Main Inorganic Nutrients for Plants
Plants need inorganic nutrients such as nitrogen, phosphorus, potassium, magnesium and calcium, absorbed from the soil as mineral ions. Each has a specific role, and a shortage of any one produces a recognisable deficiency symptom.
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Macronutrients and their functions
| Nutrient | Main function | Deficiency symptom |
|---|---|---|
| Nitrogen (as nitrate) | Forms proteins, nucleic acids and chlorophyll | Stunted growth; older leaves turn yellow (chlorosis) |
| Phosphorus (as phosphate) | Forms ATP, DNA and cell membranes; supports root growth | Poor root development; purplish leaves; stunted growth |
| Potassium | Activates enzymes; regulates stomatal opening and water balance | Yellowing at leaf edges; weak stems; poor flowering and fruiting |
| Magnesium | Central atom of the chlorophyll molecule | Yellowing between leaf veins (interveinal chlorosis) |
| Calcium | Forms part of the cell wall; needed for cell division | Stunted, deformed growing points and root tips |
Why plants need mineral ions, not just water
Water alone provides hydrogen and oxygen but cannot supply nitrogen, phosphorus, potassium and other elements needed to build proteins, nucleic acids, chlorophyll and cell structures. Plants must absorb these as dissolved mineral ions from the soil through their roots.
Diagnosing deficiency from symptoms
Deficiency symptoms often appear first in older or younger leaves depending on whether the nutrient can be moved (translocated) from old tissue to new growth. Nitrogen and magnesium are mobile, so their deficiency symptoms usually show in older leaves first, as the plant relocates the limited supply to new growth.
How it is examined
You may be shown a photograph or description of a plant with a deficiency symptom and asked to identify the missing nutrient, or be asked to explain why a nutrient is needed by linking it to a specific molecule or process, such as magnesium in chlorophyll or nitrogen in protein synthesis.
Worked exam-style question
Question. A farmer notices that the older leaves of a maize crop are turning yellow starting from the leaf tip, while the younger leaves remain green, and the stems appear weaker than normal with poor grain formation. (a) Identify the nutrient most likely deficient in the soil.
(b) Explain two functions of this nutrient that link to the symptoms described. (c) Explain why the symptoms appear in the older leaves first rather than the younger leaves.
Model answer. (a) The nutrient most likely deficient is potassium. (b) Potassium activates enzymes involved in metabolism, so a shortage slows down these reactions and weakens the stem; potassium also regulates stomatal opening and water balance, and supports flowering and fruiting, so its shortage explains the poor grain formation.
(c) Potassium is a mobile nutrient that can be translocated from older tissue to new growth, so when supply is limited, the plant moves its potassium out of the older leaves to support new growth, which is why deficiency symptoms show in the older leaves first.
Practice question
Try this. A tomato plant grown in poor soil develops purplish leaves and shows very poor root development, though the leaves remain green rather than yellow. Identify the nutrient most likely deficient, and explain the link between this nutrient and the two symptoms described.
Exam tip
Key terms
These Chapter 18 terms connect inorganic nutrients to the process they support:
- Photosynthesis, the process that depends on nutrients such as magnesium and nitrogen.
- Chlorophyll, the green pigment whose structure requires a magnesium atom.
- Chloroplast, the organelle containing chlorophyll, affected when magnesium is deficient.
- Limiting factor, a factor, including nutrient supply, that restricts the rate of plant growth.
Source:SRC-DSKP-EN
Frequently asked questions
Why does magnesium deficiency cause yellow leaves?
Why is nitrogen important for plant growth?
Why do calcium and phosphorus deficiencies both affect growing points, but for different reasons?
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