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Criteria for Adjusting Lime Content of Growing Media

Growers may occasionally experience high pH problems with their growing media causing iron chlorosis in petunias and calibrachoa; or the pH being too low, causing iron-manganese toxicity in geranium and marigolds. Sometimes growers will adjust the limestone rates of the growing medium to help in these cases, but a better approach is determining why these problems are occurring in the first place. The pH of the growing medium does not mysteriously end up outside the normal range by itself.

Issues with High pH of Growing Medium:  The indicator crops that exhibit high pH problems are bacopa, calibrachoa, dianthus, nemesia, pansy, petunia, snapdragon, verbena, vinca, etc. Growing medium pH above 6.5 can cause iron or manganese deficiency which appears as an interveinal chlorosis in the new leaves. Not all iron and manganese deficiency is caused by high media pH. Often these deficiencies are caused by insufficient application of iron or manganese. Test the crop before committing to a future adjustment in the limestone.

Factors that cause the growing medium pH to rise above the normal range for a crop include:

  • Water alkalinity. Think of alkalinity as the limestone content in the water. Every time water is applied, the growing medium is being limed. If the alkalinity is above 200-250 ppm CaCO3, then acid should be injected to minimize high pH problems with the growing medium.
  • Fertilizer. All calcium-containing or cal-mag fertilizers are potentially basic and will cause the pH of the growing medium to go up. Depending on the water alkalinity, even fertilizer with low to moderate potential acidity may not generate sufficient acid to offset the alkalinity or “limestone” coming from the water.
  • Fertilizer Rate and Frequency. The higher the fertilizer application rate and/or the more frequently that it is applied, the greater influence the fertilizer has on the growing medium pH. Even highly acidic fertilizers, when used at low rates or low frequency, have little influence on the pH of the growing medium. Besides, application of insufficient fertilizer negatively impacts crop quality and produces uneven plant growth


Growing Medium pH Becomes Too Low: When the pH of a growing medium becomes too low, most micronutrients become more available for plant uptake. The most common problem associated with low growing medium pH is iron-manganese toxicity in geraniums, lisianthus, marigolds, New Guinea impatiens and pentas. Factors that can decrease the pH of the growing medium include:

  • Low water Alkalinity: If alkalinity is low, then there is little “limestone” applied to the growing medium at each watering. The pH of the growing medium is then greatly influenced by the fertilizer. If acid is injected, reduce the amount added so the alkalinity can help offset pH drop in the growing medium.
  • Fertilizer: Consider switching to a less potentially acidic or potentially basic fertilizer. Most potentially basic fertilizers are formulated with calcium which is needed since low alkalinity water sources are low in calcium. Controlled release fertilizers are quite acidic and if they release at high rates, growing medium pH often drops. If so, avoid using them or find one with a high nitrate / low ammonium ratio as this will reduce acidity coming from the fertilizer.
  • Fertilizer Rate and Frequency: A fertilizer that is applied at higher rates or as a constant feed has a greater impact on growing medium pH. Using a potentially basic fertilizer at the proper application rate may eliminate problems with low pH of the growing medium.


Adjust Limestone Content of Growing Media. There are times when it is not advisable to change the limestone rate, especially if water alkalinity is within a range of 60-200 ppm CaCO3 or if adjustments suggested above will correct problems. Changes in limestone rates make more sense if your water alkalinity is outside this range.

For more information, contact your Premier Tech Horticulture Grower Services representative.

PRO-MIX® is a registered trademark of Premier Horticulture Ltd.

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