Plants need 17 essential mineral elements in addition to carbon, hydrogen and oxygen from air and water. CBSE Class 11 and NEET test essentiality criteria, deficiency symptoms and specific functions. Expect one NEET question a year on a specific nutrient.
Core Concepts
Macronutrients and micronutrients
Macronutrients are needed in amounts over 10 mmol/kg dry weight — C, H, O, N, P, K, Ca, Mg, S. Micronutrients are needed in trace amounts — Fe, Mn, Cu, Zn, B, Mo, Cl, Ni.
The 17 essential elements (remembered by the mnemonic: C HOPKINS CaFe, Mighty good but CuZn Mo Clu Ni — C, H, O, P, K, I is not essential but N, S, Ca, Fe, Mg, B, Cu, Zn, Mo, Cl, Ni):
| Category | Elements | Needed in |
|---|---|---|
| Framework elements | C, H, O | Large amounts (from CO, HO) |
| Macronutrients | N, P, K, Ca, Mg, S | >10 mmol/kg dry weight |
| Micronutrients | Fe, Mn, Cu, Zn, B, Mo, Cl, Ni | <10 mmol/kg dry weight |
Criteria of essentiality
An element is essential if (1) the plant cannot complete its life cycle without it, (2) it is directly involved in metabolism, and (3) no other element can substitute for it. Arnon and Stout proposed these criteria in 1939.
These criteria were established through hydroponics — growing plants in nutrient solutions where individual elements can be removed one at a time. If removing an element prevents the plant from completing its life cycle (seed to seed), that element is essential.
Functions of key elements
Detailed element-function table:
| Element | Function | Deficiency symptom | Mobile/Immobile |
|---|---|---|---|
| N | Proteins, nucleic acids, chlorophyll | General chlorosis of older leaves, stunted growth | Mobile |
| P | ATP, nucleic acids, phospholipids | Purple/dark green leaves, poor root growth | Mobile |
| K | Enzyme activator, stomatal regulation, osmotic balance | Leaf tip burn (necrosis), weak stems | Mobile |
| Ca | Cell wall (calcium pectate), second messenger | Distorted young leaves, blossom end rot | Immobile |
| Mg | Chlorophyll core, enzyme activator | Interveinal chlorosis of older leaves | Mobile |
| S | Amino acids (cysteine, methionine), coenzymes | Similar to N deficiency but affects young leaves | Immobile |
| Fe | Electron transport, chlorophyll synthesis | Interveinal chlorosis of young leaves | Immobile |
| Mn | Water splitting in PS II, enzyme activator | Interveinal chlorosis, grey speck | Immobile |
| Cu | Plastocyanin, cytochrome oxidase | Die-back of shoot tips, light green leaves | Immobile |
| Zn | Auxin synthesis (IAA), carbonic anhydrase | Little leaf, rosetting, mottled chlorosis | Immobile |
| B | Cell wall (cross-links pectin), pollen tube growth | Hollow stem (in cauliflower), poor fruit set | Immobile |
| Mo | Nitrogenase (nitrogen fixation), nitrate reductase | Whiptail in cauliflower, marginal chlorosis | Mobile |
| Cl | Water splitting in PS II, osmotic regulation | Wilting, chlorosis | Mobile |
| Ni | Urease enzyme | Leaf tip necrosis (recently added to essential list) | Mobile |
Deficiency symptoms
N deficiency — general yellowing (chlorosis) of older leaves. Mg deficiency — chlorosis between veins. Fe deficiency — chlorosis of young leaves (Fe is immobile). K deficiency — leaf tip burn.
Why mobile and immobile elements show different symptoms:
Mobile elements (N, P, K, Mg, Cl, Mo, Ni) can be remobilised from older leaves to younger growing points. So deficiency symptoms appear first in older leaves — the plant sacrifices old tissues to feed young ones.
Immobile elements (Ca, S, Fe, Mn, Cu, Zn, B) cannot be remobilised. So deficiency symptoms appear in young leaves — older leaves retain their supply while new growth suffers.
This mobile vs immobile distinction is a guaranteed NEET question format: “N deficiency shows symptoms in old leaves because N is mobile” or “Fe deficiency shows symptoms in young leaves because Fe is immobile.” Memorise which elements are mobile and which are not.
Specific deficiency examples
Nitrogen deficiency: Most common. General chlorosis (yellowing) starting from older leaves. Stunted growth, thin stems, early leaf drop. Corrected by urea or ammonium nitrate.
Iron deficiency (iron chlorosis): Interveinal chlorosis of young leaves — veins stay green but areas between turn yellow. Common in alkaline soils where iron is less available. Corrected by foliar spray of ferrous sulphate.
Boron deficiency: Hollow stem in cauliflower, heart rot in beetroot, poor pollen germination, reduced fruit and seed set. Boron is needed for pollen tube growth — its deficiency causes sterility.
Zinc deficiency: “Little leaf” disease — leaves are abnormally small. Also causes “rosetting” — shortened internodes give a bushy appearance. Zinc is needed for auxin (IAA) synthesis.
Molybdenum deficiency: “Whiptail” disease in cauliflower — leaves become thin and distorted. Mo is needed for nitrate reductase (converts NO to NH in the plant).
Toxicity
Excess of any mineral can be toxic. Manganese toxicity causes brown spots on leaves. Aluminium toxicity in acidic soils inhibits root growth. Heavy metals (Cd, Pb, Hg) are toxic at very low concentrations.
