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. Electrical 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 pre-plant 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 no other sources, such as the water supply. Electrical conductivity can also be used to monitor the accuracy of a fertilizer injector used for fertigation.
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. Electrical conductivity plays a vital role in plant growth and development, as it serves as an indicator of the availability of essential nutrients in the media, as well as the overall fertility of the media.
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 ECs 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 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.
Limitations of EC Measurement
It is important to understand that there are several limitations to this type of measurement. First, we are not actually measuring the amount of individual nutrients (e.g., nitrogen, phosphorous, potassium, and so on), but rather we are taking an indirect measurement of an effect that is caused when salts are dissolved in water. Each fertilizer material has a different Salt Index (Chapter 22, Fertilizers for Greenhouse Crops). The fertilizer salt index is a measure of salt concentration induced in a soil solution. This means that different mixtures of dissolved compounds will produce different EC readings. Not all dissolved salts are fertilizers.
Closed Irrigation Systems
Plants take up different fertilizer ions at different rates from the substrate. For example, plants rapidly take up nitrogen (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 the grower has a closed irrigation system where the nutrient solution is recycled.
Managing Substrate EC
If the reading is low, this is an indicator that the crop needs some supplementation of nutrients by increasing the rate of fertilization. Likewise, 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
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