Secondary Nutrients

Factors Affecting Increased Need for Calcium, Magnesium and Sulfur

As yields increase, it's now more important than ever to ensure our fields are receiving adequate levels of the 17 essential nutrients. Some of the factors responsible for this increased need of calcium, magnesium and sulfur follow.


High Crop Utilization of Calcium, Magnesium and Sulfur

Although calcium, magnesium and sulfur are considered secondary elements, they are just as important for plant nutrition as any of the other 14 essential plant nutrients.

Comparison of Total Calcium, Magnesium, and Sulfur Crop Uptake

Nutrients Taken Up, lb/acre

Increased Use of Higher-Analysis Fertilizer Materials

To produce high analysis bulk blended or fluid fertilizers, magnesium- and sulfur-free fertilizer materials are often used. The nutrient content of these materials – Diammonium Phosphate, Urea, Ammonium Nitrate, Nitrogen, Phosphoric Acid and Muriate of Potash – clearly indicates the lack of sulfur and magnesium.

Nutrient Content of High-Analysis Fertilizer Materials

Increased Crop Yields

Crop yields have dramatically increased during the past ten years. Corn producing at 200 bu/acre will utilize about 65 lb/acre of magnesium and 33 lb/acre of sulfur. In contrast, when the corn yield is 120 bu/acre, the magnesium and sulfur utilization drops to 30 and 20 lb/acre, respectively.

Decreased Use of Sulfur-Containing Pesticides

In the past, farmers could rely on insecticides and fungicides used for controlling insects and disease in crops as a source of sulfur. Today, many of these insecticides and fungicides have been replaced by sulfur-free compounds.

Government Restrictions on Emissions to Atmosphere

Government regulations have improved the air quality. We can no longer rely on high sulfur dioxide (SO₂) amounts in the atmosphere. Restrictions on coal-burning furnaces and the removal of sulfur from natural gas used in home heating and industry, have affected sulfur levels. As well as, catalytic converters in new vehicles removing sulfur from sulfur-containing gasoline. All these factors result in less sulfur being returned to the soil by rainfall. (map of sulfur in the 80s vs now)


Calcium is a low-key essential nutrient that carries a heavy load in plant growth. Too often, it takes a backseat as soil fertility programs are developed for many high-yield and high-quality crops. Peanut and tomato growers are probably exceptions in their emphasis on good calcium nutrition.

Functions of Calcium in Soil

In soil, calcium replaces hydrogen (H) ions from the surface of soil particles when limestone is added to reduce soil acidity. It is essential for microorganisms as they turn crop residues into organic matter, release nutrients, and improve soil aggregation and water holding capacity. Calcium helps enable nitrogen-fixing bacteria that form nodules on the roots of leguminous plants to capture atmospheric nitrogen gas and convert it into a form that plants can use.

Functions of Calcium in Plants

Calcium, along with magnesium and potassium, helps to neutralize organic acids, which form during cell metabolism in plants. Calcium also plays a role in other key plant functions:

  • Improves the absorption of other nutrients by roots and their translocation within the plant

  • Activates a number of plant growth-regulating enzyme systems

  • Helps convert nitrate-nitrogen into forms needed for protein formation

  • Is needed for cell wall formation and normal cell division

  • Improves disease resistance.

Calcium Deficiency

Calcium deficiencies are most likely to occur in acid, sandy soils from which calcium has been leached by rain or irrigation water. It may also occur in strongly acid peat and muck soils where total calcium is low.

Calcium deficiency is not likely for most crops when the soil is properly limed to adjust soil pH to optimum levels for crop production. As soils become more acidic, crop growth is often restricted by toxic soil concentrations of aluminum and/or manganese — not a calcium shortage. Soil testing and a good liming program are the best management practices (BMPs) to prevent these problems.

Calcium deficiency can be prevented by following several BMPs such as soil testing on a regular basis and correcting soil acidity with proper liming. Balance the plant nutrition program by keeping calcium, potassium and magnesium available in a balanced supply. An overabundance of one can lead to a shortage or uptake (antagonism) of another. Also apply calcium for specific plant functions. For example, calcium applied when peanuts begin to set pods can help improve seed development.

