Corrective procedures for modifying substrate pH and electrical conductivity

When the pH or substrate electrical conductivity (EC) drifts into unwanted territory, adjustments must be made. Below are the standard corrective procedures used to modify the substrate pH and EC for greenhouse grown crops in soilless substrates. For first time users, please test on a few plants to help you master the procedure.

by Brian E. Whipker, Josh Henry, and W. Garrett Owen

Caladium – typical symptomology of high substrate pH (upper foliage interveinal chlorosis). Photo by: Brian E. Whipker.

Bacopa – more advanced symptomology of elevated substrate pH in which the leaves have an overall yellow coloration. Photo by: Brian E. Whipker.

Gerbera - typical symptomology of low substrate pH induced iron/manganese toxicity on the lower leaves. Photo by: Brian E. Whipker.

1. Low Substrate pH Correction
When iron (Fe) and manganese (Mn) toxicity becomes a problem, adjust the substrate pH by raising it to levels within the recommended pH range. Corrective procedures to raise low pH levels are listed below. Switching to a basic fertilizer when the substrate pH is nearing the lower limit will help stabilize the pH. If the pH is below the recommended range, then corrective procedures will need to be implemented. Flowable lime is one option. Using a rate of 2 quarts per 100 gallons of water will typically increase the substrate pH by roughly 0.5 pH units. Two quarts can be used through an injector. Additional applications can be made if needed. Potassium bicarbonate (KHCO 3 ) can also be applied. A rate of 2 pounds per 100 gallons of water will increase the substrate pH by roughly 0.8 pH units. This treatment will also provide excessive potassium (K) and cause a spike in the substrate EC. A leaching irrigation with clear water is required the following day to restore the nutrient balance (the ratio of K:Ca:Mg ) and lower the EC. As always, remember to recheck your substrate pH to determine if reapplications are needed.

pH Adjustment Recommendations

Flowable Lime

Use 1 to 2 quarts per 100 gallons of water. Rinse foliage.
Avoid damage to your injector by using rates of 2 quarts per 100 gallons of water, or less.
Can split applications.

Hydrated Lime

Mix 1 pound in 3 to 5 gallons of WARM water. Mix twice. Let settle. Decant liquid and apply through injector at 1:15.
Caustic (rinse foliage ASAP and avoid skin contact).

Potassium Bicarbonate (KHCO3)

Use 2 pounds per 100 gallons of water.
Rinse foliage immediately.
Provides 933 ppm K.
Leach heavily the following day with a complete fertilizer to reduce substrate EC and restore nutrient balance.
Rates greater than 2 pounds per 100 gallons of water can cause phytotoxicity!

Read the full study here.