Water Wisdom for Hydroponics

Water is the basic ‘carrier’ for the nutrient salts, additives and growth promoters which bathe the root zone and also for many of the products sprayed or drenched onto hydroponic crops. Many growers have been caught out by assuming that a crystal clear, odourless water supply is pure and therefore perfect for a hydroponic system. The same can be said for some municipal or city water supplies which often have chemicals added to bring the water up to drinking standard. However these ‘drinking water standards’ apply to making water safe for people, but not necessary ‘safe’ for hydroponic crops. In many ways plants are more sensitive to certain water treatments and contaminants than humans are, so checking out your water supply should always be the first step when setting up a hydroponic system.

Fortunately, many of the common water quality problems can be easily and effectively solved, even by small growers with limited equipment. Water testing can be expensive for a full mineral, pathological and microbial analysis.

However growers using municipal/city water supplies should be able to get a copy of their water quality analysis report which outlines a huge number of water variables. Those using well or bore, stream, pond, dam or other water may need to get an analysis done, and even rain water can contain high levels of plant damaging contaminants. Some hobbyist growers are often forced to use distilled (demineralised) water for their hydroponic system, due to problems with the local water supply. Many other growers, both large and small collect and store rain water for their hydroponic crops to avoid problems with inconsistent water quality from other sources.

Water quality problems can cause reductions in plant growth, mineral toxicity or deficiency symptoms, strange growth deformities, accumulation of unwanted elements, pathogen attack, bacterial contamination, oxygen starvation of the roots, uncontrollable pH problems, irrigation and equipment scaling and blockages to name a few. The most common problems with water supplies for hydroponics - their identification and solutions are outlined below:

1. Chlorination of water supplies

Chlorination is common in many city water supplies where it is used to control bacteria in drinking water. Luckily, chlorine dissipates into the atmosphere as a gas fairly easily, so most people will be able to smell chlorine from a treated water supply and this gives an early warning.

Chlorination of water supplies for microbial control has in the past caused a fair bit of crop damage in hydroponics. Many growers chose to chlorinate their own water to kill pathogens before making up the nutrient solution. High levels of chlorination (15 - 25ppm chlorine) are required to get effective control of bacteria and immediately after treatment the water should not be used. Whatever the reason or source of chlorination of water, the active chlorine needs to dissipate down to very low levels before being used in a hydroponic system. The reason for this is that chlorine damage can occur on sensitive crops such as lettuce, salad greens, strawberries and others. Chlorinated water sources are easily treated by aeration of the water in a ‘holding tank’ for 48-72 hours (depending on the initial concentration), with good ventilation. After this time, the active chlorine levels should have fallen to below 1ppm - it is then safe to come into contact with the plant’s root systems.

2. Unwanted minerals in water supplies

Most water sources are not ‘pure’ in that they contain varying quantities of dissolved minerals. Some of these minerals are beneficial in that plants will actively take them up and use them - silica is one such example. However, others such as sodium, which is extremely common in our environment, are unwanted and can cause major growth problems in hydroponics. Plants don’t require sodium, so if salt (sodium chloride) is present in the water supply, even in small amounts, it will accumulate in a recirculating nutrient solution and cause growth problems. Some plants such as tomatoes can tolerate much more sodium than others such as lettuce. Sodium needs to be kept below 80ppm for optimum growth of most crops in recirculating systems, but below 30ppm for lettuce. Sodium, like other minerals can be removed via reverse osmosis or distillation. Smaller growers with high levels of sodium in their water should chose non recirculating hydroponic systems or frequent dumping and replacement of the nutrient solution will be required.

Nitrate, magnesium, calcium, potassium, sulphur and trace elements such as boron, copper, manganese and zinc can also be present in various water sources. In most cases, the nutrient solution formula can be adjusted to account for the presence of these plant usable elements so that accumulation and toxicities don’t occur. Trace elements can cause the most problems where they are present in the water supply, as plants only take up small quantities of these and accumulation can occur even where these elements have been taken out of the nutrient formulation. In these cases demineralisation is required or dilution of the water source with a more pure water supply can help when making up the nutrient solution or topping up nutrient tanks with water.