Critical concentration: For each element, there is a concentration below which deficiency occurs and above which toxicity occurs. The range between them is the adequate zone. Micronutrients have a very narrow adequate zone — the margin between deficiency and toxicity is small.
Hydroponics
Growing plants in nutrient solution without soil. Used to study nutrient essentiality and for commercial production of vegetables. Uses less water than soil cultivation.
Advantages: Precise control of nutrients, no soil-borne diseases, higher yields per unit area, can be done in non-arable areas. Disadvantages: High setup cost, requires technical knowledge, power failure can kill plants quickly.
Mineral uptake mechanisms
Passive absorption: Movement of ions along the concentration gradient (from high to low). No energy needed. Includes diffusion and ion exchange.
Active absorption: Movement against the concentration gradient. Requires metabolic energy (ATP). Involves carrier proteins in the root cell membrane. Most mineral uptake is active because soil solution concentrations are lower than cell sap concentrations.
Worked Examples
Nitrogen is mobile — the plant can move it from old leaves to new. So old leaves yellow first as N is withdrawn. For Fe (immobile), young leaves are hit first.
Remove one element at a time from the nutrient solution. If the plant cannot complete its life cycle, that element is essential. This is how Arnon and Stout tested hundreds of elements.
A citrus tree shows abnormally small leaves with mottled chlorosis and shortened internodes (rosette appearance). Diagnosis: zinc deficiency. Zinc is needed for IAA (auxin) synthesis. Without auxin, leaves remain small and internodes do not elongate. Confirmed by: symptoms appear in young tissues (Zn is immobile), and foliar zinc spray corrects the problem.
Common Mistakes
Calling carbon a mineral nutrient. It is not — it comes from air as CO.
Saying sodium is essential for all plants. It is essential for C4 plants only.
Confusing chlorosis (yellowing) and necrosis (tissue death).
Assuming all deficiency symptoms appear in old leaves. Only mobile elements show symptoms in old leaves. Immobile elements (Fe, Ca, B, Cu, Zn, Mn, S) show symptoms in young leaves.
Writing that micronutrients are less important because they are needed in small amounts. They are equally essential — the plant cannot complete its life cycle without any of them.
Exam Weightage and Revision
NEET 2023 asked about boron deficiency symptoms. NEET 2022 tested the function of molybdenum. CBSE boards ask about criteria of essentiality and deficiency symptoms. This chapter gives 1-2 NEET questions every year — all from the element-function-symptom table.
When a question gives a scenario, identify the core mechanism first, then match it to the concepts above. Most wrong answers come from reading the scenario too quickly.
Make a table — element, role, deficiency symptom. About 12 rows. Covers the chapter.
Practice Questions
Q1. Why does iron deficiency appear in young leaves but nitrogen deficiency in old leaves?
Iron is immobile — once deposited in a leaf, it cannot be remobilised to newer growth. So old leaves retain their iron while young leaves suffer. Nitrogen is mobile — the plant pulls nitrogen from old leaves and sends it to growing points. Old leaves yellow first.
Q2. What is the role of magnesium in plants?
Magnesium is the central atom in chlorophyll (like iron in haemoglobin). Without Mg, chlorophyll cannot be synthesised and photosynthesis stops. Mg also activates enzymes like RuBisCO and many kinases involved in phosphorylation reactions.
Q3. Name the element required for water-splitting in photosystem II.
Manganese (Mn). The oxygen-evolving complex (OEC) in photosystem II contains a cluster of 4 Mn atoms that catalyse the splitting of water: . Chloride (Cl) is also needed as a cofactor in this complex.
Q4. What is whiptail disease and which element’s deficiency causes it?
Whiptail is a disorder in cauliflower caused by molybdenum deficiency. The leaf lamina fails to develop properly, leaving only the midrib — giving a thin, whip-like appearance. Mo is needed for the enzyme nitrate reductase; without it, the plant cannot reduce NO to NH.
Q5. How does zinc affect plant growth?
Zinc is required for the synthesis of auxin (IAA) — specifically the enzyme tryptophan synthase needs Zn. Without auxin, cells do not elongate, leading to small leaves (little leaf disease) and shortened internodes (rosetting). Zinc is also needed for carbonic anhydrase and alcohol dehydrogenase.
FAQs
Why do plants need such tiny amounts of micronutrients? Micronutrients typically function as cofactors in enzymes. Since enzymes are catalytic (reused), only small amounts of the cofactor are needed. For example, one molecule of Mn-containing superoxide dismutase can neutralise millions of superoxide radicals.
Can excess fertilizer be harmful to plants? Yes. Excess fertilizer increases salt concentration in soil, which can cause osmotic stress (water moves out of roots), toxicity (excess of specific elements), and nutrient imbalance (excess of one element can block uptake of another). This is why precision agriculture — applying just the right amount — is important.
What is chelation and why is it important in mineral nutrition? Chelation is the binding of a metal ion by an organic molecule (chelator). In soil, iron is often in insoluble Fe form. Plants secrete chelators (siderophores, organic acids) that bind Fe and keep it in solution for root uptake. Without chelation, iron would be unavailable in alkaline soils.
Mineral nutrition links soil chemistry to plant physiology. Every deficiency has a clear cause and a clear cure — that makes it satisfying to study.