20.2: Other Nutrients

Additional nutrient and soil chemical issues remain important, although farmers understandably focus on nitrogen and phosphorus, because additions of these nutrients are commonly needed in order to maintain crop productivity, large quantities are normally used, and both have potential for environmental problems, additional nutrient and soil chemical issues remain important. While K deficiency is also fairly common, most other nutrients are not normally deficient. Micronutrient fertilizers generally are required in cases where the micronutrients are naturally unavailable in the soil, or when many years of intensive crop production has reduced much of the natural soil supply. We focus here mostly on the mineral nutrients that are critical for healthy plants, but some trace elements are also important for animal and human health, including zinc, iron, iodine, calcium, magnesium, selenium and fluorine, which need to be supplied through the food chain (soil-plant-animal/human) or added as nutritional supplements.

Overuse of fertilizers and amendments other than N and P seldom causes problems for the environment, but it may waste money and reduce yields. There are also animal health considerations with excess amounts. 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.

As of the writing of this edition, there are discussions around how glyphosate-based herbicides affect micronutrient availability. Glyphosate is the most frequently applied herbicide worldwide and, like soil organic matter, has chelating abilities. It is still an open debate whether this has a significant impact on plant micronutrient availability or affects soil, plant health or human health. However, there is no conclusive evidence that it is overall more harmful than the chemicals it replaces.

Potassium (K) is one of the N-P-K “big three” primary nutrients needed in large amounts, and in humid regions it is frequently not present in sufficient quantities for optimum crop yields. Deficiencies occur more readily when the entire crop is harvested and removed versus the grain only. Unlike N and P, K is more concentrated in stalks and stems that remain in the field as stover/straw if only the grain is harvested, thereby recycling most of the K for the next crop. K is 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 K exerted by the new cation exchange sites (see the next section, “Cation Exchange Capacity Management”), contributes to lower K availability. Problems with low K levels are usually easily dealt with by applying muriate of potash (potassium chloride), potassium sulfate or K-mag (potassium magnesium sulfate, also sold as Sul-Po-Mag or Trio). Manures also usually contain large quantities of K. Some soils have low amounts of CEC, such as sandy and sandy loams low in both organic matter and clay. But if the type of clay has low CEC, such as kaolinitic clays found in the Southeast, low CEC may make it impossible to store large amounts of readily K for plants to use. If a lot of fertilizer K is added at one time—an amount that may be reasonable for another soil—a significant portion may be leached below the root zone before plants can use it. In these situations, split applications of K may be needed. Since most complete organic fertilizers are low in K, organic growers with low CEC soils need to pay special attention to maintaining the K status of their soils.

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

Calcium deficiencies are generally associated with low pH soils and soils with a low CEC. 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 or soils that have been flooded by seawater (see “Remediation of Sodic [Alkali] and Saline Soils”). In general, there will be no advantage to adding a calcium source, such as gypsum, if the soil does not have too much sodium, is properly limed and has a reasonable amount of organic matter. 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 is 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 deficiency is common on coarse texture soils with low organic matter, in part because it is subject to leaching in the oxidized sulfate form (similar to nitrate). 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. Sulfur deficiency is becoming more common in certain regions now that there is less sulfur air pollution, which previously originated from combustion of high-sulfur forms of coal. (Now it is captured in power plant exhaust scrubbers, and the residue is sold as gypsum.) In the Great Plains, on the other hand, irrigation water may contain sufficient quantities of sulfur to supply crop needs even though the soils are deficient in sulfur. And some fertilizers used for other purposes, such as potassium sulfate, potassium magnesium sulfate and ammonium sulfate, contain sulfur. Calcium sulfate (gypsum) also can be applied to remedy low soil sulfur. The amount used on sulfur-deficient soils is typically 15–25 pounds of sulfur per acre.

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 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)

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. Zinc deficiencies are also common in other regions of the world, especially Sub-Saharan Africa, South and East Asia, and parts of Latin America. Applying about 10 pounds of zinc sulfate per acre (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 occur most frequently on sandy soils with low organic matter and on alkaline/calcareous soils. It shows up in alfalfa when it grows on eroded knolls where the topsoil and organic matter have been lost. Deficiencies are common in certain regions with naturally low boron, such as in the Northwest maritime area, and in many regions in other parts of the world. Root crops seem to need higher soil boron levels than do 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. Applications for boron deficiency are usually around 1–2 pounds of boron per acre. No more than 3 pounds of actual boron (about 27 pounds of borax) per acre should be applied 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 on 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 in 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. Reducing plant stressors such as compaction and selecting more tolerant crop varieties are also ways of reducing iron deficiency damage to crops. In addition, research in Minnesota indicates that companion planting a small amount of oats (whose roots are able to mobilize iron) with soybeans reduces iron deficiency symptoms. Manganese and iron deficiencies are frequently corrected by adding inorganic salts in a foliar application.

Copper is another nutrient that is sometimes deficient in high-pH soils. It can also be 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.

High-end fertilizer materials have been developed that combine many macro and micronutrients into a single product that can be applied as seed coatings, leaf sprays (foliar), directly to the soil or through fertigation systems, and they are especially of interest for high-value crops.