The function of limestone in growing media

Limestone is commonly used in the manufacturing of growing media. It is mostly added to offset the acidity of components such as peat moss and pine bark, and to bring a growing medium's pH up into the 5.5-6.5 range, which is ideal for most crops.

Greenhouse growers know that in order to achieve optimum nutrient availability for plant growth, the growing medium's pH for most crops should be in the 5.5-6.5 pH range. The standard base components in most soilless media, peat moss and composted pine bark, have a pH in the range of 3.5-5.0.

This is too low for most crops and requires the use of limestone to bring the growing medium's pH up into the ideal range.

The quantity and choice of limestone used to increase the growing medium's pH is a function of several factors, including:

the growing medium's components;
the type of limestone;
the particle size of the limestone used.

When the growing medium is in use, the alkalinity of the irrigation water as well as the specific crop grown will also affect the growing medium's pH; therefore, the pH must be monitored by the grower. A soilless media manufacturer only has control of the starting pH and some short-term pH stability based on the limestone source used.

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