I am considering setting up a small commercial hydroponic farm using a recirculating system. I have read that water quality is very important for hydroponics, especially when recirculating. Is this correct and can you give me some guidelines as to the quality required to grow different crops in hydroponics?
Answer by RICK DONNAN
Yes, the quality of the raw water going into a hydroponic system is very important. The most significant chemical naturally occurring in water supplies is common salt (sodium chloride, formula NaCl).
In a ‘closed’ recirculating system, every time the salt build up is displaced from the system, it is returned to become part of the feed into the system. Hence, if the Na in the added raw water is higher than the crop can take up, the Na level in the recirculating solution will continue to rise. However, in an ‘open’, free drainage, system any modest build up of Na stabilises, because any excessive build up is pushed out of the system by the next irrigation cycle.
Crop sensitivity to sodium chloride
Different crops have different sensitivities to sodium chloride, although it is sodium which usually has the major impact. The Dutch have been studying this problem for many years and their results for vegetables and cut flowers follow:
Vegetable sensitivity to salt
• Egg Plant
Cut flower sensitivity to salt
• Euphorbia fulgens
(From: Wageningen UR. (2014) Kwantitatieve Informatie voor de Glastuinbouw.)
Reducing Na levels
Unfortunately there is no way of removing only sodium ions from a solution. If the level of Na in your raw water is too high for the crop you are growing, then if you have a recirculating system your only choice is to install reverse osmosis (RO).
Reverse osmosis equipment mainly consists of tubular cells made of semi-permeable membrane. When pressurised these allow water to pass through, but block the passage of all dissolved ions including Na and Cl, as well as all nutrient ions. The result is chemically (and biologically) clean water. This is perfect for use in recirculating systems, and especially for salt sensitive crops.
One aspect of installing RO equipment is that you need to be able to handle the waste (brine) stream from the unit, which contains all the incoming salts. For example, say you have a unit that recovers 50% of the input as pure water. The brine stream will be the other 50% of the input, but contains all the salts, hence it will be twice as strong as the initial water supply.
Another aspect of RO to watch is that a major operating cost is the replacement of the expensive membranes. Ensure that you purchase your unit from someone experienced, because proper pre-treatment of your raw water is essential to prolong the life of the membranes.
Water coming off limestone will often be “hard’, that is, it contains calcium (Ca) and bicarbonate (HCO3) and perhaps magnesium (Mg) ions. These are usually OK to use provided they are the only significant dissolved solids. Because the Ca and Mg are nutrients they can be used, however, the Ca and Mg in your fertiliser formulation must be adjusted downwards to make allowance for these free nutrients.
The bicarbonate will raise the pH and hence needs to be neutralised with acid, typically phosphoric or preferably nitric. Again, your fertiliser formulation must be adjusted to make allowance for the extra phosphate or nitrate ions added as part of the acid.
Iron and boron
Although iron (Fe) is a nutrient, in water supplies it is much more of a problem than a benefit. This is because it oxidises and hence is useless as a nutrient and also can cause dripper blockages. This is why hydroponic fertilisers have their iron in a stable chelated form. Iron in the raw water can be removed by aeration followed by settling or filtration.
A problem in some countries, but rarely in Australia, is water with too high a boron (B) content. Ω
June 2016 / Issue 168