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What are Plant Macronutrients and Why do they Matter?
Nitrogen is usually the limiting factor in plant growth. This is because nitrogen is a fundamental component of amino acids, the structural building blocks of proteins responsible for regulating metabolic processes. Nitrogen plays a key role in the chlorophyll molecule (C55H72MGN4O2) and photosynthesis. Thus, an adequate nitrogen supply is characterized by dark green and vigorous vegetative growth. Conversely, a nitrogen deficiency is expressed through diminished plant growth; chlorosis (yellowing) starting at the older leaf tips and moving down the middle of the leaf; as well as, fibrous, stiff stems. Although nitrogen requirements differ from plant to plant, the highest inputs are required during vegetative growth.
Ammonium (NH4+) and Nitrate (NO3–) are the two plant usable forms of nitrogen. Ammonium is positively charged, and therefore, resistant to leaching (downward movement through the soil profile). Soil microorganisms go to work on Ammonium and convert it into Nitrate. Nitrate will readily leach below the roots zone due to its negative charge.
In natural systems, nitrogen is recycled through plant material decomposition. In a crop system, this nitrogen is removed during harvest and must be supplemented.
Phosphorus holds a key position in cellular processes, total energy transfer of the plant, and is a building block of cell walls and DNA. Thus, phosphorus promotes root, flower, fruit and seed development and growth. A large percentage of the soil’s phosphorus is unavailable for plant uptake and use, particularly in soils with high pH above 7. Adequate phosphorus is characterized by increased plant growth, stem strength, root growth and early crop maturation. A phosphorus deficiency is characterized by a reduction in plant growth, delayed maturation, small fruit set, and purple coloring of older leaves. Phosphorus requirements are highest during the flower portion of the growth cycle.
Potassium is the only essential nutrient that is not a structural component of the plant. Instead potassium functions as a regulating chemical, controlling the flow of water and carbon dioxide into and out of the plant. This is why a plant low in potassium will often wilt. In addition, potassium plays a key role in many physiological processes improving overall plant vigor, root growth, and acts as a ripener, or finisher, at the end of the fruits life. A potassium deficiency is characterized by reduced plant vigor and weakened stems. As a deficiency progresses, older leaves become chlorotic at the margins and between veins, followed by burning that starts at the edges and moves inward, and further followed by upward curling. Potassium is needed in larger quantities during the flowering stage of the growth cycle.
Calcium builds strong cell walls and thus is very important for structural stems and plant support. Additionally, calcium is responsible for transporting other nutrients, such as magnesium, across the cell membrane and into the plant. Excess potassium can inhibit the uptake of calcium creating an artificial calcium deficiency. Calcium deficiencies appear in the actively growing roots, shoots and young leaves, and the result is malformation in the affected area. For more on calcium, check out http://www.ageoldorganics.com/calcium-ca-essential-for-plant-growth-and-development/ .
Magnesium is a key component of the chlorophyll molecule and thus plays a vital key role in photosynthesis. A magnesium deficiency will inhibit chlorophyll production, and is characterized by interveinal chlorosis in older leaves first.
Sulfur is a component of important amino acids that influences the structural components and metabolic processes (photosynthesis and respiration) within plant cells. Thus, it fulfills an important role in the water equilibrium both in the plant and in the soil, and its requirements are highest during its vegetative growth phase. Additionally, it is the nutrient responsible for pungency in plants such as garlic, onions, crucifers and radishes. Although sulfur deficiency is rare, it manifests itself as chlorosis in newer growth that is spindly in nature.
*Silicon (Si), although not technically considered a macronutrient, those plants that do require Silicon require it in very large quantities. Silicon is key to increasing drought tolerance by creating a waterproof layer in the epidermal cells. Silicon thickens this barrier and helps the plant retain more water. In addition, silicon’s ability to form strong bonds helps the plant ward off insects and block pathogens. Silicon deficiency appears in younger foliage.