Micronutrients

Micronutrients, including essential vitamins and minerals, play a serious role in maintaining overall health and key physiological functions such as immune response, energy production, bone health, and cognitive development.

MICRO ELEMENT NUTRITION

With the increasing use of soil and plant testing, many soils have identified micronutrient deficiencies. Some of the factors limiting random micronutrient application include:

• High-yielding crops that require the extraction of micronutrients from the soil;
• The increasing use of high-concentration NPK fertilizers with lower levels of micronutrients;
• Advances in fertilizer technology are reducing residual micronutrient applications.
• These factors contribute to a significant increase in the use and requirement for micronutrients to achieve complete and balanced nutrition.

BORON

Boron (B) is present in soil solutions primarily as the anion BO₃⁻³, the form commonly absorbed by plants. Boron, an essential micronutrient that affects membrane stability, keeps the structural and functional integrity of plant cell membranes. Boron deficiency symptoms first appear at growing points, and some soil types are more susceptible to boron deficiency.

COPPER

Copper (Cu) activates enzymes and catalyzes reactions involved in various plant growth processes. Its presence is closely related to vitamin A production and promotes efficient protein synthesis.

IRON

Iron (Fe) is essential for crop growing and food production. Plants absorb iron in the form of ferrous cation (Fe⁺). Iron is a component of many enzymes involved in energy transfer, nitrogen reduction and fixation, and lignin formation.

MANGANESE

Manganese (Mn) plays a vital role in the functioning of plant enzyme systems. It activates several important metabolic reactions and is directly involved in photosynthesis. Manganese accelerates seed germination and maturation and increases the accessibility of phosphorus (P) and calcium (Ca).

MOLYBDENUM

Molybdenum (Mo) is a trace element current in the soil and essential for the synthesis and activity of the nitrate reductase enzyme. Molybdenum is vital for the symbiotic process of nitrogen (N) fixation by Rhizobia bacteria in the root modules of legumes. Given the importance of molybdenum in optimizing plant growth, fortunately, Mo deficiency is relatively rare in most agricultural areas.

ZINC

Zinc (Zn) is absorbed by plants as the divalent cation Zn⁺². It was one of the first micronutrients recognized as indispensable for plants and one of the most common limiting factors for crop yield. Although Zn is only required in small amounts, it is essential for high crop yields.

NICKEL

Nickel (Ni) was extra to the list of indispensable plant nutrients at the end of the 20th century. Nickel plays an important role in nitrogen absorption in plants, as it is a component of the enzyme urease. Without Ni, urea cannot be formed. Plants require it in very small amounts, with the critical concentration being approximately 11pm

CROP RESPONSE TO MICROELEMENTS

Plant requirements for certain microelements vary. The table “Relative Sensitivity of Each Crop to Microelements” shows an assessment of each crop’s relative sensitivity to microelements. The ratings “low,” “medium,” and “high” are used to indicate the relative degree of sensitivity.

APPLICATION WITH MIXED FERTILIZERS

• Micronutrient incorporation into the soil is the most common method for applying them to crops. Recommended application rates are typically less than 4.5 kg/acre (per element), making it difficult to distribute micronutrient sources evenly in the field. Therefore, NPK fertilizers, both granular and liquid, are commonly used as micronutrient carriers.
• Incorporating micronutrients into mixed fertilizers is a convenient application method that provides more even distribution using conventional application equipment. It also reduces costs by eliminating the need to apply them separately.

There are four methods for incorporating micronutrients into mixed fertilizers:

• Incorporation with granular fertilizers: Incorporation during production ensures even distribution of micronutrients in grainy NPK fertilizers.
• Bulk blending with granular fertilizers: Bulk blending harvests fertilizers that provide the recommended rates of micronutrients. Inappropriately, nutrient segregation is common, resulting in uneven distribution. Coating granular fertilizers: Coating granular NPK fertilizers with powdered micronutrients reduces the likelihood of segregation.

Foliar Applications

• Foliar applications are widely used to apply micronutrients, especially iron and manganese, to many crops. Soluble inorganic salts are typically as effective as synthetic chelates in foliar applications, so inorganic salts are often chosen due to their lower cost.
• Suspected micronutrient deficiencies can be diagnosed by foliar applications with one or more micronutrients, but tissue sampling is the most common method for identifying deficiencies throughout the growing season.
• Correction of deficiency indications usually occurs within the first few days, after which an adequate source of micronutrients can be applied to the entire field. To improve adhesion of the micronutrient source to leaves, adding sticky spreading agents to the formulation is recommended.

