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.

Nursery, Greenhouse, Turf, and Landscape - Soluble Salts

The following is information problems related to high soluble salts in landscapes, greenhouses, nurseries, and turf.

Each year we see plant damage in turf, landscapes, greenhouses, and nurseries due to high levels of soluble salts in soils or media. High soluble salts can come from a number of sources: deicing materials used in the winter; excessive fertilizer, manure, or compost use; “dumping” of fertilizer salts from certain slow release fertilizers; and irrigation water that is high in salts. Excessive soluble salts can result in reduced growth, loss of vigor, root death, yellowing of leaves, wilting, marginal leaf burn, or plant death. This occurs because soils or media with high salt levels make it more difficult for plants to take up water, or if very high will cause loss of water from roots. Foliage exposed to high salt levels will show symptoms similar to leaf scorch. There is also a potential for certain elements to become toxic in high salt conditions: examples would be sodium or ammonium.

A common problem is over application of fertilizers. This can occur with miscalculation of fertilizer rates or dilutions, poorly calibrated or malfunctioning fertilizer applicators or injectors, or excessive overlapping during application. All soluble fertilizer nutrients (nitrate, ammonium, potassium, calcium, magnesium, chloride, sodium, or sulfate ions), contribute to the soluble salt content of the soil or media. However, some soluble fertilizers have greater chance for salt injury than others. Refer to the salt index of a fertilizer to evaluate the potential to cause salt injury. Certain manures and composts can also contain high salt levels. In greenhouses or nurseries, salt levels often build up in media due to inadequate leaching or poor pot drainage. Dumping of salts from coated slow release fertilizers can also occur in greenhouse and nursery production, especially if fertilizer coatings were damaged in handling or by exposure to excess heat.

If you suspect high salts in soil or media, a soluble salt test should be done (the UD soil testing lab and other soils labs can do this test). Soluble salt content is determined with an electrical conductivity meter using a specified dilution of soil or media. An electrical conductivity meter is a useful instrument for growers, turf professionals and landscapers to have on hand for troubleshooting salt related problems.

Gordon Johnson, Extension Horticulture Agent, UD, Kent County

Greenhouse - High Soluble Salts

High soluble salt levels in greenhouse media can cause plant performance problems or if severe, plant failure. High salt levels usually occur from over fertilization in liquid feeds or excessive release of fertilizer salts from slow release fertilizers. The following is a short article on the subject.

The soluble salt content of the media and irrigation water should be tested frequently throughout the growing process. All soluble nutrients, such as nitrate, ammonium, potassium, calcium, magnesium, chloride, sodium and sulfate, contribute to the soluble salt content of the media. A soluble salts test will alert growers if the salt level starts to creep upward and can also provide an indication of the nutrient status available for plant uptake.

Greenhouse plants vary in their tolerance to salt levels and this tolerance is often dependent upon the growth stage of the plant. Excessive soluble salts can result in reduced growth and loss of vigor, root death, chlorosis and necrosis of leaves, wilting and marginal leaf burn. Excessive soluble salts are generally a result of too much fertilizer present in the soil solution in relation to the plant’s needs. High salts may occur from miscalculation of fertilizer dilutions, poorly calibrated or malfunctioning fertilizer injectors or over-application of fertilizers. Other causes of excessive salt buildup may be inadequate leaching and poor drainage. Excessive soluble salts may be reduced in the media by leaching several times with clear water.

Total soluble salt content is determined with a solu-bridge conductivity meter and is expressed as millisiemen (mS) or millimho (mmho). Millisiemen is the preferred unit for expressing soluble salt measurements. Regardless of the unit, the value is the same. Growers often determine soluble salt content by using one part media to two parts distilled water, allowing the solution to sit idle for up to one hour before taking a reading. Many testing labs determine soluble salt content on the saturation extract.

Soluble salt levels by various test and plant reactions.

Information from "Diagnosing plant problems – don’t forget about pH and soluble salt content" by Steven Gower and Jan Byrne, MSU Diagnostic Services, in the Michigan State University Greenhouse Alert Newsletter, March 4, 2008.

Greenhouse - Comparison of Numbers from Different Types of Greenhouse Media Tests

The following is a chart that shows a comparison of soluble salt numbers from the three common greenhouse media testing methods.

Information from "Current Methods of Greenhouse Media Testing and How They Differ" by Douglas Cox, Plant and Soil Sciences, University of Massachusetts.