The Roles of the Three Major Nutrients: Nitrogen, Phosphorus, and Potassium

(1) Nitrogen: Physiological functions: ① Nitrogen is a major component of proteins, nucleic acids, and phospholipids, all of which are essential constituents of cellular structures such as protoplasm, the cell nucleus, and biological membranes. ② Nitrogen is a component of enzymes, ATP, and various coenzymes and cofactors (such as NAD+, NADP+, and FAD), which play important roles in material and energy metabolism. ③ Nitrogen is also a component of certain plant hormones, such as auxins and cytokinins, as well as vitamins such as B1, B2, B6, and PP, which regulate biological processes. ④ Nitrogen is a component of chlorophyll and is closely related to photosynthesis. Symptoms of nitrogen deficiency: ① Stunted plants. When nitrogen is deficient, the synthesis of proteins, nucleic acids, phospholipids, and other substances is inhibited, affecting cell division and growth. Plants become stunted, with few branches and tillers; leaves are small and thin; and flowers and fruits are scarce and prone to falling off. ② Chlorosis and loss of green color. Nitrogen deficiency impairs chlorophyll synthesis, causing branches and leaves to turn yellow, leading to premature leaf senescence and even desiccation, which in turn results in reduced yields. ③ Symptoms first appear on older leaves. Because nitrogen is highly mobile within plants, nitrogen compounds in older leaves can be broken down and transported to younger tissues for reuse; therefore, when nitrogen is deficient, leaves turn yellow, starting from the lower leaves and gradually progressing upward. (2) Phosphorus: Physiological Functions: Enhances crops’ resistance to drought, cold, and saline-alkali conditions. ① Phosphorus is a major component of nucleic acids, nucleoproteins, and phospholipids, and is closely related to protein synthesis, cell division, and cell growth. ② Phosphorus is a component of many coenzymes, such as NAD+ and NADP+, as well as ATP and ADP. ③ Phosphorus is involved in the metabolism and transport of carbohydrates; for example, during photosynthesis and respiration, the synthesis, conversion, and degradation of sugars mostly occur only after phosphorylation. ④ Phosphorus plays a crucial role in nitrogen metabolism; for example, NAD and FAD are involved in nitrate reduction, while pyridoxal phosphate and pyridoxamine phosphate participate in amino acid conversion. ⑤ Phosphorus is involved in fat metabolism, which requires the participation of NADPH, ATP, CoA, and NAD+. Symptoms of phosphorus deficiency: ① Stunted plants. Phosphorus deficiency impairs cell division, leading to reduced tillering and branching, stunted growth of young shoots and leaves, thin stems and roots, dwarfing of the plant, flower and fruit drop, and delayed maturation. ② Leaves turn dark green or purplish-red. During phosphorus deficiency, protein synthesis decreases and sugar transport is impeded, leading to a relative increase in sugar content within the reproductive organs. This promotes the formation of anthocyanins, causing leaves to exhibit an abnormal dark green or purplish-red color. ③ Symptoms first appear on older leaves. Phosphorus is highly mobile within the plant and can be recycled; during deficiency, most of the phosphorus in older leaves is transferred to growing, tender tissues. Consequently, symptoms of phosphorus deficiency first appear on the lower, older leaves and gradually spread upward. (3) Potassium: Physiological Functions: Promotes various metabolic processes in crops. Potassium promotes photosynthesis; stimulates respiration; enhances the absorption and transport of nitrogen; and plays a crucial role in nutrient transport and organic compound synthesis within plants. Potassium improves crop quality and is an important quality-enhancing element. Potassium increases the starch content in sweet potatoes; increases cotton fiber length; enhances the strength of jute fibers; increases the sugar content in sugar beets and sugarcane; and improves the quality of tobacco leaves. Potassium enhances crop stress tolerance. It promotes the development of epidermal and vascular tissues, increases cellular water-holding capacity, reduces transpiration, and enhances drought tolerance; potassium increases sugar accumulation within plants, raises cellular osmotic pressure, and enhances cold tolerance. Additionally, potassium strengthens crops’ resistance to lodging and disease. Symptoms of potassium deficiency: ① Reduced stress tolerance. Potassium-deficient plants have weak stems that are prone to lodging, with reduced drought and cold tolerance. ② Yellowing of leaves and scorched leaf margins. Potassium deficiency causes leaves to lose water, leading to the breakdown of proteins and chlorophyll, resulting in yellowing and gradual necrosis; it may also cause scorched leaf margins and slow growth. However, since growth in the central part of the leaf remains relatively rapid, the entire leaf may become cup-shaped or shriveled. ③ Symptoms first appear on older leaves. Potassium is a highly mobile and reusable element; therefore, deficiency symptoms first appear on the older leaves at the base of the plant.