I am using coco growing media for my potted plants and plug trays. Nitrate of lime is very expensive in my country of Venezuela. Can I use gypsum to buffer the coco peat? What other options have I got to do that and how?

Thank you for your question. When discussing buffering or buffering agents in a soil, it is usually in reference to the soil’s ability to maintain a consistent pH level, even when an acidic or alkaline solution is added. For most horticulturists, the buffering capacity of a soil or medium is referring to the medium’s ability to maintain a pH level during and after feedings of liquid fertilizers. There are many different organic materials growers can use as buffering agents in a soil mixture. If you take a look at any prepackaged soil, chances are it will contain a few ingredients whose purpose is solely pH adjustment.

One of the most popular buffering agents for horticulture is dolomite lime. This is because many of the organic materials used in soil tend to be acidic and dolomite lime helps to neutralize some of that acidity. Other calcium sources, such as oyster shell, can also be used to help neutralize acidity. Gypsum is a great additive for soils that already have a relatively stable pH. However, it is not the best choice for soils that lean toward acidity. Although gypsum is a source of calcium, it will not raise the pH of the soil like dolomite lime or oyster shell.

The buffering agent you should add to a coco-based growing medium will depend on the coco product itself. Not all coco products are the same. Most horticultural coco products are by-products of the coconut industry; the way the coconuts are processed will affect the quality of the coco by-product material. However, buffering a coco-based soil should be similar to a peat-based soil. You can test the concentration of hydrogen ions in the runoff water to determine the soil’s buffering capacity. The greater the concentration of hydrogen ions, the lower the pH will be. Nutrients are most available to plants when the soil’s pH is between 6-7. If the coco medium tests lower than 6, it may be necessary to use a buffering agent like dolomite lime or oyster shell to raise the pH. If the coco medium tests above 7, it may be necessary to add peat moss or another buffering agent, such as elemental sulfate, which will lower the pH. Gypsum could be used as a buffering agent as long as the coco material itself is fairly balanced in pH. Gypsum is a great buffering agent for maintaining the current pH of the medium but it is not a good choice for making adjustments to the pH of the medium. I hope this answers your question.