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.