Electrical Conductivity (EC)

Function:

Soluble salts refer to the total dissolved salts in the root substrate (medium) at any given time and is measured in terms of electrical conductivity (EC). All fertilizer materials contribute to the EC content of the root substrate, most commonly these are nitrates (NO₃), ammonium (NH₄), phosphates (PO₄), potassium (K), calcium (Ca), magnesium (Mg), boron (B), iron (Fe), zinc (Zn), copper (Cu), sulfates (SO₄), sodium (Na), bicarbonate (HCO₃) and chlorides (Cl). Organic materials also contribute to the EC content after they have been changed from an insoluble to soluble form. Nitrogen and potassium are the fertilizer materials that most often contribute to the EC concentration of the root substrate. In some areas, high sodium in the irrigation water can also be a problem.

Deficiency:

At the opposite end of the spectrum, when the EC content of the root substrate is too low, plant growth is stunted from lack of fertilizer.

Smaller growth of poinsettia due to low EC/nitrogen levels.
Click on image for larger version. (image ©Brian Whipker, NCSU)

Excess:

Excess salts accumulate when leaching during irrigation is insufficient, too much fertilizer is applied, or the irrigation water contains a high amount of dissolved elements. Excessively high EC readings are associated with poor plant growth. Plant symptoms often begin on the lower leaves as leaf chlorosis and progress to necrotic leaf tip margins. If the root substrate is allowed to dry down, plants may also exhibit wilting symptoms because of dieback of the root tips, which further inhibit water and fertilizer uptake. High EC has also been linked with the increased incidence of Pythium root rot.

Poor plant growth of poinsettia due to excessive EC / nitrogen levels.
Click on image for larger version. (image ©Roy Larson, NCSU)

Confirm your actual EC levels by conducting a routine root substrate (medium) test.

Monitoring and Management Strategy for Electrical Conductivity (EC) Fertilization Rate:

1.5 to 2.5 mS/cm. Optimal range is 25% to 50% lower with flood irrigation or reduced leaching fertilization programs.

Options:

Preplant:

Incorporation of fertilizer in the root substrate (medium) for a starter charge.

Continual Fertilization:

1. Use a fertilizer that provides N. Examples include calcium nitrate (Ca(NO₃)₂) with potassium nitrate (KNO₃), 20-10-20, 15-5-25, Excel® 15-5-15 Cal-Mag and others.
2. If using calcium nitrate (Ca(NO₃)₂) and potassium nitrate (KNO₃), remember to supply P, Mg, and micro-nutrients to the plants.
3. If using 20-10-20 or 20-20-20, remember to supply Ca and Mg to the plants.

Corrective Fertilization:

Too High EC:

1. Decrease the fertilization rate or the frequency of fertilization.
2. Leaching will reduce the salts level. The root substrate should be irrigated normally, then followed immediately with another irrigation. After this, the root substrate should be allowed to dry to the usual stage, and if further leaching is required, a double-irrigation treatment can be repeated. Check the EC level again to make sure it is within the acceptable range.

Too Low EC:

Increase the fertilization rate or the frequency of fertilization. See target levels listed under nitrogen.

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