While the “micro” in micronutrients may imply “of little importance,” a deficiency in any macro or micronutrient can significantly lower yield. Boron, copper, chlorine, iron, manganese, molybdenum, nickel or zinc are all as essential as nitrogen or phosphorus, although in comparison, only required to apply at a “micro” rate.
Micronutrients play a key role in many plant functions, from root growth to flower formation and from photosynthesis to fruit set. Deficiencies in any one micronutrient adversely affect plant health, so it’s important to know what’s in your soil, if it is in a form that is available to plants and what nutrient management strategies need to be implemented to ultimately achieve a strong, healthy crop.
A combination of soil and tissue testing provides the most accurate picture. Soil tests uncover what is present in the soil, while tissue tests determine what is being absorbed by the plant. Together, they provide farmers with information to build the most efficient crop fertility program.
In addition to the big three primary nutrients (nitrogen, phosphorus and potassium), there are three secondary nutrients (sulfur, calcium and magnesium) and eight micronutrients (boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc). Not only do these micronutrients play important roles in the bigger picture of plant health, they also benefit soil microorganisms and impact the diets of the humans and animals that consume these crops.
So what do each of these small, but mighty, micronutrients do for the plant and in the soil?
Boron (B): Boron plays a complex role in the metabolism of plants. Besides being important in healthy cell growth, boron is instrumental in the formation of pollen, translocation of sugars and nutrients within the plant and protein synthesis. Reduced fruit, death of terminal buds and stunting of growth are all symptoms of boron deficiency, though symptoms vary by crop.
Copper (Cu): Copper is an important catalyst for many chemical processes within plants, including photosynthesis, nitrogen and carbon metabolism and cell wall formation. Stunted growth and chlorosis of new leaves are signs of copper deficiency.
Chlorine (Cl): Chlorine helps regulate water content within the plant. Chlorine is necessary for the plant to open and close the stomata (pores on the undersides of leaves) to conserve water or release oxygen and other gases. It is also vital to photosynthesis, because it helps split the water molecule. It is uncommon for plants to be deficient in this micronutrient.
Iron (Fe): Iron has many functions within the plant. Nitrogen fixation in legumes, photosynthesis, respiration, defense against diseases, and hormone synthesis all rely on sufficient iron supplies. But iron’s most valuable role is chlorophyll formation. Yellow tissue between green veins on newer leaves (interveinal chlorosis) is a sign of iron deficiency. The leaves will eventually die if the issue is not corrected.
Manganese (Mn): Chlorophyll formation and plant metabolism require sufficient manganese. Like iron, manganese deficiency often shows up as interveinal chlorosis. Manganese aides in photosynthesis and helps plants tolerate stress.
Molybdenum (Mo): Molybdenum helps plants absorb nitrogen and phosphorus, and helps legumes fix nitrogen. As a component of enzymes such as nitrogen assimilating nitrate reductase and the N-fixing nitrogenase enzyme found in legume nodules, plants deficient in molybdenum can appear as nitrogen deficient due to a plant’s reduced capacity to incorporate the nutrient. This will show the same signs of N deficiency, including pale, scorched, deformed leaves and poor overall growth.
Nickel (Ni): Plants require less nickel than any other micronutrient. But like molybdenum, nickel is vital to the uptake and use of nitrogen, and critical for leguminous plants. Although it rarely occurs, nickel deficiency causes leaf yellowing, interveinal chlorosis and death of leaf tips.
Zinc (Zn): Experts believe zinc to be present in more than 1,000 plant proteins, where it helps form indole acetic acid (IAA), one of the main plant growth regulators. Because IAA promotes stem elongation, zinc deficiency often results in reduced plant height. Leaf distortion, interveinal chlorosis and brown spots on the upper leaves are other symptoms.
The amounts of micronutrients needed, and symptoms of deficiencies, vary from crop to crop. And similarities between symptoms, paired with a variety of soil textures and pH levels may make diagnosing a deficiency difficult. To ensure plants get the right nutrients at the right time, consult with a retail agronomist for thorough soil and tissue analysis.