Electrical conductivity (EC) and fertilizer concentration

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Inorganic fertilizers are mixtures of various salts (e.g., calcium nitrate and monopotassium phosphate), and the fertilizer formula on the front of the package (e.g., 13-3-16) indicates the percentage (by weight) of some, but not all, components. The package label should give the percentage of all constituents by weight. A fertilizer solution's electrical conductivity (EC) reflects the amount of dissolved salts but is not specific for any particular chemical (e.g., nitrate). Reasonably priced EC meters are available, can be calibrated, and can indicate when a problem with fertilizer concentration is present. Here are some fertilizer problems that can cause inaccuracy in our fertilizer concentrations.

1. When we purchase a liquid or granular fertilizer, we hope the contents match the label on the container; however, fraud and production errors occur.

2. Liquid fertilizers may have some salts that precipitate out, changing the concentration of dissolved salts in the fertilizer that goes into the final fertilizer solution.

3. Granular fertilizers repeatedly exposed to air can absorb water. The water may not harm the fertilizer but may change its weight and volume, altering the fertilizer concentration when we dose it by weight or volume. Most granular fertilizers contain an indicator that turns blue, pink, or green when enough water has been absorbed, so beginning color change in some of the granules can be a warning.

4. Many granular fertilizers consist of particles of varying size and weight. The container must be thoroughly mixed, and a large enough sample must be taken to obtain the correct balance of salts. A single teaspoon or tablespoon may not reflect the fertilizer formula.

5. We make mistakes in measuring.

Checking for these issues is easy with an EC meter IF you know the expected EC of your fertilizer solution. This value must come from the company that produced the fertilizer. The best vendors tell you on the package the expected EC for a fertilizer mixture at a given ppm nitrogen (N) concentration.

For example, the FirstRays K-Lite label has a table showing that a 100 ppm N solution in pure water should have an EC of 0.71 mS/cm, and the bulk Green Care label for 13-3-15 (“MSU orchid fertilizer for pure water”) notes that a 100 ppm N solution should have an EC of 0.80 mS/cm. If we aren’t using pure water, we need to add the EC of our water to these values to get the target EC for our 100 ppm N fertilizer solution. If you target a lower concentration of N, the relationship is linear, so a 50 ppm N solution should have half the EC.

The fertilizer formula substantially changes the EC. A 2004 paper by Bill Argo from Green Care showed that the EC of a 100 ppm N solution of 15 different fertilizers ranged from 0.07 to 1.2 mS/cm. He also noted “some slight differences between the values of the same formulation from different companies. You should always obtain a fertilizer chart from your manufacturer.” Unfortunately, many fertilizer vendors do not provide an EC chart on the label, which leaves us guessing.

I make enough 10% stock solution of my granular fertilizers to last about a week. I measure the EC of almost every fertilizer solution I make, but I think it would be acceptable to measure only the first time I use each new stock solution. If I am substantially off my target EC, I must be sure the meter is calibrated. Otherwise, something is wrong with the fertilizer in the container, or I incorrectly prepared the stock solution. Less likely is that I added the incorrect amount of 10% stock to my water.
 
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