ADVANTAGES OF FOLIAR APPLICATION

• Application rates are considerably lower than those of soil application. Uniform application is easily achieved.
• The response to nutrient application is almost immediate, so shortages can be corrected during the growing season.

DISADVANTAGES OF FOLIAR SPRAYING

• Too high a concentration of salts in the solution can cause leaf burn.
• Nutrient requirements are often high when plants are small and the leaf area is insufficient for uptake.
• If spraying is delayed until shortage symptoms appear, maximum yield may not be achieved.
• Spraying costs are higher if more than one application is required, unless combined with pesticide spraying.

TRACE ELEMENT APPLICATION RATES

BORON

• Recommended boron application rates are fairly low (0.5 to 2 lb/acre), but they must be followed carefully, as the range between boron deficiency and toxicity is narrow for most plants. Therefore, uniform application of boron throughout the field is critical.
• Boron-containing NPK fertilizers will provide a more uniform application than most blended fertilizers. Foliar applications also provide a fairly uniform application, but are generally more expensive.
• Boron fertilization programs should include soil testing, first to assess the level of available boron and then to determine any residual effects (accumulation). The most common soil test for boron is the hot water solubility test. This test is more difficult to perform than most other soil micronutrient tests, but most soil response data to boron correlate with it.

COPPER

• Recommended copper application rates range from 1.4 to 4.5 kg/acre as CuSO₄ or finely ground CuO. The residual effects of copper applications are very pronounced, with responses lasting up to eight years after application. Because of these residual effects, soil testing is necessary to monitor for potential copper accumulation to toxic stages in soils where copper fertilizers are applied.
• Plant testing can also be performed to monitor copper levels in plant tissues. Copper applications should be reduced or discontinued when available levels exceed the deficiency range.

IRON

• Soil applications of most iron sources are normally ineffective on crops, so foliar applications are recommended. Iron deficiency is treated by applying 3-4% FeSO₄ at a rate of 76-160 liters/acre. The application rate should be high enough to wet the leaves.
• More than one foliar application may be necessary to correct iron chlorosis. To improve foliar adhesion and iron availability, it is recommended to include a binding agent in the application formulation.

MANGANESE

• Recommended application rates range from 2 to 20 lb/acre of manganese, usually in the form of MnSO₄. Application rates of MnO₄ will be similar when applied as a fine dust or in NPK fertilizers.
• Banding manganese sources with acidifying fertilizers provides more efficient use of the applied manganese, as the oxidation rate of the applied manganese to the unavailable tetravalent form (e.g., MnO₂) is reduced.
• For the same reason, applied manganese has no residual effects, so annual applications are necessary. Foliar sprays of MnSO₄ are also used, but application rates are lower than those for soil applications.

MOLYBDENUM

• The recommended application rates for molybdenum are much lower than for other micronutrients, so uniform application is important. Molybdenum deficiency can be corrected by broadcast application of phosphate fertilizers containing molybdenum before planting or to pastures. Soluble sources of molybdenum can also be sprayed onto the soil surface before tillage to ensure even distribution.
• Seed dressing is the most common method for applying molybdenum. Molybdenum sources are applied to the seed with a binding agent or conditioner. This method ensures uniform application and allows the seed to be coated with sufficient molybdenum to provide the required content.

ZINC

• Recommended application rates for zinc typically range from 0.5 to 3.5 kg/acre. Banding or broadcast application methods are used, but foliar applications are also effective. Banding zinc sources with starter fertilizers is a common practice in row crops. Foliar applications of 0.5% ZnSO₄ applied at a rate of 20-30 gallons/acre will also provide an adequate amount of zinc, but multiple applications may be required.
• As with copper, the residual effects of zinc are significant, with results observed for at least 5 years after application. Because of these residual effects, available zinc levels in the soil tend to increase after multiple applications. In many states, recommended application rates for zinc have been reduced due to these residual effects.

Overview

Although needed in small amounts, their impact is enormous: deficiencies can lead to serious health problems, while adequate intake helps prevent disease and promotes optimal bodily function. Eating a balanced, nutrient-dense diet is the most effective way to ensure adequate micronutrient intake. In some cases, medically supervised supplementation may be necessary. Eating foods rich in micronutrients is critical for lifelong health and well-being. Micronutrients occur in small amounts, but are highly beneficial. Prioritizing a nutritious diet and addressing deficiencies promptly are critical steps to maintaining long-term health, vitality, and disease prevention.