What Affects Greenhouse Media pH?

• Media components contribute significantly to pH. Organic materials are generally more acidic than inorganic materials. Peat moss is commonly used for acidic media.
• Irrigation water quality. Alkalinity (bicarbonates) increase pH. Remember that a water pH of 7.0 can be more alkaline than water with a pH of 8.0.
• Lime. Commonly incorporated into many greenhouse media to increase pH or stabilize pH under conditions that favor a pH reduction.
• Acid injection into irrigation water.
• Crop being grown.
• Fertilizer type used. Fertilizers with nitrate forms of nitrogen increase pH over time. Fertilizers with ammonium and urea forms of nitrogen gradually decrease pH over time.

Increase Media pH during Greenhouse Crop Production

• Eliminate acid injection into irrigation water.
• Use an alkaline-reaction fertilizer type.
• Add potassium bicarbonate at the rate of 13.5 oz per 1,000 gallons of irrigation water. This also supplies 40 ppm potassium. Use in cases of low water alkalinity (bicarbonates).
• Drench with flowable lime. Typical rate is 1-2 quarts per 100 gallons water.

Increase Media pH During Production of Subsequent Greenhouse Crops

• Use alkaline-reaction fertilizer type.
• Increase media incorporation of liming materials.
• Replace peat moss in media with coconut coir, pine bark or vermiculite.
• Add potassium bicarbonate via method previously described.

Decrease Media pH During Greenhouse Crop Production

• Use a stronger acid-reaction fertilizer type.
• Inject acid into irrigation water. This neutralizes alkalinity. Ideal irrigation water pH should be 5.8-6.0 when media pH problem is corrected.
• Drench with aluminum sulfate or iron sulfate. Apply 1-3 pounds per 100 gallons of water. Iron sulfate is first choice, but aluminum sulfate acts slightly faster. Rinse off residue after drenching.

Decrease Media pH During Production of Subsequent Greenhouse Crops

• Inject acid into irrigation water to lower water pH to the 5.8-6.0 range. Best and easiest method.
• Use a stronger acid-reaction fertilizer type.
• Decrease rates of incorporated liming materials. Be sure to have adequate calcium and magnesium available, however.