As we have previously discussed the three primary nutrients for plants (nitrogen, phosphorous, and potassium), the next category of plant nutrients are the ‘secondary’ nutrients. This list consists of calcium (Ca), magnesium (Mg), and sulfur (S).
The main function of calcium in plants is to provide structural support to cell walls. Calcium serves as a secondary messenger when plants are physically or biochemically stressed. I bet many home gardeners have had blossom end rot on your tomatoes. This condition results from a lack of calcium and is exaggerated by alternating periods of water stress. Rapid early growth of the plants can cause the rot because the calcium is needed by the tomatoes when they are actively growing and the plants may not be able to take up sufficient calcium quickly enough through the roots.
To remedy this rotting problem, you likely used the “tomato rot stop” spray from the local garden center. This product is a liquid calcium that can be sprayed on the plant as a foliar fertilizer and will often fix the problem.
However, calcium, as a nutrient, should not be confused with or given the same credit as lime. We add lime to soils to reduce acidity and to provide calcium over time. All liming materials are inorganic materials containing calcium. In our region we add lime (crushed limestone) which is calcium carbonate (CaCO3). As it breaks down and reacts in the soil, lime both reduces the acidity and provides the needed calcium to the soil.
Magnesium is crucial to the photosynthesis process as well as activating several plant enzymes. Magnesium is easily added to the soil with dolomitic lime or other soluble sources when the pH is correct and lime is not needed.
In plants, a common diagnosis for magnesium deficiency is yellowing between the leaf veins. The technical name for this is interveinal chlorosis. Cattle can be affected with a magnesium deficiency by a condition called grass tetany. Stockmen may see this when cattle are grazing rapidly growing grass deficient in magnesium, often in the spring.
The last of the secondary nutrients is sulfur (S). In the soil, plants primarily absorb this nutrient in the form of a sulfate ion, SO4. Sulfur is major building block in the chlorophyll process and with the development of proteins.
There are many sources of fertilizers that can add sulfur: ammonium sulfate, potassium sulfate, gypsum, and zinc sulfate. Adding sulfur, depending on the source, can certainly acidify the soil. It takes three pounds of lime to offset the acidification you get from adding just one pound of pure sulfur. This could provide a real conundrum when adding necessary sulfur to soils that are in the correct pH range.
Older gardeners have shared with me that they have used Epsom Salts as a fertilizer. I’ve heard several say, “that’s what my grandparents used in their garden.” And yes, Epsom salt is magnesium sulfate (MgSO4) – a natural mineral that would seem to solve two of the three secondary nutrients. If you read some on of the online social media, it would seem it could save the entire garden!
But to be clear, this salt will not help blossom end rot. As mentioned earlier, that is from a lack of calcium and exacerbated by soil moisture fluctuations. In fact, adding Epsom salts to calcium deficient plants with this problem will make it even worse.
And if you are lacking in magnesium or sulfur, research-based information will not encourage you to use Epsom salt. I know that I have never seen a university’s soil report list Epsom salt as a solution.
Next time, we’ll wrap up this series as we look at the micronutrients. There are several and they are certainly needed in very small amounts.
Finally, if you are waiting for any recommendation on how much fertilizer to put out on your yard, garden, or field, you won’t hear any from me. To get it right, really correct, you’ll need to fill out a soil sample. Texas A&M and many other land-grant universities have a form online that you can download and print, or you can easily get the form from your local Extension office.
