Macronutrients: Functions, Deficiency & Soil Behavior

This chapter details the primary and secondary macronutrients. Mastering this specific format directly answers the requirements of PYQ 2017 Q2(a) and 2016 Q2(a).

10.1 Nitrogen (N)

• Classification: Primary macronutrient. It is the most universally limiting nutrient in Indian agriculture. While 78% of the atmosphere is N₂ gas, this form is completely unavailable to most plants directly.
• Absorbed Form: Absorbed as NO₃⁻ (nitrate) in aerobic, well-drained soils and as NH₄⁺ (ammonium) in anaerobic or flooded soils. Both are equally usable by the plant but are metabolized differently.
• Mobility in Plant: Highly mobile. The plant easily remobilizes nitrogen from older leaves to support new growing tissues.

• Key Functions (Biochemical & Physiological):
• Structural: It is a fundamental component of amino acids, which form proteins. Since all enzymes are proteins, nitrogen is structural to all cellular machinery and membranes.
• Photosynthetic: Nitrogen is the core component of the porphyrin ring in chlorophyll. High nitrogen means more chlorophyll, a deeper green color, and greater photosynthetic capacity.
• Genetic: It is a core component of the purine and pyrimidine bases in DNA and RNA, making it essential for cell division.
• Energy Metabolism: It is a structural component of all major energy carrier molecules, including ATP, ADP, NAD, and NADP.
• Yield Determinant: Nitrogen regulates the Leaf Area Index (LAI), tiller number in cereals, and final grain protein content. It holds the most direct relationship with crop yield of any element.

• Deficiency Symptoms: Because nitrogen is highly mobile, symptoms appear on the old (lower) leaves first. It manifests as a uniform yellowing (chlorosis) that progresses upward. Plants exhibit stunted growth, thin upright stems, reduced tillering, and premature senescence. In maize, it causes a diagnostic "V-shaped" yellowing starting from the leaf tip.
• Soil Behavior: In the soil, 97 to 98% of nitrogen is locked in organic forms (humus and microbes) and is not directly available. Only 2 to 3% exists in the available inorganic forms (NO₃⁻, NH₄⁺). It undergoes continuous transformations including mineralization, nitrification, and severe losses via denitrification and volatilization.
• Fertilizer Sources: Urea (46% N) is the most widely used source in India. Other sources include Ammonium Sulfate (21% N + 24% S), DAP (18% N + 46% P₂O₅), and Calcium Ammonium Nitrate (CAN). Notably, the nitrogen-use efficiency in India is very poor; only 30 to 40% of applied N is recovered by the crop, with the rest lost to the environment.

10.2 Phosphorus (P)

• Classification: Primary macronutrient. It is the second most limiting nutrient globally. India suffers from exceptionally low phosphorus-use efficiency (averaging only 15 to 20%).
• Absorbed Form: Absorbed primarily as H₂PO₄⁻ (dihydrogen phosphate) in acidic to neutral soils (pH < 7), and as HPO₄²⁻ (hydrogen phosphate) in neutral to alkaline soils (pH > 7). The H₂PO₄⁻ form is absorbed much more rapidly.
• Mobility in Plant: Moderately mobile. It is remobilized from older leaves to developing seeds, which is why mature grain has a very high phosphorus content.

• Key Functions (Biochemical & Physiological):
• Energy Currency: Phosphorus forms the high-energy bonds in ATP (Adenosine Triphosphate), the universal energy currency that drives all metabolic reactions.
• Nucleic Acids: Phosphodiester bonds link the nucleotides in the DNA and RNA backbone. Without P, there is no genetic synthesis and no cell division.
• Membrane Structure: Phospholipids (glycerol + fatty acids + phosphate) form all cellular membranes, making P essential for membrane integrity and fluidity.
• Root Development: It heavily promotes early root growth, lateral root formation, and root hair density.
• Seed Formation and Maturity: It is critical for seed filling, often stored as phytic acid in seeds. It accelerates crop maturity and vastly improves final grain and fruit quality.

• Deficiency Symptoms: The classic visual symptom is a distinct purple or reddish-purple discoloration on leaves and stems (especially the undersides) due to the accumulation of anthocyanin pigments. Plants show severe overall stunting, very poor lateral root development, delayed tillering, and severely delayed maturity with poor seed set.