Symptoms of Calcium Deficiency


New leaf growth may slow and leaf tips may stick together. Remember that calcium does not readily translocate within the plant so deficiency symptoms will appear on the new growth.


Slow root development. Roots may develop a dark color and in severe cases the growing point may die.

All photos are provided courtesy of the International Plant Nutrition Institute (IPNI) and its IPNI Crop Nutrient Deficiency Image Collection. The photos above are a sample of a greater collection, which provides a comprehensive sampling of hundreds of classic cases of crop deficiency from research plots and farm fields located around the world. For access to the full collection, you can visit IPNI's website.

Sources of Calcium

A good liming program is an efficient supplier of calcium to most crops. High-quality calcitic limestone is effective when pH adjustments are needed. If magnesium is deficient also, dolomitic limestone may be used, or calcitic limestone may be applied along with a magnesium source such as potassium-magnesium-sulfate. Gypsum (calcium sulfate) provides calcium when soil pH is adequate. Some common sources of calcium are shown at right.

Common Calcium Sources


Energy is required for proper plant growth. Wheat and other crops require magnesium to capture the sun's energy for growth and production through photosynthesis. Magnesium is an essential component of the chlorophyll molecule, with each molecule containing 6.7 percent magnesium. Chlorophyll, the green pigment in plants, is the site where photosynthesis occurs. Without chlorophyll, plants could not manufacture food, and life on Earth would cease to exist.

Magnesium also acts as a phosphorus carrier in plants. It is necessary for cell division and protein formation. Phosphorus uptake could not occur without magnesium, and vice versa. So, magnesium is essential for phosphate metabolism, plant respiration and the activation of several enzyme systems.


Hidden in the heart of each molecule of chlorophyll is an atom of magnesium.

Magnesium in Soils

The Earth's crust contains about 1.9 percent Mg, largely in the form of Mg-containing minerals. As these minerals slowly weather, some Mg is made available to plants. The supply of available Mg has been or is being depleted in some soils through leaching, plant uptake and removal processes.

Magnesium availability to plants is often related to soil pH. Research has shown that Mg availability to the plant decreases at low pH values. On acid soils with a pH below about 5.8, excessive hydrogen and aluminum can influence Mg availability and plant uptake. At high pH values (above 7.4), excessive calcium may have an overriding influence on Mg uptake by plants.

Sandy soils with low cation exchange capacity have a low Mg-supplying power.

Application of high-calcium limestone can aggravate an Mg deficiency by increasing plant growth and increasing the demand for Mg. High applications of ammonium and potassium may also interfere with balanced nutrition through competitive ion effects. If soil test levels are below 25 to 50 parts per million (ppm) – 50 to 100 lb/acre – exchangeable Mg is usually considered low, and Mg application is warranted.

A Rule of Thumb:

For soils with a cation exchange capacity (CEC) higher than about 5 milliequivalents (ME) per 100 grams, it may be desirable to maintain the soil Ca-to-Mg ratio at about 10 to 1. For sandy soils with a CEC of 5 ME or less, it may be desirable to maintain the Ca-to-Mg ratio at about 5 to 1.

Symptoms of Magnesium Deficiency


Magnesium is mobile within the plant and easily translocated from older to younger tissues. When deficiencies occur, the older leaves are affected first.


Loss of color between the leaf veins, beginning at the leaf margins or tips and progressing inward. This can give the leaves a striped appearance.


Leaves may become brittle and cup or curve upward and they may become thinner than normal.


Tips and edges of leaves may become reddish-purple in cases of severe deficiency (especially with cotton).


Low leaf Mg can lead to lowered photosynthesis and overall crop stunting.

All photos are provided courtesy of the International Plant Nutrition Institute (IPNI) and its IPNI Crop Nutrient Deficiency Image Collection. The photos above are a sample of a greater collection, which provides a comprehensive sampling of hundreds of classic cases of crop deficiency from research plots and farm fields located around the world. For access to the full collection, you can visit IPNI's website.

How to Treat Mg Deficiencies

If Mg deficiencies are detected in growing crops through plant tissue analyses, a soluble magnesium source may be applied and watered into the soil by irrigation or rainfall. This will permit root access and plant uptake. Small amounts of Mg can also be applied to growing crops through foliar fertilization to correct or prevent developing deficiencies. The preferred approach is to soil-apply the required amounts of Mg before crops are planted or before they begin active growth.

