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What are Plant Micronutrients and Why do they Matter?
Boron affects flowering, pollen germination, fruit set, nitrogen metabolism, and hormone movement. It is required for calcium uptake. Additionally, boron is key to transporting potassium across the cell membrane that regulates leaf stomata, thus it is an essential part of a plant’s water retention system. Boron is immobile in plants. A boron deficiency is characterized by terminal bud necrosis, lateral branch growth, and is followed by lateral bud necrosis. Leaves thicken, curl and become brittle.
Chlorine (Cl–) is the counter balance to potassium (K+) in operating the stomata, and therefore, crucial for their proper functioning. Additionally, it plays an indispensable role in photosynthesis by providing the electrons required to break the bonds holding the water molecule together. Chlorine occurs in soil mostly as chloride salts sand is absorbed by the plant as chloride ions (Cl–). A chlorine deficiency will show itself in younger leaves first with the plant tips wilting and then turning bronze. Leaves will sometimes show mottled chlorosis. In the roots, you will see stunted growth. An excess of chlorine appears in older leaves as leaf margin chlorosis.
Copper is a vital component in enzymes, chlorophyll production, a catalyst for respiration, and metabolism of proteins and carbohydrates. Copper is sometimes tied up and unavailable for plant use because of high organic content or low pH. Copper deficiencies are characterized by terminal leaf bud necrosis, sluggish growth, chlorotic leaves and new growth leaf necrosis.
Iron is involved in so many processes that plants have literally developed a way to prevent it from becoming unavailable. They do this by releasing ions into the soil to lower its pH, ensuring that it does not become unavailable to them. Iron is a component of essential enzymes and proteins; indispensable in nitrogen fixation; a catalyst in the synthesis of chlorophyll; and, a carrier of electrons, which makes it important for respiration and oxidation-reduction reactions (electrons removed from one molecule and given to another molecule). Iron deficiency is characterized by interveinal chlorosis beginning in younger leaves.
Manganese frees up oxygen for photosynthesis by accepting electrons from water and acts as a coenzyme that helps break down carbohydrates. Manganese deficiency is characterized by interveinal chlorosis (green veins and yellow leaves) in new growth. Eventually, leaves turn white and fall off. Manganese may be unavailable due to high soil pH.
Molybdenum is essential in protein and organic phosphorus compound synthesis. Molybdenum is also required for Rhizobia and Frankia bacteria to fix nitrogen in the root nodules of legumes. Molybdenum deficiencies appear as interveinal chlorosis in older leaves, and may be characterized by rolled or cupped leaves and yellow spots. Crucifer leaves become narrow, a condition sometimes called “whiptail”, while cereal glumes will not fill out. The soil pH greatly affects the availability of this micronutrient. If the soil is too acidic, molybdenum reacts with iron and aluminum silicates to form insoluble compounds that are unavailable to plants. Liming the soil will help raise the pH and increases its plant availability.
Zinc is used in the formation of auxins (the main plant growth hormone), chloroplasts, carbohydrates, and proteins. It is also part of oxidation and reduction reactions. Zinc deficiency is characterized by chlorotic older leaves, lack of stem elongation and diminished growth; and, most often occurs in soils with pH above 7.5.