Water Quality Management for Use in Hydroponics

Water is one of the most important resources for growing plants. Both availability and quality of water is critical for successful crop production. In many parts of Canada, USA and indeed throughout the world water quality can be a major problem for plant growth in soil generally and soil less medium more specifically because of contamination by various organic and inorganic substances. Even water supplies suitable for domestic or agriculture use is not necessarily good for hydroponics /soilless medium. The best domestic water supplies commonly contain substances that affect plant growth, because the parameters considered suitable for drinking are not necessarily acceptable for growing plants.

Since most soilless culture systems requires sizeable quantities of relatively pure water, water quality is of utmost important. Water quality can vary from source to source. Three sources of water are commonly used by growers’ i.e. well water, Municipal water and pond water. Each water source has separate problem /issues of water quality. For example, well water frequently contain high levels of dissolved elements, especially Calcium and Magnesium, which can lead to high alkalinity that varies with depth of water. City water commonly obtained from lakes and rivers has a low level of dissolved chemicals and has no disease and algae, but they have a high level of chloride and fluoride that can cause a leaf margin necrosis. While pond water used by growers usually contains low levels of dissolved solids but they might have higher algae and other disease problems.

Keeping all this in view water quality parameters are divided into three categories

· Physical

· Biological

· Chemical

Physical

This includes suspended solid particles and temperature of water. Suspended solids such as soil particles are potential problems because they can clog the drippers and can also cause abrasion of pipes. Water temperature can be an important consideration particularly when growing foliage plants where high or low temperature can put plant under stress and will lead to leaf spotting and ultimately to poor quality of plant. Both of these properties are easily manageable by using a filter to remove suspended solids and maintaining temperature of water supplies to plants.

Biological

This includes algae and microorganisms present in water. Algae and microbes are not of major concern if they are used in soil medium but they are of major concern when used in soilless medium because they can induce diseases through root and put plants under stress. They can also block the drippers. These problems mostly occur if somebody is using pond water for irrigation. This can be controlled by using a safe enzyme product which can help in getting rid of all these algae and other organic material from water.

Chemical

These properties are considered more important when dealing with irrigation water because chemicals in irrigation water can have a major negative impact on plant growth and this becomes even more important while using this water in a soilless medium because of its high potential for change with relatively low buffering capacity. The chemical constituents of irrigation water can affect plant growth directly through toxicity or deficiency, or indirectly by altering plant availability of nutrients. For hydroponics water quality the most critical chemical water quality parameters are Soluble Salts, Hardness (Calcium and Magnesium concentration), Sodium, Chloride, and to some extent pH.

Soluble salts

Soluble salts in water are measured by electrical conductivity. The ideal value of irrigation water is less than 1.0 but one can use irrigation water up to 1.5 EC depending upon the level of different salts. These salts are present in higher quantities mostly in well/ground water. Some of these salts like sodium, chloride, boron, iron, fluoride are of greater concern because they are either directly toxic to plants, reduce/impede the water uptake by plant roots and also cause leaf burning spots that ultimately lower the value of the produce. Water that has higher EC value is referred to as saline, but EC measurement does not indicate the relative amounts of any specific salts or ion. Additional information on specific ions is required to get exact idea of any problem in water. EC will give a general idea whether water has soluble salts or not and how much is the quantity present in water.

Hardness

It is an indication of the combined concentration of Calcium and Magnesium in water. The amounts of these two elements in water are variable. Well/ground water normally has more Calcium and Magnesium compared to pond or municipal water. Normally the ideal range for Calcium and Magnesium in water is 40-80 and 20-30 ppm respectively. If water has higher concentrations of Calcium and Magnesium it is termed as hard water and can become a potential problem since both of them can combine with Bicarbonates to form insoluble Calcium and Magnesium Carbonates. This will also impact on media pH (increase the pH of medium) and restrict the availability of many other nutrients. Calcium can also react with phosphorous to form insoluble Calcium Phosphate. If water has higher concentration of these salts, then growers should be careful about using more Calcium and Magnesium in their feed solution.

Sodium and Chloride

Sodium and Chloride concentration in water are often associated with each other. If sodium concentration will be high in irrigation water then most probably chloride concentration will also be high. Well and municipal water sources may contain higher levels of Sodium and Chloride. The ideal concentration of Sodium is below 30 ppm. Both of these salts are very toxic at higher concentrations. They can cause direct as well as indirect damage to plant growth. They can interfere with the availability of Calcium, Magnesium, Nitrate and many other elements and if absorbed in higher quantities they can form leaf spots/burning.

pH

pH is a measure of acidity or basicity of water/measure of hydrogen ion activity in water. Its range is from 0-14 with 7 as a neutral. Values less than 7 considered as acidic while higher than 7 is considered as basic or alkaline. Most of the plants grow best when the medium pH is 5.6-6.2. The main effect of water pH on plant growth is through control of nutrient availability. A low pH may be responsible for excess Iron and Manganese availability leading to toxicities or Calcium and Magnesium deficiencies. A high pH is worse as it can cause deficiency of many of the nutrients. In general, the pH of irrigation water is not as critical as the pH of medium but the addition of high or low pH water can change the pH rapidly especially in soilless medium that already has very low buffering capacities.