Sources of Magnesium


Sulfur in Soil

Sulfur is supplied to plants from the soil by organic matter and minerals, but it is often present in insufficient quantities and at inopportune times for the needs of many high- yielding crops. Most S in the soil is tied up in the organic matter and cannot be used by plants until it is converted to the sulfate (SO₄²⁻) form by soil bacteria. That process is known as mineralization.

Sulfate is mobile in the soil, just as nitrate-nitrogen is mobile, and in some soils, it can be leached beyond the active root zone with heavy rainfall or irrigation. Sulfate may move back upward toward the soil surface as water evaporates, except in the sandier, coarse- textured soils that may be void of capillary pores. This mobility of sulfate-sulfur makes it difficult to calibrate soil tests and to use them as predictive tools for sulfur fertilization needs.

Sulfur tends to be held by clay soil particles more than nitrate-nitrogen. When early spring rains occur, soils with a sandy topsoil, but containing relatively high amounts of clay in the subsoil, may have sulfate-sulfur leached out of the topsoil but retained in the subsoil. Therefore, crops grown on these types of soils may show early S deficiency, but as the roots penetrate into the subsoil, the deficiency may disappear. On deep sandy soils with little or no clay in the subsoil, plants will likely respond to sulfur applications.

Sulfur in Plants

Sulfur is part of every living cell and required for synthesis of certain amino acids (cysteine and methionine) and proteins. Sulfur is also important in photosynthesis and crop winter hardiness. In addition, sulfur is important in the nitrate-reductase process, during which nitrate-nitrogen is converted to amino acids.


Sulfur is required for the synthesis of vitamins, and is a constituent of certain amino acids, which are the building blocks from which proteins are created. Without proteins, plants wither and die.

Sulfur Deficiency

In the field, sulfur deficiency and nitrogen deficiency are often easily confused. Symptoms of both deficiencies may appear as stunted plants, with a general yellowing of leaves. Sulfur is immobile within the plant and does not readily move from old to new growth. With sulfur deficiency, yellowing symptoms often first appear in younger leaves, whereas with nitrogen deficiency, the yellowing appears on the older leaves first. In less severe situations, visual symptoms may not be noticeable.

The best way to diagnose a deficiency is with a plant tissue analysis that includes an assay for both sulfur and nitrogen. Sulfur concentrations in most plants should range from about 0.2 to 0.5 percent. Desirable total nitrogen to total sulfur ratios have been considered, and range from 7:1 to 15:1. Wider ratios may point to possible sulfur deficiency, but should be considered along with actual N and S concentrations in making diagnostic interpretations.

When sulfur is deficient, nitrate-nitrogen may accumulate. This can pose significant health threats to grazing ruminants or those consuming hay high in nitrates. When nitrates accumulate in the plant, seed formation can be inhibited in some crops such as canola. Balancing sulfur with nitrogen nutrition is important to both plant and animal health.

Crops such as hybrid bermudagrass, alfalfa and corn that have a high dry- matter production generally require the greatest amount of sulfur. Also, potatoes and many other vegetables require large amounts of S, and have produced best when S is included in the fertility program. Without adequate S fertilization, crops that receive high rates of nitrogen may develop sulfur deficiencies.


Sulfur Sources

Some irrigation waters may contain significant quantities of sulfur. For example, when the irrigation water exceeds about 5 parts per million (ppm) sulfate S, a sulfur deficiency is unlikely. Most fertilizer sources of sulfur are sulfates and are moderately to highly soluble in water. The most important water-insoluble sulfur source is elemental S, which must be oxidized through bacterial action to the sulfate form before it can be utilized by plants. This oxidation is favored by warm soil temperatures, adequate soil moisture, soil aeration and fine sulfur particle sizes. If elemental sulfur is used, it should be incorporated into the soil well in advance of the crop needs.

Sources of Sulfur

SZ, S10 and S15 are trademarks and K-Mag and MicroEssentials are registered trademarks of The Mosaic Company.

Adapted from "The Efficient Fertilizer Use Manual",
Secondary Nutrients chapter by Dr. Cliff Syndor and Dr. Bob Thompson