How to adjust substrate pH with limestone

ONE CHALLENGE GREENHOUSE GROWERS consistently face is managing substrate pH for containerized plants. Substrate pH affects micronutrient availability in the root zone. If the pH is too high, micronutrients are less available and deficiencies can become visible (Fig. 1). Alternatively, if the pH is too low, micronutrients become too available and toxicities can occur (Fig. 2). While there are several post-planting strategies to alter substrate pH, pre-plant adjustment with limestone affords growers a great opportunity to accommodate species-specific requirements in the greenhouse.

There are a number of factors that influence which type and how much lime should be added. First, determine what your target pH is, based on crop needs. Next, determine what the pH of your un-amended substrate is. When you take your target pH and subtract the pH of your unamended substrate, this will give you the amount you need to increase your pH.

As previously mentioned, plants vary in their pH requirements. Many greenhouse crops grow well at a pH from 5.8 to 6.2. However, there are a number of species that require lower or higher substrate pH values. The “geranium group” is comprised of plants that are efficient at taking up micronutrients and, therefore, grow well at higher pH levels, from 6.0 to 6.6. Alternatively, the “petunia group” grows well when substrate pH is from 5.4 to 6.0, as these species are inefficient at taking up micronutrients.

Sphagnum peat moss is the primary component for most greenhouse substrates. With a pH of 3.0 to 4.0, it is too acidic to use without amending for production. When other components such as perlite, bark and/or vermiculite are added to peat to produce the substrate mix, the pH will still be too low and will then need to be raised so it is within a range acceptable for plant growth. For soilless substrates used in containerized crop production, adjusting the pH of growing substrate is accomplished by adding limestone.

You will also want to take your water quality and fertilizer source(s) into consideration. If your water is high in alkalinity, you can reduce the amount of lime added since the alkalinity will raise the substrate pH over time. Additionally, the different form(s) of nitrogen will affect pH. If you are using a nitrate (NO₃^-)-based fertilizer, this will cause the pH to increase over time and reduce the amount of lime you’ll need to add. Alternatively, ammonium (NH₄⁺)-based fertilizers decrease the pH, increasing the amount of limestone needed.

Once you have determined how much you need to increase the substrate pH, you’ll need to select which type of limestone to use. There are two types of limestone used for substrate pH adjustment- calcitic limestone and dolomitic limestone. Dolomitic limestone [CaMg(CO₃)₂] is the most commonly used limestone, while calcitic limestone (CaCO₃) is also used. Calcitic limestone is generally a faster-acting lime source, whereas dolomitic limestone will also provide magnesium to your plants.

The best way to know how much lime to add is to develop a lime addition curve. Make several small test batches of substrate and add different amount of limestone to each batch. After the substrate has had a chance to equilibrate with the limestone, measure the pH of the different substrates. Using these data, plot the response of substrate pH to the various lime additions. Looking at this graph should give you a very good idea of how much lime you should add to your specific substrate to get the final pH you want.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu