What We Mean When We Talk about Fertilizers: Calcium, Magnesium, and Sulfur

(News article for July 10, 2021; edited. This article is part of a series. Click to see the introductory article and ones about nitrogen fertilizers, phosphorus and potassium fertilizers, and micronutrients.)

This article will round out our discussion of macronutrients, or elements that plants contain in relatively large concentrations.

Plant-available calcium (Ca) levels in soil are often naturally sufficient for plants. When soil calcium is low, soil pH is often low, too. Applying lime to raise pH increases soil calcium. Calcitic lime is typically 32 to 36% Ca, while dolomitic lime is approximately 22 to 30% Ca.

When calcium is needed but pH is not low, gypsum (calcium sulfate) is one option for increasing calcium levels. Gypsum is generally 19 to 22% Ca.

Calcium nitrate, triple superphosphate, and single superphosphate provide some calcium, as well. When these are applied, they should be used at rates needed to meet nitrogen or phosphorus needs, as applicable. If more calcium is needed, another source should be used.

A calcium-related issue that people commonly encounter is blossom end rot in tomatoes. This can also affect peppers, watermelons, and squash. It’s associated with low calcium in the fruit, but its occurrence doesn’t necessarily mean that soil calcium is low. Blossom end rot can occur because the water supply to plants is irregular or too much ammonium-form nitrogen is in the soil. Both of these affect calcium uptake.

It’s sometimes thought that applying Epsom salt helps prevent blossom end rot, but Epsom salt does not provide calcium. In fact, the magnesium in Epsom salt may compete with calcium for plant uptake and thus lead to more blossom end rot.

When magnesium (Mg) is needed in the soil and soil pH is low, using dolomitic lime – or lime that contains both magnesium carbonate and calcium carbonate – kills two birds with one stone. Dolomitic lime is generally 6 to 12% Mg.

When magnesium is needed but pH isn’t low, there are other options. Some fertilizer suppliers sell a 36% Mg product. Epsom salt (magnesium sulfate heptahydrate) has a lower magnesium concentration (typically about 10%) but is more commonly available.

Sulfur (S) is similar to nitrate-form nitrogen in that it does not tend to remain in soil if not taken up by plants but leaches out with rain or irrigation water. We often see in soil reports that sulfur levels are low.

Sulfur is one of the few plant nutrients that is occasionally applied in the elemental form. Typically, elemental sulfur is used when it’s applied to reduce soil pH, or make it more acidic. It should be used with caution – and with the guidance of a soil test report – to avoid reducing soil pH excessively.

There is often a way to provide sulfur by choosing a nitrogen, phosphorus, or potassium source that contains it. Examples of sulfur-containing fertilizers include ammonium sulfate (22 to 24% S), 33-0-0 (the urea-ammonium sulfate blend; 11 to 12% S), single superphosphate (approximately 11 to 12% S), potassium sulfate (approximately 16 to 18% S), and sulfate of potash magnesia (Sul-Po-Mag or K-Mag, approximately 23% S). Many slow-release fertilizers also contain some sulfur.

When one wants to provide sulfur without adding nitrogen, phosphorus, or potassium, gypsum (approximately 16 to 18% S) can be used to provide it, as well as calcium. Likewise, Epsom salt provides sulfur (approximately 13 to 14%) along with magnesium.

Soil test reports provide information about levels of calcium, magnesium, and sulfur in the soil but do not always indicate how much, if any, should be added to improve soil fertility. If your report indicates that one of these is low in your lawn, garden, or horticultural crop field, let me know, and I can help you determine how to address it.

Let me know if you have questions.

Contact Mary Helen Ferguson.

7/22/2021 6:22:03 PM
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