Building Soils for Better Crops, Third Edition

Other Fertility Issues: Nutrients, CEC, Acidity, and Alkalinity


The potential available nutrients in a soil, whether natural or added inmanures or fertilizer, are only in part utilized by plants . . .



Although farmers understandably focus on nitrogen and phosphorus—because of the large quantities used and the potential for environmental problems—additional nutrient and soil chemical issues remain important. Overuse of other fertilizers and amendments seldom causes problems for the environment, but it may waste money and reduce yields. There are also animal health considerations. For example, excess potassium in feeds for dry cows (cows that are between lactations) results in metabolic problems, and low magnesium availability to dairy or beef cows in early lactation can cause grass tetany. As with most other issues we have discussed, focusing on the management practices that build up and maintain soil organic matter will help eliminate many problems or at least make them easier to manage. The risk for sulfur deficiency varies with the soil type, the crops grown on the soil, the manure history, and the level of organic matter in the soil. A deficiency is more likely to occur on acidic, sandy soils; soils with low organic matter levels and high nitrogen inputs; and soils that are cold and dry in the spring, which condition decreases sulfur mineralization from soil organic matter. Manure is a significant supplier of sulfur, and manured fields are not likely to be S deficient; however, sulfur content in manure can vary. —S. PLACE ET AL. (2007)

Potassium (K) is one of the N-P-K “big three” primary nutrients needed in large amounts, but in humid regions it is frequently not present in sufficient quantities for optimum yields of crops. It’s generally available to plants as a cation, and the soil’s cation exchange capacity (CEC) is the main storehouse for this element for a given year’s crop. Potassium availability to plants is sometimes decreased when a soil is limed to increase its pH by one or two units. The extra calcium, as well as the “pull” on potassium exerted by the new cation exchange sites (see the next section, “Cation Exchange Capacity Management”), contributes to lower potassium availability. Problems with low potassium levels are usually dealt with easily by applying muriate of potash (potassium chloride), potassium sulfate, or sul-po-mag or K-mag (potassium and magnesium sulfate). Manures also usually contain large quantities of potassium.

Magnesium deficiency is easily corrected if the soil is acidic by using a high-magnesium (dolomitic) limestone to raise the soil pH (see “Soil Acidity”). If K is also low and the soil does not need liming, sul-po-mag is one of the best choices for correcting an Mg deficiency. For a soil that has sufficient K and is at a satisfactory pH, a straight Mg source such as magnesium sulfate (Epsom salts) would be a good choice.

Calcium deficiencies are generally associated with low pH soils and soils with low CECs. The best remedy is usually to lime and build up the soil’s organic matter. However, some important crops, such as peanuts, potatoes, and apples, commonly need added calcium. Calcium additions also may be needed to help alleviate soil structure and nutrition problems of sodic soils (see “Remediation of Sodic (Alkali) and Saline Soils”). In general, if the soil does not have too much sodium, is properly limed, and has a reasonable amount of organic matter, there will be no advantage to adding a calcium source, such as gypsum. However, soils with very low aggregate stability may sometimes benefit from the extra salt concentration and calcium associated with surface gypsum applications. This is not a calcium nutrition effect but a stabilizing effect of the dissolving gypsum salt. Higher soil organic matter and surface residues should do as well as gypsum to alleviate this problem.

Sulfur deficiencies are common on soils with low organic matter. Some soil testing labs around the country offer a sulfur soil test. (Those of you who grow garlic should know that a good supply of sulfur is important for the full development of garlic’s pungent flavor.) Much of the sulfur in soils occurs as organic matter, so building up and maintaining organic matter should result in sufficient sulfur nutrition for plants. Although reports of crop response to added sulfur in the Northeast are rare, it is thought that deficiencies of this element may become more common now that there is less sulfur air pollution, originating mainly in the Midwest. Some fertilizers used for other purposes, such as sul-po-mag and ammonium sulfate, contain sulfur. Calcium sulfate (gypsum) also can be applied to remedy low soil sulfur. The amounts used on sulfur-deficient soils are typically 20 to 25 pounds of sulfur per acre.

Zinc deficiencies occur with certain crops on soils low in organic matter and in sandy soils or soils with a pH at or above neutral. Zinc problems are sometimes noted on silage corn when manure hasn’t been applied for a while. Zinc also can be deficient following topsoil removal from parts of fields as land is leveled for furrow irrigation. Cool and wet conditions may cause zinc to be deficient early in the season. Sometimes crops outgrow the problem as the soil warms up and organic sources become more available to plants. Applying about 10 pounds of zinc sulfate (which contains about 3 pounds of zinc) to soils is one method used to correct zinc deficiencies. If the deficiency is due to high pH, or if an orchard crop is zinc deficient, a foliar application is commonly used. If a soil test before planting an orchard reveals low zinc levels, zinc sulfate should be applied.

Boron deficiencies show up in alfalfa when it grows on eroded knolls where the topsoil and organic matter have been lost. Root crops seem to need higher soil boron levels than many other crops. Cole crops, apples, celery, and spinach are also sensitive to low boron levels. The most common fertilizer used to correct a boron deficiency is sodium tetraborate (about 15% boron). Borax (about 11% boron), a compound containing sodium borate, also can be used to correct boron deficiencies. On sandy soils low in organic matter, boron may be needed on a routine basis. Apply no more than 3 pounds of actual B (about 27 pounds of borax) per acre at any one time—it can be toxic to some plants at higher rates.

Manganese deficiency, usually associated with soybeans and cereals grown on high-pH soils and vegetables grown on muck soils, is corrected with the use of manganese sulfate (about 27% manganese). About 10 pounds of water-soluble manganese per acre should satisfy plant needs for a number of years. Up to 25 pounds per acre of manganese is recommended if the fertilizer is broadcast on a very deficient soil. Natural, as well as synthetic, chelates (at about 5% to 10% manganese) usually are applied as a foliar spray.

Iron deficiency occurs in blueberries when they are grown on moderate to high-pH soils, especially a pH of over 6.5. Iron deficiency also sometimes occurs on soybeans, wheat, sorghum, and peanuts growing on soil with a pH greater than 7.5. Iron (ferrous) sulfate or chelated iron is used to correct iron deficiency. Both manganese and iron deficiencies are frequently corrected by using foliar application of inorganic salts.

Copper is another nutrient that is sometimes deficient in high-pH soils. It is also sometimes deficient in organic soils (soils with 10–20% or more organic matter). Some crops—for example, tomatoes, lettuce, beets, onions, and spinach—have a relatively high copper need. A number of copper sources, such as copper sulfate and copper chelates, can be used to correct a copper deficiency.

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