Plants need water for growth and healthy development. If plants are deprived of the required amounts of water, over a period they will whither and die. Unlike plants grown in soil, plants grown hydroponically in grow rooms and indoor greenhouses, are completely dependent on supplied water to meet their nutritional needs. It is therefore important that growers gain a basic understanding of certain vital aspects concerning water supply and plant nutrition.



Water is an excellent solvent and whether it is drawn from a tap, a lake, river or other natural sources, contains dissolved impurities along with mineral salts. The salts commonly found in water include common salt (sodium chloride), calcium chloride and carbonates. Plant growth is not adversely affected as long as salinity levels (levels of dissolved salts) are not excessive, but beyond certain threshold limits it can cause extensive tissue damage.

Chemicals used in standard hydroponic nutrient solutions contain only minor quantities of common salt (sodium chloride) which does not normally constitute a problem. Usually, it is the amount of dissolved salts in the water used for making nutrient solutions and the water added to replace the water transpired by plants that can be potentially damaging to plant tissue. Unless this water is purified to remove the salts it can lead to high concentration in the nutrient solution which can cause damage. The dissolved salts are present as anions and cations, which are negatively and positively charged atoms or groups of atoms. These always exist in pairs as so that the net electric charge is zero.

Salinity can be defined as the total amount of soluble salts in the nutrient solution, which is a measure of its conductivity. It can also be defined specifically with reference to the level of sodium chloride in the solution.

Crops vary as to their sensitivity to sodium and chloride ions. Tomatoes, for example can be grown in saline nutrient solutions; in fact common salt (sodium chloride) is added to nutrient solutions to increase conductivity. Higher conductivity of nutrient solution provides improved fruit quality, though yield decreases. On the other hand, plants like lettuce are prone to damage by common salt and high salt concentrations would be disastrous.

Salinity Meters

Growers today, have several types of electronic meters for measuring the salinity of water or nutrient solutions. The more widely used of these are electro-conductivity (EC) and Total Dissolved Solids (TDS) meters. EC meters measure the ability of an aqueous solution to carry an electric current by measuring the electric current between two electrodes. A solution that is rich in nutrients has higher electro-conductivity than a solution with less ionic salts. These meters measure the electro-conductivity in either milliSiemens/cm (mS/cm) or micro Siemens/cm (mS/cm). EC meters are more commonly used by commercial growers because their measurement more accurately corresponds with the best estimate of the strength of nutrient solutions.

TDS meters measure the concentration of a solution as the total weight of dissolved solids. Widely used by hobbyists, they actually measure the electro-conductivity of a solution. They measure the electric current between two electrodes – with a greater concentration of nutrients electric current flows faster than in a solution with a lower concentration. An in-built conversion factor in the microprocessor logic converts the measured current into parts per million (ppm) of total dissolved solids. There is no direct relationship between micro Siemens and ppm for mixtures but with an appropriate conversion factor applied to the measured current a rough estimate of TDS is obtained.

TDS meters can only give a rough estimate of TDS as there are several factors at play including the presence of air bubbles, probe positioning, temperature etc. TDS measurements are indicative of range and pin point accuracy is not possible. However, in the majority of typical applications a range indication is good enough.

Meter Ranges

EC and TDS meters are available in different ranges and not all are suitable for hydroponics applications. There are TDS meters that measure up to 1999ppm, and EC meters that only measure up to 1990mS. These meters, will not be useful for several crops where the optimum electro-conductivity exceeds these limits. For use in hydroponics TDS meters should ideally have a range from 0 to 10,000ppm while EC meters should have a range from 0 to 19.9mS (19,999mS).

Conclusion

Salinity is an important determinative factor in plant growth and growers need to ensure that the water they use has a low salinity. Both EC and TDS salinity meters have been in use in hydroponics in different parts of the world in accordance with the regional preferences and practices. Both types of salinity meters have enabled thousands of growers ensure correct water salinity levels for optimizing plant growth in hydroponics systems. Though much can be said about the need for accuracy, consistency etc. of the measurements, it may be best at times to let practical experience guide user preferences.