Between the macronutrients and micronutrients sit the secondary macronutrients. These essential nutrients are needed in lesser amounts than the primary macronutrients: nitrogen (N), phosphorus (P), and potassium (K), but in greater quantities than the micronutrients. We’re going to shine the light on the mighty middle children of nutrients: calcium (Ca), magnesium (Mg), and sulfur (S).
Considered the queen and king of nutrients, calcium is essential for crop quality and growth. Calcium works as a signaling molecule through calcium channels in cell membranes. It helps in stress tolerance, signaling for immune response, and regulates the opening and closing of stomata.
Calcium teams up with boron in a number of functions. Together, they help move nutrients from roots to other parts of the plant. They strengthen cell walls and membranes, which results in improved crop quality and extended shelf life. Calcium and boron aid in nitrogen fixation and legume nodulation. It’s a fine balance, as too much of either nutrient will inhibit the other.
Calcium is taken up by plants as the ion Ca2+ and transported from the root to the shoot, as plants pull up more water from the roots and transpire out of its leaves. Sometimes deficiencies are caused by soil factors, such as a recent lime addition that has not had enough time to react with the soil and become plant available, or environmental conditions, such as drought. Calcium is an immobile nutrient, so deficiencies are seen in young tissue and fruit: as curling or necrosis (death) of tips and leaf margins and blossom end rot.
Calcium is needed at all times of plant growth. You can foliar spray calcium to prevent or correct a deficiency. For instance, a foliar calcium spray on very small tomato fruits can prevent blossom end rot. Use of calcium in fertigation can also be effective. When a soil test recommendation indicates a need for more calcium and an increase in pH, you can add calcitic lime or other liming substances, such as aragonite, oyster shell flour, and wollastonite, and when magnesium is also needed you can add dolomitic lime. Gypsum can be added when the soil needs more calcium but not an increase in pH. Many other fertilizers such as bone meal, rock phosphate, crab shell, and Harmony contain calcium.
Calcium deficiency in a variety of crops (Barker, UMass).
Magnesium has vital roles throughout a plant, and is influential in crop quality and growth. Magnesium is the central molecule of chlorophyll and has other critical roles in photosynthesis. It is essential in protein biosynthesis, and having adequate amounts of magnesium will help with nitrogen uptake and nitrogen utilization efficiency. Magnesium aids in transportation of carbohydrates from photosynthesis (sugars) from leaves to sinks (locations where more nutrients are needed, such as roots, fruits, and growing tips). Mg inhibits toxicities from heavy metals and aluminum. It is a modulator or cofactor in over 300 enzyme reactions and helps with stress tolerance.
Magnesium roles in plants (Chaudhry et al., 2021).
Plants absorb magnesium as the ion Mg2+. Most magnesium is found in the mineral structure of soil and when released it is mobile and easily soluble in soil water. Although magnesium is a cation (positively charged), negatively-charged soil particles do not hold magnesium tightly, and as a result, magnesium can be leached from the soil with high rainfall or excess irrigation. High potassium levels in soils and plants can inhibit uptake and transport of magnesium by plants. High soil levels of calcium and ammonium (NH4+) can also inhibit the uptake of magnesium. Researchers are actively studying these antagonistic interactions to figure out the cause and provide a solution.
Older leaves show magnesium deficiencies due to magnesium being mobile in plants. Interveinal chlorosis, or yellowing between leaf veins, necrotic (dead) spots on leaves, and stunting are signs of magnesium deficiency.
You can correct a magnesium deficiency with epsom salts (magnesium sulfate) via sidedress, fertigation, or foliar spray. For amending soils, use dolomitic lime if you need calcium and an increase in pH; otherwise use epsom salts or KMag (aka SulPoMag or Langbeinite).
Sulfur is the nutrient needed in the fourth largest amount after the macronutrients. Like calcium and magnesium, sulfur is essential for crop production and quality. It is important in protein and oil production, as well as general metabolism and stress tolerance. Cysteine and methionine are sulfur-containing amino acids, the building blocks of proteins, and play a main role in many biochemical processes. Iron (Fe) and sulfur form clusters, and those clusters are key in chlorophyll synthesis, nitrate reduction, and other metabolic functions. Plants produce sulfur compounds for helping with stress from other organisms (biotic stress) - think of the spicy and stinky compounds of alliums (alliins) and brassicas (glucosinolates). Sulfur is part of antioxidants and helps with antioxidant defense to abiotic stress, such as drought and high temperatures.
Organic matter is the largest pool of sulfur, but only a small portion of sulfur from organic matter is available at one time. Through weathering or microbial breakdown of organic matter, sulfur is released. Sulfur/sulfate-containing fertilizers are more readily available sources of sulfur. Sulfur is only taken up by plants from the soil in the sulfate (SO42-) form. Sulfur-specific transporters take in sulfate from the soil and transport it within the plant to where it’s needed. Extra sulfate is stored in vacuoles (storage structures in plant cells). In the plant, sulfate is transformed into other chemicals before being used. A small amount of gaseous sulfur, (hydrogen sulfide or H2S), can be absorbed from the vegetative portion of the plant.
Sulfur has not been mentioned much in the past but deficiencies have become more common as a result of a decrease in atmospheric deposition of sulfur dioxide from industrial sources, following the enactment of the Clean Air Act in the 1970s, and less use of manure and sulfur-containing fertilizers. Sulfur deficiency is seen first in the younger leaves, as the nutrient has poor mobility within the plant. Chlorosis or yellowing of the entire leaf is the most common sign. In corn, striping of the leaves is a common symptom of S deficiency. Stunting and a decrease in yield and grain can also occur from a sulfur deficiency.
For in-season sulfur corrections, use a sulfate-containing fertilizer: Epsom salt (magnesium sulfate), potassium sulfate, or any sulfate form of a micronutrient if those micronutrients are needed as well. These can be applied via sidedress, fertigation or foliar sprays. When soil test results show a need for sulfur, add elemental sulfur when a decrease in pH is needed, or if no pH adjustment is necessary, use any sulfate-containing fertilizer that provide other required nutrients, such as gypsum (Ca), epsom salt (Mg), and potassium sulfate (K). When using elemental sulfur, add in the spring or when the soil is warm; microbes are required to convert it to the plant-useable sulfate form.
Crop production, yield, and quality are all affected by calcium, magnesium, and sulfur. Even though these nutrients are required in smaller amounts than the primary nutrients, they are equally as important in contributing to a successful crop. To assure adequate levels, test your soil and add fertilizer and amendments according to recommendations.
If you have soil test results and recommendations and would like assistance, please feel free to reach out to us:
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