3. Microbial or pathogen contamination and BOD

Ground water from wells that has percolated through the soil can become infected with a number of soil-borne pathogens, the most common being Pythium. Pathogen spores may also be present in other water sources such as ponds, streams and dams if water treatment or sterilisation hasn’t been carried out. Pythium species are common and widespread pathogens which can be difficult to completely eradicate. However, Pythium is an opportunist pathogen which will not actively attack healthy plants unless a high spore load is present in the water source. Other pathogens may also be present in some water supplies, along with high numbers of ‘beneficial’ microbes which often play a role in suppressing plant pathogens. If Pythium or other disease spores are identified in a water source these can be controlled by a number of methods. Smaller growers often chose to chlorinate water or add hydrogen peroxide as a water sterilisation procedure. So long as the water is held for a few days after these treatments to allow these chemicals to drop below root damaging levels, before the nutrient solution is made up, then these methods work well. As an example, recent studies have found it takes a level of 100ppm or more of hydrogen peroxide to kill pathogens such as Fusarium wilt and Pythium in water and nutrient solutions. However levels of 85 - 100ppm almost killed young lettuce seedlings, while even a level of 8 - 12 ppm considerably reduced the growth of young plants. So treatment of the water before adding this to the hydroponic system is the best option for pathogen control.

BOD (Biological Oxygen demand) is often used as an indicator of microbial contamination as high microbial populations use up dissolved oxygen in the water supply. A high BOD is common in water that has a concentration of organic matter on which the microbes feed.

4. Iron and Iron bacteria

Iron is often found in the form of iron hydroxide in water supplies and is usually associated with ground water near deposits of iron sand or iron ores. Iron in the hydroxide form is not readily available to plants and its presence in water is a problem as it causes blockages in irrigation components and drippers/emitters. Iron problems in the water supply will show up as a rusty red deposit. If not removed, iron bacteria can result which causes a slimy deposit that can also lead to equipment blockages. These bacteria not only need iron, but also organic matter, oxygen, manganese and a warm temperature in order to multiply to levels that will cause problems - all elements that are provided in a hydroponic system. Iron hydroxide can be removed by aeration and settling, or by flocculation with a number of different agents. Iron bacteria contamination can be removed by sterlisation of the water or nutrient solution.

5. Hard water sources causing pH problems and equipment ‘scaling’

Alkalinity of water describes the strength of a high pH. A high pH with high alkalinity will need proportionally more, stronger acid to lower the pH than if the same pH had a low alkalinity. High alkalinity would be greater than 300ml/l of calcium carbonate, low alkalinity would be less than 100 mg/l of CaCO3. Carbonate and Bicarbonate ions are derived from limestone or sea water.

Water is termed ‘hard’ when it has large amounts of dissolved calcium bicarbonate and to a lesser extent magnesium and other elements. When these elements come into contact with pipes and equipment they rapidly change from soluble calcium bicarbonate to insoluble calcium carbonate otherwise known as lime scale. Hard water will scale up irrigation pipes, heating elements and pumps and this can cause major headaches in hydroponic systems. Hard water sources can be treated with water softeners which use an ionising resin that is back flushed while adding sodium chloride. However this process leaves sodium in the water which is not desirable for hydroponics. Computerised water conditioner units, such as those used to prevent scaling in domestic water supplies, are a better option for water treatment in hydroponic systems.

6. Herbicide and other contaminates

There have been cases of herbicide contamination of not only steam, pond and ground water but also city water supplies which have seriously affected hydroponic growers. The first sign of such contamination may be severe damage on sensitive crops such as tomatoes. Activated carbon filtration will remove chemical residues on a small scale, provided the carbon is replaced often enough to remain active as a filter.

7. Rain water problems

Although some rainfall is reasonably pure and clean, not all rainwater is free from water quality problems. If rain has fallen through areas of pollution, acid rain will result. Coastal rainfall may be highly contaminated with sodium. Rainwater collected from galvanised iron roofs may have high zinc levels. Water stored in new cement tanks can have minerals which have leached from the concrete. Larger hydroponic growers are advised to have an analysis of their rainwater supply carried out to check for potential problems.

Summary

High quality water is essential for any hydroponic grower. Often persistent growth problems in hydroponics can be traced back to a problem within the water supply which is effecting the composition of the nutrition solution and plant growth. Finding out what is in your particular water supply is one of the first steps for any hydroponic grower whether they are using municipal water of ‘drinking standard’ or other sources. Luckily most of the water problems encountered in hydroponics can be easily solved with simple treatments before the nutrient solution is made up. Small growers have the option of using de-mineralised or rain water if necessary, while large commercial growers may invest in some of the more expensive water treatment options. Clean, pure water will give all hydroponic plants the best start and sometimes switching to a better quality water source when problems have been identified, can give significant improvements in plant growth. Hydroponic growers should not just assume their water supply is O.K - if in doubt, always check it out!

References and Sources of Information:

Nederhoff E, 2000. ‘Hydrogen peroxide unsuitable for root disease control in hydroponic vegetables’ NZ Commercial Grower Volume 55 No 9, page 46..

Nederhof E, 2000. ‘Hydroponic peroxide for cleaning irrigation systems’. NZ Commercial Grower Volume 55 no 10, page 32.

Morgan L, 2000. ‘Water, Water Everywhere...’ Growing Edge Magazine, Volume 1 number 6 page 29.