The chemical properties of the media, such as pH and electrical conductivity (EC), have the biggest effect on nutrient availability. Testing labs can provide growers with media pH and EC, as well as water alkalinity readings. Growers can also purchase meters in order to perform on-demand testing on their own. Fertilizer and/or acid-injection programs are then based on the information learned from these tests.
Alkalinity is the key parameter in understanding what your water will do to your media pH over time. Alkalinity is a measure of the buffering capacity of water to neutralize acids. Buffering is an indication of how resistant water is to pH change. The higher the alkalinity, the more upward pressure the water will put on media pH, all other factors being equal. Ideally, alkalinity for container-grown crops falls in the range of 80 to 120 ppm, while plug producers generally are looking for values of 50 to 80 ppm. Water quality should be tested at least once per year.
The most practical and economical way to eliminate high alkalinity in water is to neutralize it with acid. Acid injection is recommended for alkalinity levels higher than 150 ppm. Use caution when handling acid, protecting your skin and eyes from spills or splashes. Always add acid to water and never the reverse to avoid a potentially dangerous chemical reaction. Consult your supplier before beginning an acid-injection program. In cases of borderline high alkalinity, fertilizers can sometimes be used to make the adjustment, such as the Everiss pHLow formulations.
Media pH determines the availability of essential plant nutrients in the growing media, especially the micronutrients. With the exception of molybdenum, most micronutrients are more readily available at a lower media pH. A good range, in most cases, is 5.3 to 6.0. Media pH can also influence plant diseases. It is known that thielaviopsis (black root rot) can be inhibited at pH lower than 5.8, and fusarium can be inhibited at pH higher than 6.5.
Soluble salts in irrigation water are measured by EC and are usually expressed in horticulture using the following units: mS/cm or mmhos/cm. Irrigation water with an EC of greater than 1.0 may be problematic, as it can lead to a buildup of soluble salts in the growing media. Media EC (soluble salts) indicates how much total fertilizer is in the growing media. Crops vary in their tolerance of soluble salts in the media. High EC levels can damage plant roots and lead to root-rot outbreaks, while low EC values indicate the crop may be underfed. Fertilizer manufacturers provide a known EC value per 100 ppm of fertilizer on the bag. With this information, growers can use an EC meter to verify the ppm N delivered by their injectors.
The Testing Methods
The importance of tracking your media pH and EC on a graph is frequently emphasized but, unfortunately, not a consistent greenhouse management practice. You don’t want to wait until a crop displays symptoms of a nutrient toxicity or deficiency to begin tracking this information. A variety of meters are available to perform this testing. Three common methods are used to test media pH and EC: the 2:1 Technique, the Pour-Thru Method and the Saturated Media Extract (SME).
A minimum of six to 10 samples is needed from a crop to give a representative sample. The person responsible for taking samples needs to be consistent from sample to sample. Keeping detailed records concerning sampling done before or after fertilization avoids significant differences in pH and EC readings. For the greatest accuracy, draw samples from an area approximately halfway down the root ball. The volume of media needed for sampling for 4-inch and larger pots is about a tablespoon per pot, or a pinch, in toward the center of the pot. Pool these samples together to make the collective sample. Fill the measuring cup at approximately the same level of compaction that is found in the pots being tested. For plugs and smaller cells, the Squeeze Method, developed by NC State University, is recommended. Consult your supplier for details.
2:1 Technique: This method is quickly and easily done. The following materials will be needed before you begin sampling: distilled water, measuring cup, meter(s) that measure EC and pH, and a glass or plastic container. Two parts distilled water are mixed with one part media. Easy quantities to work with would be 1 cup of distilled water and 1/2 cup of media. Stir vigorously for a few minutes and then let it sit for one to four hours before taking your reading. Use meter(s) to test water on the top after particles settle out. If the meter used requires filtration, coffee filters or cheesecloth can be used. Clean equipment after use and store properly.
Pour-Thru Method: This method has an advantage over the other methods in that no media needs to be collected. Irrigate the crop thoroughly to saturation and wait one hour before testing. Place a clean saucer under the pot to collect leachate. Distilled water needs to be applied to each pot to yield 50 ml (1.7oz) of leachate. A chart is available from the NCSU Floriculture website as a guide to determine the amount of water needed to obtain the 50 ml of extract for different container sizes. Collect the leachate, 50 ml, for your readings. Using your calibrated meter, test extracts as soon as possible, within two hours, as the pH reading can change over time.
Saturated Media Extract Method: This method is more accurate because it provides a more exact representation of field conditions but is more time consuming. Media samples are collected and put into a glass or plastic container. Slowly stir in distilled water until the media takes on a paste consistency with a little moisture glistening on the surface. Let stand for 60 minutes. Filter and then use your meter(s) to test the leachate for pH and EC.
When comparing test results performed by a commercial lab that uses a SME method, comparable 2:1 test results will be 1/2 to 1/3 of those reported by the lab. Prepared by Dr. Douglas Cox at UMass, this chart compares EC results from three methods of media testing.
|EC values associated by media test method|
|0 to 0.03||0 to 0.8||0 to 1.0||Very low|
|0.3 to 0.8||0.8 to 2.0||1.0 to 2.6||Low|
|0.8 to 1.3||2.0 to 3.5||2.6 to 4.6||Normal|
|1.3 to 1.8||3.5 to 5.0||4.6 to 6.5||High|
|1.8 to 2.3||5.0 to 6.0||6.6 to 7.8||Very High|