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| Mg and S |
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| The Sometimes Forgotten Secondary Nutrients Magnesium¡ªSulfur |
Two of nature's products ¡ªmagnesium and sulfur¡ª are very important in the industrial world. Magnesium, in its metallic form, is the lightest of all building or structural metals. Sulfur is just as important industrially. Its many uses range from its use in producing sulfuric acid to a component of protein building blocks.
In addition to being important commercially, calcium, sulfur and magnesium are also vital to plant and animal existence. They are three of the 16 essential plant nutrients.
Table 8.1: Essential Plant Nutrients
| CARBON |
CALCIUM |
BORON |
IRON |
| OXYGEN |
MAGNESIUM |
COPPER |
MOLYBDENUM |
| HYDROGEN |
SULFUR |
CHLORINE |
ZINC |
| NITROGEN |
PHOSPHORUS |
POTASSIUM |
MANGANESE |
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| Figure 8.1 The Law of the Minimum |
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To produce at optimum yields, all crops must have an adequate supply of all of the 16 essential plant nutrients. If one or more is lacking in the soil, crop yields will be reduced even though an adequate amount of the other 13 elements are available. This is somewhat analogous to the fact that a wooden bucket will hold no more water than its shortest stave. Crop yields may be limited by the element that is in shortest supply.
Increased Need for Magnesium and Sulfur
In today's agriculture with the emphasis on higher crop yields, there is an increased need for magnesium and sulfur. Some of the factors responsible for this increased need are:
- Increased use of higher analysis fertilizers.
- Increased crop yields.
- High crop utilization of sulfur and magnesium.
- Decreased use of sulfur containing insecticides and fungicides.
- Government restrictions on sulfur emissions to the atmosphere.
- Many soils are deficient in sulfur and magnesium.
- Increased awareness of sulfur and magnesium needs.
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| Figure 8.1 The Law of the Minimum |
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Plant growth is an energy requiring process. During germination alone, a bushel of wheat seed needs about 900 cubic feet of air and produces the same amount of energy needed by a tractor to plow an acre of land. Magnesium is required by wheat, and all other crops, to capture the sun¡¯s energy for growth and production through photosynthesis. 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 is an essential component of the chlorophyll molecule, with each molecule containing 6.7 percent magnesium. 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.
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. Where Mg is deficient, growers are noticing good responses to fertilization with Mg.
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 a 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.
Although no ideal basic cation saturation range in soil has been scientifically proven at which crop yields are maximized, a rule of thumb may be used to ensure that Mg is not limiting. 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 example, if soil test results show 2000 lb/acre of Ca, the soil Mg levels should be about 200 lb/acre. 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. For example, if a soil has a CEC of 5 ME and contains 800 lb/acre of extractable Ca, the Mg level should be about 160 lb/acre.
Note: In certain forage regions of the U.S., adequate P nutrition is essential for stimulation of adequate Mg uptake by plants and translocation from roots to tops. Low Mg in forages can lead to a condition termed grass tetany or hypomagnesia, associated with low blood serum levels of Mg in cattle. This condition reduces animal performance and sometimes results in death. Beef and dairy producers can help avoid potential Mg deficiency in their animals with adequate P and Mg fertilization.
In contrast, there are other limited areas of the U.S. where certain soils contain much more extractable Mg than K. Fertilization with higher-than-necessary phosphorus rates in these limited areas may induce K-deficiency in crops such as cotton, even if soil test levels indicate that the soil K supply may be adequate. In these exceptional areas, excessive P may stimulate Mg uptake, which can interfere with adequate K nutrition. |
| Sulfur in Plants and Sulfur Deficiency |
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As mentioned above, sulfur is absorbed primarily in the sulfate form (SO4-2) by plants. It may also enter the leaves of plants from the air as sulfur dioxide gas. It 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. Leguminous plants need sulfur for efficient nitrogen fixation. Sulfur is also important in the nitrate-reductase process where nitrate-nitrogen is converted to amino acids.
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.
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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. |
| Plant Deficiency Symptoms |
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Magnesium is taken up by the plant as the divalent cation, Mg++. It is mobile within the plant and easily translocated from older to younger tissues. When deficiencies occur, the older leaves are affected first. The deficiency symptoms may include the following:
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.
As a rule of thumb, most crops have a critical plant tissue Mg concentration of about 0.2 percent. Some species have a higher total requirement than others: forage legumes and grasses, cotton, oil palm, corn, potatoes, citrus, sugar beets and tobacco need lots of Mg. Some varieties and hybrids of crops such as corn, soybeans, lespedeza, cotton and celery may require more Mg than others.
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. |
| Sulfur |
A chain is only as strong as its weakest link. Often overlooked, .... sulfur (S) can be that weak link in many soil fertility and plant nutrition programs. Some of the reasons for the increased observance of sulfur deficiencies and increased sulfur needs were highlighted in the introductory sections of this chapter.
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 it tied up in the organic matter and cannot be used by plants until it is converted to the sulfate (SO4-2) form by soil bacteria. That process is known as mineralization.
Sulfate is mobile in the soil, just as nitrate-nitrogen is mobile, and can be leached beyond the active root zone in some soils with heavy rainfall or irrigation. Sulfate may move back upward toward the soil surface as water evaporates, except in the sandier, coarse textured soils which 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. |
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