Greenhouse growers know that in order to achieve optimum nutrient availability for plant growth, the growing medium pH should be within 5.5-6.2, depending on the crops grown. The standard components in most growing media, peat moss and composted pine bark have a pH of 3.5 - 5.0. The pH of a growing medium with these components is too low for crops; therefore, limestone has to be added to increase the pH.

The amount and choice of limestone used to increase the growing medium pH is a function of several factors, including the bulk density of the growing medium, components used and pH requirements for the majority of crops grown in the growing medium. Once a crop is planted into the growing medium, factors such as the alkalinity of the water, the fertilizer(s) used as well as the specific crops grown, also affect the growing medium pH and must be monitored and maintained by the grower throughout the crop cycle.

“Powdered dolomitic limestone used in commercial growing medium.”

Variable Starting pH?

A growing media manufacturer only has control over the bulk density, components and the limestone source and particle sizes used. In general, growing media manufacturers will add limestone at a rate which will provide a pH of 5.5-6.2 once the packaged growing medium pH has stabilized in the bag. What is meant by stabilized? Typically a growing medium that is less than one month old with low moisture has an initial pH of 4.5-4.9. The limestone added to the growing medium is in a granular form and must dissolve in the water contained in the growing medium before it can raise the pH to the desired range. The higher the moisture content, the faster it will rise. Once within the range, the true growing medium pH can be determined; however, most growing media are packaged at a moisture content between 45 - 55% by weight and the available water is far too low to activate the lime in a reasonable length of time. The most accurate way to determine the pH of the growing medium is to measure it 7-10 days after planting and thoroughly watering in the crop.

Particle Size of limestone

When selecting a limestone source, the size of the particles dictate how fast the lime will start to adjust the pH of the growing medium and how long it will last. Most growing media manufacturers use powdered limestone that has a texture similar to flour. These fine limestone particles begin dissolving quickly after planting to adjust growing medium pH within a week (if it is wet), and will last for 2-4 months. This correlates well with the bench time for most greenhouse crops. If the limestone particles are coarser and have the texture of salt, the limestone may take up to 2 months to begin adjusting the pH of the growing medium, but will last for over a year. This would not work for most greenhouse crops.

Common Types of limestone

The two most common forms of limestone incorporated into growing media are calcitic and dolomitic limestone:
  • Calcitic limestone is calcium carbonate and it neutralizes acids in the growing medium. It provides some calcium, but almost no magnesium. Calcitic limestone dissolves faster than dolomitic limestone so it quickly adjusts growing medium pH within one week, but has shorter residual of up to 1-2 months.
  • Dolomitic limestone is a combination of calcium carbonate and magnesium carbonate and serves two purposes in the growing medium. Primarily, it neutralizes acids in the growing medium but also provides some additional magnesium and calcium for plant uptake. Dolomitic limestone dissolves slowly in the growing medium resulting in longer term pH adjustment (up to 2-4 months) and buffering to improve pH stability.
Most commercial growing media manufacturers rely solely on dolomitic limestone and vary the particle size to control release rates.

Premier Tech Horticulture however uses a combination of both forms. Together dolomitic and calcitic limestones create a more even and consistent growing medium pH. When using both forms of limestone, better control of pH can be achieved; calcitic limestone quickly releases and adjusts the growing medium pH while dolomitic limestone slowly releases over the course of the crop resulting in limited pH fluctuations during production.

For more information
Premier Tech Horticulture Office
1, avenue Premier
Rivière-du-Loup (Québec)
Canada G5R 6C1
Tel.: +1 418 867-8883
Toll free: +1 855 867-5407