MYTH Series: Epsom Salt Decreases the pH of Growing Media

Epsom salt looks like sea salt and also readily dissolves in water. It is a great source of both magnesium and sulfate for plants. Source: www.aquaseasalt.com>

Often, fertilizers that contain sulfate are considered acidic and can easily drop the pH of a growing medium. This is rarely true in soilless growing media. Consider Epsom salt, which is magnesium sulfate. Epsom salt is used by many growers as a fertilizer supplement to provide magnesium and sulfate when they are low in the fertilizer solution.

Often, plants green up after Epsom salt is applied and several growers use it since magnesium and sulfate are low in most water sources and many fertilizers contain insufficient levels of both elements. But does Epsom salt cause the pH of a growing medium to drop?

Plant Physiology 101

Besides the alkalinity of the water, the plant has a significant impact on changing the pH of the growing medium. Fertilizer components disassociate in the growing medium solution into individual elements. In the case of Epsom salt, it dissociates into magnesium (Mg⁺²) and sulfate (SO4^-2).

When the plant absorbs an element through its roots, it must maintain an electrical balance, so when it absorbs an element, it releases an element of equal charge back into the growing medium. Often, the only elements the plant can exchange are hydrogen (H⁺¹), which is acid and can reduce the pH of the growing medium, and hydroxide (OH^-1), which is alkaline and increases the pH of the growing medium.

In the case of Epsom salt, for every magnesium (Mg⁺²) ion taken up by the roots, it gives off two hydrogen (H⁺¹) ions, which can decrease the growing medium's pH around the root. For every sulfate (SO4^-2) ion, the root releases two hydroxide (OH^-1) ions, which raises the pH of the growing medium around the root. So what does this mean?

In most cases, a plant will take up the same number of magnesium ions as sulfate ions so the net result is that there is no change in the pH of the growing medium around the plant root. In almost all cases, Epsom salt has little influence on the pH of the growing medium.

Does Epsom Salt Decrease the pH of Growing Media?

As stated, if a plant acquires the same amount of magnesium as sulfate, the net result is that there is no net change in the pH of the growing medium. There are a few exceptions when it comes to soilless growing media. In cases where a soilless medium is contaminated with soil, soil-borne bacteria are introduced into the growing medium that 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 growing medium through the drainage holes. These higher bacteria populations can then convert sulfates indirectly into sulfuric acid, causing the pH of the growing medium to drop. Unlike gypsum, the amount of sulfate coming from Epsom salt is much lower, so the pH drop, if any, would be little.

In conclusion, Epsom salt has little influence on the pH of growing media. It is simply applied as a fertilizer supplement to provide additional magnesium and sulfate missing from the water and fertilizer.