Functions, Mode of Absorption and Deficiency Symptoms of Macronutrients

Macronutrients are required in the largest quantities and each has a well-defined role, absorbed form, and deficiency picture. Nitrogen is needed in the greatest amount of all - it is a component of proteins, nucleic acids, amino acids, vitamins, hormones, alkaloids, chlorophyll and cytochrome, and is absorbed mainly as nitrate (NO3-); its deficiency causes chlorosis, stunted growth and anthocyanin (purple pigment) formation. Phosphorus is a constituent of cell membranes, proteins, nucleic acids, ATP, NADP, phytin and sugar phosphates, absorbed as H2PO4- and HPO4(2-) ions; deficiency causes stunted growth, anthocyanin formation, necrosis, reduced cambial activity, and poor root growth and fruit ripening. Potassium maintains cell turgidity and osmotic potential, controls stomatal opening and closing, drives phloem translocation, activates enzymes, and balances anions and cations by ion exchange; it is absorbed as K+ ions, and its deficiency causes marginal chlorosis, necrosis, reduced cambial activity, loss of apical dominance, lodging in cereals, and curled leaf margins. Calcium is needed for calcium pectate synthesis in the middle lamella, mitotic spindle formation and cell division, membrane permeability, lipid metabolism, and as an activator of phospholipase, ATPase, amylase and adenyl kinase; absorbed as exchangeable Ca2+ ions, its deficiency causes chlorosis, necrosis, stunted growth, premature leaf and flower fall, inhibited seed formation, black heart of celery, and hooked leaf tip in sugar beet, banana (Musa) and tomato. Magnesium is a component of chlorophyll, activates the carbohydrate-metabolism enzymes RuBP carboxylase and PEP carboxylase, and is involved in DNA and RNA synthesis and ribosomal subunit binding; absorbed as Mg2+ ions, its deficiency causes interveinal chlorosis, necrosis, anthocyanin (purple) formation, and sand drown of tobacco. Sulphur is …