• Soil Behavior (The Fixation Problem): Phosphorus is the most heavily "fixed" of all nutrients, forming insoluble compounds that plants cannot use. In acid soils (pH < 5.5), it is fixed by iron and aluminum into insoluble Fe-P and Al-P. In alkaline soils (pH > 7.5), it is fixed by calcium into insoluble tricalcium phosphate. Optimal availability exists only in a narrow band between pH 6.0 and 7.0. Because it does not move by mass flow, it relies entirely on slow diffusion (moving only 1 to 3 mm), making mycorrhizal fungi critical for its acquisition.

• Fertilizer Sources: DAP (46% P₂O₅) is the most popular in India. Other sources include Single Super Phosphate or SSP (16% P₂O₅ + 11% S), Triple Super Phosphate or TSP (46% P₂O₅), and raw Rock Phosphate (25–30% P₂O₅), which is insoluble and only used to reclaim highly acidic soils.

10.3 Potassium (K)

• Classification: Primary macronutrient. It is the third most applied nutrient globally, though historically heavily underused in India (where the N:P:K application ratio is often 7:2:1 instead of the ideal 4:2:1).
• Absorbed Form: Absorbed strictly as the K⁺ (potassium) ion. It is freely soluble and the most mobile macronutrient in the soil.
• Mobility in Plant: Highly mobile. Like nitrogen, it is easily remobilized, meaning deficiency symptoms show on the old (lower) leaves first.

• Key Functions (Biochemical & Physiological):
• Enzyme Activation: It acts as a mandatory cofactor or allosteric activator for over 60 different plant enzymes, including pyruvate kinase and starch synthase.
• Osmotic Regulation: It is the primary osmoticum in plant vacuoles. It regulates cell turgor pressure and controls the opening and closing of stomata via guard cell K⁺ fluxes, making it the absolute controller of plant water-use efficiency.
• Photosynthesis and Phloem Loading: It activates ATP synthase in chloroplasts for optimal CO₂ fixation. Furthermore, K⁺ actively drives the loading of sucrose into the phloem, transporting sugars from the leaves to the growing grains or tubers.
• Disease Resistance: High potassium levels build thicker cell walls that resist fungal penetration, making plants significantly more resistant to diseases like rice blast and potato late blight.

• Deficiency Symptoms: The hallmark symptom is marginal scorch (necrosis) on older leaves—the leaf tips and outer edges turn yellow and then brown, looking like "burnt edges," while the middle of the leaf remains green. It also causes weak stems (leading to lodging), poor grain filling, and severe drops in fruit and tuber quality.
• Soil Behavior: Potassium exists in four pools. Solution K (<1%) is immediately available. Exchangeable K (1–2%) is held on clay/OM CEC sites. Slowly Available K (1–10%) is trapped or "fixed" between the layers of 2:1 clays like illite. Structural K (90–98%) is locked inside primary minerals like feldspar and releases over centuries.
• Fertilizer Sources: Muriate of Potash (MOP / KCl) containing 60% K₂O is the most economical and widely used. Sulfate of Potash (SOP / K₂SO₄) containing 50% K₂O is preferred for chloride-sensitive crops like potatoes, tobacco, and specific vegetables.

10.4 Calcium (Ca)

• Classification: Secondary macronutrient. It is rarely deficient in Indian soils, except in highly acidic, heavily leached regions.
• Absorbed Form: Absorbed as the Ca²⁺ (calcium) ion. It is absorbed almost entirely at actively growing root tips.
• Mobility in Plant: Immobile. Once placed in tissue, it cannot be remobilized. Therefore, deficiency symptoms appear strictly on new, young growing tissues.

• Key Functions (Biochemical & Physiological):
• Cell Wall Structure: It forms pectin-calcium cross-links in the middle lamella, acting as the primary biological glue that holds cell walls together and maintains structural integrity.
• Membrane Integrity: Ca²⁺ chemically stabilizes membrane phospholipids, preventing cellular contents and electrolytes from leaking out.
• Second Messenger: It acts as the primary "message carrier" inside cells for signal transduction in response to various plant hormones and environmental stresses.

