK DONNAN
The quality of the raw water going into a hydroponic system is very important, especially if the system is recirculating (‘closed’). The ion most likely to cause problems is sodium (Na+).
Build-up of sodium
Most recirculating systems are managed on the basis of maintaining constant EC (electrical conductivity—a measure of solution strength) of the recirculating solution. However, EC tells you nothing about the individual ions that make up that solution. Unfortunately, when non-essential ions, such as Na, are added in with the raw water at strengths higher than the plants take up, then their concentration will rise. This is shown in Figure 1, which indicates their increase with time.
When this happens there is a double whammy. The non-essential ions, such as Na, are increasing towards toxic levels. At the same time, the effective nutrient content of the solution is shrinking. That is, the plants are heading towards a mixture of being both poisoned and starved. Not a good combination.
Maximum Na level in recirculating solutions
Table 1 giving recommended maximum levels of sodium in recirculating solutions for different hydroponic crops is based upon information given in ‘Bemestingsadviestbasis Substraten’, published by Proefstation voor Bloemisterij en Glasgroente, The Netherlands.
The original information is given in molar units (millimole/litre). A mole is the molecular weight of a molecule expressed as gram/litre. For an atom such as Na, molecular weight is the same as atomic weight, which for Na is 23. Because ppm (parts per million), also expressed as milligram/litre), is the unit commonly used in Australia and some other countries, I have also converted the molar units to ppm.
In the previous issue, I mentioned using reverse osmosis to remove the ions from input raw water that had too high a content of unwanted ions, such as Na. There is another possible source of water sometimes pure enough to use. This is rainwater, but care needs to be taken, apart from availability, dependent upon the frequency and quantity of rainfall.
Rainwater which has flowed over the ground could to be contaminated with soil borne pathogens. Consequently, it is dangerous to use stream or dam water without it being sterilised. Sometimes growers get away with not sterilising if they are in an area with no other horticultural or agricultural activity. Often, this may last for a year or two (the honeymoon period), but usually disaster eventually strikes.
Rain collected off greenhouse, etc, roofs is usually cleaner, but is still risky if not sterilised, especially if in a dusty area.
Other aspects to watch are that if near the sea, sea mist and drift can result in high levels of salt collecting in the rain water. Also, do not collect water from galvanised roofs—a little zinc dissolved from the galvanised layer can give levels of zinc in the water which are toxic to plants. Ω
PH&G July 2016 / Issue 169