Correcting Nutritional Disorders in Greenhouse Crops

It is critical to maintain the pH of the media or substrate within a range where it can provide an adequate supply of nutrients to the plants being grown. Greenhouse growers should conduct weekly in-house testing to determine media pH or consider the media's regular evaluation (soil testing) by an accredited commercial laboratory.

Each crop grown in a greenhouse performs best when grown within its optimal pH range. For many greenhouse crops, the desired or target range for a soilless root substrate is 5.8 to 6.2. If the substrate contains soil, the desired or target pH range is 6.0 to 6.6. When media pH falls outside of the optimal range for the crop being grown, nutrient availability or uptake may be adversely impacted.

Growers should monitor the pH of the root substrates or media on a weekly basis when possible. If the media pH starts to creep above the optimal range the grower should consider using an acidic fertilizer (21-7-7 or 20-10-20) or use acid injection to lower media pH. Acidic fertilizers contain a higher percentage of ammoniacal nitrogen or urea-type nitrogen, and under cooler growing conditions these acidic fertilizers may increase the risk of ammonium toxicity in some crops. If the media pH begins to creep too low a grower can switch to a basic fertilizer type (calcium nitrate + potassium nitrate) or by drenching the media or substrate with prescriptive amounts of liquid limestone, hydrated lime, or potassium bicarbonate to increase the media pH.

When rapid corrective actions are needed to adjust the media pH in growing plants, great care must be exhibited by the grower, or plant injury can occur. Iron sulfate, aluminum sulfate, and hydrated lime will burn most plant tissues. Never apply any of these materials directly to the leaves and/or foliage of any plant, or injury may be observed. Iron sulfate, aluminum sulfate, and hydrated lime should only be applied to the media or root substrate. If any plants are contacted by the solution, it is critical to syringe or flush the foliage with clear water to prevent plant injury.

Plant sensitivity to products utilized for corrective pH adjustment can vary, so it is recommended that growers treat a small grouping of plants first to check for phytotoxicity or injury prior to treating an entire greenhouse. Iron sulfate, aluminum sulfate, and hydrated lime will change the media pH quite rapidly, so observe plants for several days after treating the root substrates for any symptoms of phytotoxicity or plant injury. When corrective treatments are applied to adjust media pH, please remember that the pH change may not last very long and that the grower should continue to monitor the media or substrate pH weekly and remain prepared to re-treat the media if the pH creeps outside of the optimal range for the crop being grown.

Lowering Media pH

• There are several options that growers can utilize to lower media pH. Option 1 requires the grower to dissolve 1 to 2.5 pounds of iron sulfate in 100 gallons of water. The resulting solution should be applied to the root substrate as a drench, and all foliage and/or leaves should be rinsed or syringed off with clear water to prevent plant injury. Iron sulfate will increase the media substrate EC level, which could release toxic levels of minor elements from the substrate's exchange sites, heightening plant injury concerns.
• Option 2 may be an easier way for a grower to lower media pH. A grower can consider adding sulfuric acid to clear irrigation water to reduce the pH of the irrigation water to 4.5. The pH-adjusted irrigation water is applied as a drench to the root substrate. Plant foliage should be rinsed or syringed off with clear water to prevent plant injury. Substrate or media pH should be re-tested on a weekly basis, and growers should consider re-treatment if the substrate pH begins to creep outside of the optimal zone for the crop being grown.

Raising Media pH

• If a grower needs to raise media or substrate pH there are several things that a grower can do to raise the pH to the optimal range for the crop being grown. The first option involves the use of flowable limestone as a substrate drench. Most horticulture specialists recommend that growers start with a 1–2-quart rate of flowable limestone per 100 gallons of water. This solution should be applied as a substrate drench, and all leaves and foliage that come in contact with the solution directly should be rinsed to prevent a white precipitate from forming on the leaves and/or plant injury.
• The second option to raise media or substrate pH involves mixing 2 pounds of potassium bicarbonate in 100 gallons of water. The solution should be applied as a substrate drench with at least 30% leaching. After the application has been made, all foliage should be rinsed with clear water to limit foliar residues and prevent plant injury. When growers utilize potassium bicarbonate to raise substrate pH this treatment may provide 933 ppm potassium, but it will not supply calcium or magnesium to the crop being grown. It is recommended that growers restore the nutrient balance by applying a complete basic-type fertilizer using moderate leaching one day after the potassium bicarbonate drench has been made to the root substrate.
• The third option for raising substrate pH can be the most potentially injurious to crops being grown if good care is not exercised by the grower. Mixing one pound of hydrated limestone in 100 gallons of water is recommended. Allow the lime to settle in the container after mixing, and then apply the clear solution to the root substrate. Leaves and/or foliage that come in direct contact with the hydrated limestone and water solution can be injured or burned so it is important that all leaves/foliage be rinsed off after the application has been completed.
• The fourth option requires growers to mix 1 pound of hydrated limestone in a 3–5-gallon plastic bucket filled with warm water. The solution should be allowed to settle out, and the clear portion of the solution should be poured into another plastic bucket. The clear solution can be applied through a fertilizer injector at a 1:100 to 1:128 injector setting. Hydrated limestone is quite caustic and can injure skin and damage metal. Hydrated limestone may also displace ammonium from the root exchange sites of the substrate into the soil solution, which may cause plant injury. If high levels of fertilizer containing ammoniacal nitrogen have been used by the grower it may be prudent to avoid using hydrated limestone for increasing media pH. Leaves and/or foliage of plants contacted with a solution of hydrated limestone and water should be rinsed thoroughly to prevent plant injury.

(Adapted from NC State University Floriculture Research; Nutritional Disorders: Low and High pH by Brian Whipker, James l. Gibson, Paul V. Nelson, and Dharmaligam S. Pitchay, NC State University)