Irrigation Water for Greenhouses
Water Electrical Conductivity
Electrical conductivity (EC, also known as conductivity or soluble salts) is a term used to measure the total concentration of salts in irrigation water. A salt is a combination of positively charged elements (cations) and negatively charged elements (anions). The most common cations of interest in water are calcium (Ca2⁺), magnesium (Mg2⁺) and sodium (Na⁺); the most common anions are bicarbonate (HCO3¯), chloride (Cl¯), and sulfate (SO4¯). Fertilizer, fungicidal drenches, and breakdown of organic substrate components can also contribute additional salts. As the EC reading of the solution increases, so does the concentration of ions. Irrigation water is classified by its salt hazard in Table 18.2 It should be noted, however, that plant sensitivity to salinity varies with the species.
Units of Measurement for EC
Currently, the units used to measure electrical conductivity (ECw) are millisiemens/cm (mS/cm), microsiemens/cm (µS/cm), or decisiemens/m (dS/m), where S stands for Siemens. The conversion for all these units is 1000 µmhos = 1000 µS/cm=1 mmhos = 1 mS/cm = 1 dS/m. Other units for expressing ECw of irrigation water include millimhos per centimeter (mmhos/cm), micromhos per centimeter (µmhos/cm), and milliequivalents per liter (meq/L).
Salinity Effects on Plants
Irrigation water high in soluble salts transfers the salt to the potting medium and can accumulate over time. Crops vary in their tolerance to salinity and some may be adversely affected at ECs less than 2 dS/m. The first of sign of salinity is usually stunted growth, with plant leaves often having a bluish-green color. As salt levels in the soil increase to more toxic levels, scalding or burning on the tip and edges of the older leaves occurs. The leaf dies and falls off and finally, the plant dies. In other cases, the younger leaves may appear yellow, or the plants may show signs of wilting, even though the soil appears adequately moist. Saline irrigation water can affect plant growth in two ways: salinity effect and toxicity effect.
Monitoring Container Leachate for EC and pH
Monitoring container leachates can help growers take preventive steps to reduce possible damage to roots due to high electrical conductivity before visible symptoms occur. Some greenhouses monitor EC on a weekly basis, to determine how they will irrigate or fertigate each zone the following week. For greenhouse production, growers can leach out the salts or fertilizer rates can be reduced, if the EC is too high. If the EC is too low, fertilizer rates can be increased.
Electrical Conductivity Meters
An EC meter measures the concentration all soluble salts dissolved in a solution but does not determine which salts are present at specific concentrations. Because of the impact of temperature, most commercial EC meters automatically compensate their output to account for temperature differences between samples. Some meters will output TDS. Taking an EC measurement is simply a function of immersing two electrodes in the sample, applying a voltage and measuring the response. This is then converted to conductivity.
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