Mineral Nutrition
Essential Elements: Macronutrients and Micronutrients
Besides carbon, hydrogen and oxygen (which come from air and water), plants need a number of mineral elements that they absorb from the soil as ions. An element is called essential if the plant cannot complete its life cycle without it, it has a specific role that no other element can replace, and it is directly involved in the plant's metabolism. Scientists discovered these requirements using hydroponics — growing plants in a nutrient solution rather than soil — and leaving out one element at a time to see the effect.
Essential elements are grouped by the amount needed:
- Macronutrients — needed in large amounts. These are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur.
- Micronutrients (trace elements) — needed in very small amounts. These include iron, manganese, copper, zinc, boron, molybdenum and chlorine.
By function, the elements act as: components of biomolecules and structures (e.g. nitrogen in proteins, magnesium in chlorophyll), activators or inhibitors of enzymes (e.g. magnesium, zinc), and regulators of the osmotic balance of cells (e.g. potassium). Even though micronutrients are needed in tiny amounts, they are just as essential — a shortage of any one causes problems.
Recall the definition.
- The plant cannot complete its life cycle without it.
- It has a specific, irreplaceable role and is directly involved in metabolism.
Sort by quantity required.
- Nitrogen — needed in large amounts — macronutrient.
- Iron — needed in trace amounts — micronutrient.
Plants are grown without soil.
- Hydroponics — growing plants in a defined nutrient solution.
- One element is left out at a time to observe the effect.
Key Points
- Essential element: needed to complete the life cycle, irreplaceable specific role, directly in metabolism.
- Studied by hydroponics (soil-less culture).
- Macronutrients (large amounts): N, P, K, Ca, Mg, S (+ C, H, O).
- Micronutrients (trace): Fe, Mn, Cu, Zn, B, Mo, Cl.
- Roles: structure, enzyme activation, osmotic balance.
Roles of Minerals and Deficiency Symptoms
Each essential element has particular jobs, and its shortage produces a characteristic deficiency symptom. Knowing the symptom helps farmers diagnose what a crop is lacking. Some important examples:
- Nitrogen (N) — a part of proteins, nucleic acids, chlorophyll and many vitamins. Deficiency causes chlorosis (yellowing of leaves) and stunted growth.
- Phosphorus (P) — part of DNA, RNA, ATP and membranes. Deficiency causes poor root growth and dark/purplish leaves.
- Potassium (K) — helps open and close stomata, activates enzymes and maintains osmotic balance. Deficiency causes scorched, curled leaf margins.
- Magnesium (Mg) — the central atom of the chlorophyll molecule and an enzyme activator. Deficiency causes chlorosis between the veins.
- Iron (Fe) — needed to make chlorophyll (though not part of it) and in respiration. Deficiency causes chlorosis of young leaves.
- Calcium (Ca) — needed for the cell wall (middle lamella) and cell division. Deficiency affects growing tips.
An important pattern: symptoms of mobile elements (like N, P, K, Mg) appear first in the older leaves, because the plant moves these elements out of old leaves to the growing young parts. Symptoms of immobile elements (like Ca, Fe) appear first in the young leaves. The most common visible symptom overall is chlorosis — the loss of green colour caused by reduced chlorophyll.
Link nitrogen to the green pigment.
- Nitrogen is part of chlorophyll.
- Without enough nitrogen, less chlorophyll forms, so leaves turn yellow.
Recall the chlorophyll structure.
- Magnesium sits at the centre of the chlorophyll molecule.
Mobile elements are redistributed.
- They appear first in the older leaves.
- The plant moves these elements from old leaves to growing young parts.
Key Points
- N: proteins, nucleic acids, chlorophyll → deficiency = chlorosis, stunting.
- P: DNA/RNA/ATP; K: stomata, osmotic balance; Mg: central atom of chlorophyll; Fe: chlorophyll synthesis; Ca: cell wall.
- Mobile elements (N,P,K,Mg): symptoms first in old leaves; immobile (Ca,Fe): first in young leaves.
- Most common symptom = chlorosis (loss of chlorophyll).
Nitrogen Metabolism and Biological Nitrogen Fixation
Nitrogen is one of the most important nutrients, yet plants cannot use the abundant nitrogen gas (N₂) of the air directly. The air is about 78% nitrogen, but the two atoms are held by a very strong triple bond. Converting this unusable N₂ into a form plants can absorb (like ammonia or nitrate) is called nitrogen fixation.
Most of this is done biologically by certain microbes — this is biological nitrogen fixation, carried out by the enzyme nitrogenase. The nitrogen fixers are of two kinds:
- Free-living fixers in the soil, such as the bacterium Azotobacter and the cyanobacterium Anabaena / Nostoc.
- Symbiotic fixers, the most famous being Rhizobium, which lives in the root nodules of legumes (peas, beans, gram). The bacterium fixes nitrogen for the plant, and the plant supplies it with food and shelter — a true symbiosis.
Inside the plant, the fixed nitrogen (as nitrate) is first converted to ammonia and then built into amino acids and other compounds — the start of nitrogen assimilation. Because legumes enrich the soil with nitrogen through their nodules, farmers practise crop rotation, growing a legume crop to naturally restore soil fertility and reduce the need for chemical fertilisers.
Think about the nitrogen molecule.
- N₂ has a very strong triple bond and is unreactive.
- It must first be fixed into ammonia or nitrate before plants can use it.
Recall the symbiotic fixer.
- Rhizobium lives in legume root nodules.
Legumes host nitrogen fixers.
- Rhizobium in their nodules fixes atmospheric nitrogen into the soil.
- This enriches the soil with nitrogen (used in crop rotation).
Key Points
- Nitrogen fixation: converting unreactive N₂ into ammonia/nitrate that plants can use.
- Biological fixation by the enzyme nitrogenase: free-living (Azotobacter, Anabaena) and symbiotic (Rhizobium in legume root nodules).
- Fixed N → ammonia → amino acids (assimilation).
- Legumes enrich soil → basis of crop rotation.