Management strategies

Working with water quality issues is technical and may require the addition of expensive equipment, chemical treatment or strategies to handle it without investing money on any treatment solution. In most cases some form of water treatment will be necessary to make and maintain useful solutions. The treatment of water depends upon the problems present in water.

I. Soluble salts

There are several approaches to deal with this problem (saline water). But the most common two approaches that were used to tackle these excess soluble salts are:

1. Blending of poor quality water with good to dilute the toxic concentration of soluble salts. The blending ratio may be 1:1, 2:1 (bad quality: good quality) or any other ratio depending on the level of soluble salts concentration in irrigation water.

2. Management of irrigation scheduling. Use of water volume as required by plants followed by every 3rd or 4th irrigation with good quality water to leach down excess salts from root zone along with use of fertilizer product with a lower salt index.

Both of these two approaches require a technical knowledge and very strong day to day water management strategies which is not an easy task. Secondly, growers using high salt water would not want to let plant mediums dry out because the concentration of salts will increase around the roots that will lead to a shock to the plant and poor growth. The final solution to tackle this problem is done by using two commonly used and available water treatment systems.

· Reverse Osmosis

· Deionization

Reverse Osmosis

This type of system is usually the most cost effective and commonly used for small greenhouses for container crop production. It has the ability to remove up to 95% of total soluble salts from the irrigation water and it cost about three to four cents per gallon. This system works by Osmosis which is a passage of a solvent through semi permeable membrane separating two solutions of different salt concentration. The amount of purified water delivered in a given time and the degree of salt removal depends on several things like pressure of system, membrane type, quantity of soluble salts in water and temperature. Efficiency also depends on the integrity and cleanliness of membrane which is often degraded by chlorine and clogged by suspended sediments. For this reason water to be purified should be pretreated for suspended solids, calcium carbonates to avoid damage to membrane. It is always advisable to use some filtration before passing that water through a reverse Osmosis system. This will not only enhance the efficiency and quality of water but also extends the life of membranes being used in the system.

Deionization

It is a process of removing ions from solution by using exchange resins. Although this is a very effective system and produces high quality water, but water quality that is generally not required for plant growth. Secondly the cost is three to four times higher than water produced by reverse osmosis because one needs to change the resin quite regularly if the water quality is poor.

However, both these systems could be used and justified if the water quality is poor, the crop value is high and alternate water resources are not available.

II. Hard water

Calcium and Magnesium need to be removed from hard water if the concentration is high. If the concentration of Calcium and Magnesium is low that can be managed by using fertilizer with low or no Calcium and Magnesium. But if the concentration is high, these can be removed by water softening technique. Care should be taken while using the softening technique because most of the systems use sodium for softening and that can become toxic for plant growth. Potassium should be used for softening to replace excess Calcium and Magnesium. Although total salt level does not change, the ion balance becomes favorable for better plant growth. Even using potassium (Potassium Chloride) can cause problems due to chloride concentration if it’s being used in larger quantities.

The best management practices for hard water are to use 50% water softening along with use of a fertilizer low in Calcium and Magnesium.

III. pH

As mentioned earlier the pH of water is not of great concern if it is slightly lower or higher than the required pH in medium. But if the water pH is very high and that water has very strong buffering capacities (presence of carbonates and bicarbonates) then it is advisable to maintain the pH of the water before irrigation. If it’s not maintained, continuous use of this water will lead to an increase in the pH of medium and becomes unfavorable for plant growth. The use of acids or pH down products available in the market (Liquid or Powder) is recommended to bring down the pH.

At the end I would like to say that in hydroponics, high quality crops can only be produced by using high quality water and this best quality water can be achieved by choosing the best water treatment program for a greenhouse operation based on identified problems in water by lab analysis. Therefore, a complete analysis of water should be done to identify the problem followed by the selection of a treatment method that is economical and effective. After that, frequent water and medium testing should be done for some time to determine if the water treatment yields the desired results. The time spent in obtaining information and proper follow-up can more than pay for itself in improved crop quality.

Dr. Tahir Mahmood

Director Research and Development

Grotek Manufacturing Inc.