Greenhouse and Nursery - Monitoring Soluble Salts

The following is a good article on monitoring soluble salts in greenhouse or nursery media.

Once again another growing year is upon us and I want to remind all of you who are growing plants in containers the importance of routine monitoring for soluble salts and pH. It is the easiest way to check the fertility status for your plants and can save you $’s when you catch problems early. While electrical conductivity (EC) does not tell you exactly which nutrients are available, it can help you make decisions about your fertility and irrigation programs. Additionally, a pH test will aid in determining nutrient availability and will alert you as to whether potential problems may show up.

The PourThru procedure is easy. You simply wait about an hour after irrigation when containers are at water holding capacity. Simply find three to five containers randomly from a block or irrigation zone within the nursery and tilt them at an angle sufficient enough to have water come out the bottom of the container. Sample each container individually. Collect the released water from each container in a small cup. You need only enough water to cover the EC and pH probes for an accurate reading. If you cannot retrieve a sample, simply pour a small amount of water evenly over the top of the container. You need just enough to get that sample out the bottom of the pot. You are only trying to displace the water at the bottom of the pot so you may only need a few ounces. You could use distilled water, but it’s not necessary.

North Carolina State University recommendations from Dr. Ted Bilderback suggests EC’s around 0.5 dS/m if you are using controlled release fertilizer and around 1.0 dS/m if you are using soluble fertilizer through your irrigation. Don’t forget to check the EC’s of your irrigation water and subtract that value from your PourThu EC readings. For example, if you have an EC reading of 0.5 dS/m and your irrigation water is 0.2 dS/m, then your actual reading is 0.3 dS/m, or the EC contribution of the fertilizer. If you are lower than 0.2 dS/m, you may be over-watering or you may need to apply more fertilizer. However if your EC’s are far above the suggested value, you may need to irrigate. A sustained level of 4.0 dS/M will cause root damage. You will need to irrigate and dilute the salt build-up in your containers. An EC of 1 dS/m in the morning can easily jump to 4 dS/m in the afternoon as plants pull more water than nutrients (salts) out of the potting medium, especially during high temperature days. Remember to try and keep your leaching fractions (fraction of applied water that comes out the bottom of the pot) to less than 15% whenever possible.

Remember that the following EC units all mean the same thing: mmhos/cm (milli-mhos per centimeter) = mS/cm (milli-siemens per centimeter) = dS/m (deci-siemens per meter) = 1000 μS/cm (micro-siemens per centimeter).

PourThru’s should ideally be performed weekly or even biweekly, however that may be impractical. At the very least, test before you top dress with controlled release fertilizer (CRF). If your EC’s are in range you may not need to fertilize. Also, a good rule of thumb is to monitor during hot dry periods.

Don’t forget to keep an eye on your pH. The pH range for soilless potting media is between 5.4 and 6.3 for optimum nutrient availability. Some say that since you apply fertilizers in soluble form, pH is not very important for nutrient availability in organic potting media, but monitoring for large swings in pH is wise, regardless.

Electrical Conductivity and pH meters can be purchased through nursery supply businesses, catalogs and via internet orders where you can shop for the best prices. You can even purchase combination meters that read both EC and pH. You want an EC meter with a range of 0 to greater than 4 dS/M or 0 to 4000 μS/cm. It is extremely important to purchase pH and EC standard solutions to calibrate your meters and an electrode holding solution for the pH electrode.

Monitoring your container root zone EC and pH goes a long way for preventing nutrient related problems including deficiencies and toxicities. Even pathogen and insect infestations may be kept in check, all by keeping your roots healthy.

Article by Andrew Ristvey, Extension Nursery Specialist, University of Maryland.