MYTH Series: Gypsum Changes the pH of Growing Media
What material is the most commonly used to raise the pH of a growing medium? The answer is limestone.
However, some believe that gypsum increases the pH of growing media as well. Why? Gypsum is calcium sulfate and it is assumed that calcium causes the pH of growing media to rise. Like gypsum, limestone contains calcium in the form of calcium carbonate. However, it is not calcium that increases the pH of growing media, but rather carbonate, by neutralizing acid (hydrogen ions).
Likewise, calcium coming from gypsum does not increase a growing medium's pH. Individually calcium and sulfate can alter the pH of the growing medium immediately surrounding the roots, but the plant takes up both elements in nearly equal amounts through its roots. The result is a neutral effect on the pH of the growing medium. Gypsum should not be used as a limestone substitute, but as a fertilizer supplement to provide calcium and sulfate, both of which plants need for healthy growth.
Gypsum is ground up rock that has a gritty to powdery feel. Depending on how finely ground it is, it can provide plants calcium and sulfate for 1-2 months and is typically applied at 1-2 lb/yd3 (0.6-1.2 kg/m3) of soilless growing medium. Gypsum is one of the few sources of calcium that does not cause the pH of growing media to rise.
Although there is little change in a growing medium's pH when using gypsum, there are some minor exceptions. In cases where a soilless medium is contaminated with soil, soil-borne bacteria that are introduced into the growing medium can convert sulfur into oxidized forms producing sulfuric acid. This can occur when crops are placed on a soil floor and muddy water wicks up into the container through the drainage holes. It can also occur when muddy water splashes onto the growing medium's surface. These higher bacteria populations can then quickly convert sulfur-based materials indirectly into sulfuric acid, causing the pH of the growing medium to drop.
In conclusion, gypsum, under most circumstances, has little influence on the pH of growing media. If there is a need to increase the pH of a growing medium prior to planting, incorporate limestone, which contains the carbonate ion that actually causes the pH of growing media to rise.