• Deficiency Symptoms: Because it cannot move, young leaves and growing tips become distorted, necrotic, or fail to grow. Classic agricultural symptoms include "blossom-end rot" in tomatoes and peppers (calcium deficiency in the expanding fruit), "tip burn" in lettuce and cabbage, and "hollow heart" in potatoes.
• Soil Behavior: Calcium dominates the CEC sites in most Indian soils, often making up 60 to 70% of the base saturation. It is highly mobile in humid regions (leaching away to cause acidity) and accumulates in arid soils to form hard CaCO₃ (kankar) nodules.
• Fertilizer Sources: Agricultural lime (CaCO₃, CaO) is used to supply calcium and reclaim acid soils. Gypsum (CaSO₄·2H₂O) is used to supply calcium and sulfur, and is the primary amendment for reclaiming sodic soils. SSP also provides a steady byproduct source of calcium.

10.5 Magnesium (Mg)

• Classification: Secondary macronutrient. Deficiencies are being increasingly reported across high-intensity cropping areas in India.
• Absorbed Form: Absorbed as the Mg²⁺ (magnesium) ion.
• Mobility in Plant: Mobile. It can be remobilized from old tissue to young tissue, so deficiencies appear on the old (lower) leaves first.

• Key Functions (Biochemical & Physiological):
• Chlorophyll Structure: This is its most important function. Magnesium is the exact central atom of the porphyrin ring in the chlorophyll molecule. Without magnesium, chlorophyll cannot exist.
• Enzyme Activator: It forms the Mg-ATP complex, making it the required activator for all ATPase enzymes. It is the most universal cellular cofactor after potassium.
• Protein Synthesis: It is structurally required to maintain the physical integrity of ribosomes; without it, protein synthesis stalls.

• Deficiency Symptoms: The classic symptom is distinct interveinal chlorosis on older leaves. The tissue between the leaf veins turns yellow, while the veins themselves remain starkly green. In maize, this often creates a diagnostic "Christmas tree" pattern.
• Soil Behavior: Magnesium is held on soil CEC sites but competes antagonistically with potassium, calcium, and ammonium for plant uptake. Heavy potassium fertilization can easily induce a magnesium deficiency.
• Fertilizer Sources: Magnesium sulfate (MgSO₄·7H₂O, commonly known as Epsom salt), Kieserite (MgSO₄·H₂O), and Dolomitic lime (CaMg(CO₃)₂, which supplies both calcium and magnesium while correcting soil acidity).

10.6 Sulfur (S)

• Classification: Secondary macronutrient. Sulfur deficiency is a rapidly growing crisis in Indian agriculture, largely caused by the nationwide shift away from sulfur-containing SSP toward high-analysis DAP and Urea (which contain zero sulfur).
• Absorbed Form: Absorbed from the soil as the SO₄²⁻ (sulfate) anion. A very minor amount can be absorbed directly as SO₂ gas through the stomata from atmospheric deposition.
• Mobility in Plant: Semi-mobile. It moves somewhat, but not fast enough to support new growth during a severe shortage.

• Key Functions (Biochemical & Physiological):
• Amino Acids: It is a mandatory structural component of cysteine and methionine (the sulfur-containing amino acids), making it essential for forming the disulfide bonds that give proteins their shape.
• Vitamins & Coenzymes: It is a component of thiamine (B1), biotin (B7), and Coenzyme A, which is central to fatty acid synthesis.
• Flavor Compounds: Sulfur drives the production of alliins (giving onions and garlic their pungency) and glucosinolates (giving mustard and brassicas their distinct flavor and oil quality).
• Photosynthesis: It forms the iron-sulfur proteins (like ferredoxin) critical for photosynthetic electron transport.

• Deficiency Symptoms: The symptoms strongly resemble nitrogen deficiency (a uniform yellowing or chlorosis). However, because sulfur is semi-mobile, the yellowing appears on the young (new) leaves first, whereas nitrogen deficiency appears on the old leaves. It also results in poor overall protein and oil quality.
• Soil Behavior: Like nitrogen, most soil sulfur is tied up in organic matter and must be mineralized by microbes into sulfate before plants can use it. Sulfate is an anion and leaches easily in high-rainfall, sandy soils.
• Fertilizer Sources: SSP (16% P₂O₅ + 11% S) remains the best dual-purpose source in India. Other sources include Ammonium Sulfate (24% S), Gypsum (17% S), and pure Elemental Sulfur (which provides a slow release and is excellent for lowering the pH of alkaline soils).