Plant Nutrition of Greenhouse Crops
Electrical Conductivity of Growing Medium
Improper nutrient management is one of the primary causes of poor crop quality and plant losses in greenhouses. Elctrical conductivity (EC) is a measure of the dissolved soluble salts concentration in a solution, such as the irrigation water in a growing substrate. Soluble salts include salts from preplant amendments, water-soluble fertilizers, controlled-release fertilizers, irrigation water, residual fertilizer, media components, and compounds resulting from microbial decomposition of organic matter. Some fungicide products applied as a media drench also contribute to soluble salts. Soluble salts are a fairly reliable measure of the nutrient status of a crop as long as the major source of salts is from the fertility program and not other sources, such as the water supply. EC can also be used to monitor the accuracy of a fertilizer injector. Nutrient deficiencies and toxicities are often related to high substrate pH due to the use of poor-quality irrigation water. However, initial symptoms of nutrient deficiencies or toxicities are not always visible until they become a problem that increases costs and reduces quality (and, therefore, profitability).
Why is EC Useful?
More and more, researchers and extension specialists give fertilizer recommendations based on the EC of the growing medium, instead of recommendations based on fertilizer concentrations. This is because fertilizer recommendations based on the EC of the growing medium give a better indication of the nutrient availability to the plant, as well as the potential for salt stress which can damage the roots and leaves. Growing medium-based guidelines also have the advantage that they are less dependent on environmental conditions.
Measuring EC
On-site EC monitoring programs allow growers to see trends during the crop production cycle and to adjust before crop vigor or quality problems occur. Some greenhouses monitor EC’s on a weekly basis, to determine how they will irrigate or fertigate each zone the following week. Monitoring on a frequent basis allows for sound management decisions—the more frequent the monitoring, the better the quality of the data and the more likely production problems will be caught in a timely fashion.
Substrate Extraction Methods
Greenhouse growers who have their own test equipment can use substrate extraction methods to measure the EC of the extract using an EC sensor. There are three methods available to collect pore water for substrate EC measurements: (1) The 1:2 Dilution Method. With this method, you collect a sample solution from the top of the container by mixing one part of the substrate in two parts of distilled water and allowing substrate to settle at the bottom. (2) The SME Method. With this method, you collect solution by slowly mixing distilled water, stirring the substrate until it is saturated (a glistening water layer appears on top of the substrate). You then filter the solution from the saturated paste.
Electrical Conductivity Meters
Several inexpensive meters are available which can be used to monitor EC and pH separately (with two instruments) or together (with one instrument). Some sensors measure substrate EC from the region that is in contact with the tip of the probe. In this case, it is useful to insert the probe at different depths and measure EC. Exercise care while inserting the probes into the substrate, because they the tip of the electrode can easily be damaged. Other sensors measure the substrate EC in a small cylindrical volume along the length of the probe. These sensors are usually more expensive, but the measurements are more reliable. These sensors are simple to use: the grower can either dip the electrode directly into the solution or pour solution into the cup connected to the electrode.
Limitations of EC Measurement
Greenhouse growers who have their own test equipment can use media extraction methods measuring the EC of the extract. There are four accepted methods for extracting media solution for testing: the pour-through test, 1:2 extraction test, plug squeeze test, and the saturated media extract (SME) test. The pour-through extraction method has several advantages: it samples the solution from the entire root zone, is nondestructive, and can be used with media that contain slow- or controlled-release fertilizers. It also can be used to test the bark, coconut coir, or sphagnum moss media used to grow orchids.
Closed Irrigation Systems
Plants take up different fertilizer ions at different rates from the substrate. For example, plants rapidly take up N, potassium (P), phosphorus (K), and manganese (Mn) ions; however, plants take up calcium (Ca), magnesium (Mg), and sulfur (S) ions more slowly. Because of this differential uptake, a fertilizer solution’s EC in the substrate can be completely different in nutrient composition compared to the initial fertilizer solution supplied to plants. Understanding this can be very important if you have a closed irrigation system where the nutrient solution is recyled.
Managing Substrate EC
If the EC value is high in the substrate, there is no need for further fertilization, but if it is too high, then lower it by leaching, diluting with fresh water to reduce the soluble salts, or by deionization or reverse osmosis. A practical alternative for many growers is to develop an effective leaching program to prevent excessive accumulation of salts in the root zone. Leaching is best achieved on greenhouse crops by applying the proper leaching fraction.
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