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Quiz yourself on Plant Diseases and DefensesGet your flashcardsBuild my revision planTrack my progressMineral Ion Deficiencies
Plants can show signs of ill health that are not caused by pathogens at all. Sometimes leaves yellow, growth slows, or flowers fail to develop because the plant cannot absorb enough of a specific mineral ion from the soil. Plants absorb mineral ions through their root hair cells by active transport — an energy-requiring process against the concentration gradient.
Key rule: Nitrate ions are needed to make amino acids and proteins. Magnesium ions are needed to make chlorophyll. Deficiency of either causes chlorosis (yellowing of leaves).
| Mineral Ion | Needed For | Deficiency Symptoms | Why It Causes These Symptoms |
|---|---|---|---|
| Nitrate (NO3-) | Making amino acids, then proteins | Stunted growth; yellowing of older leaves first | Without nitrate, the plant cannot make proteins — including enzymes needed for growth and chlorophyll-related proteins. Older leaves are stripped of nitrogen first as the plant redirects what it has to new growth. |
| Magnesium (Mg2+) | Making chlorophyll molecules | Yellow leaves (chlorosis) across all leaf ages; reduced photosynthesis | Magnesium is a core component of the chlorophyll molecule. Without it, chlorophyll cannot be synthesised, leaves lose their green colour, and the rate of photosynthesis falls — reducing glucose supply for growth. |
| Potassium (K+) | Enzyme function; opening stomata | Poor flower and fruit development; yellow leaves with brown/scorched edges | Potassium activates many enzymes involved in photosynthesis and respiration. It is also needed to regulate guard cells so stomata open correctly. Deficiency reduces crop yield and causes leaf edge scorch. |
How to Distinguish Nitrate vs Magnesium Deficiency
- Nitrate deficiency: Yellowing starts on older, lower leaves first — the plant moves nitrogen from old to new leaves when supply is limited. Growth is noticeably stunted.
- Magnesium deficiency: Yellowing appears more evenly across leaves of all ages. Growth may be less stunted initially, but photosynthesis rate drops measurably because chlorophyll cannot form.
- Both: Cause chlorosis (yellowing) — but the distribution and other associated symptoms (stunted growth for nitrate; chlorophyll-linked colour loss for magnesium) distinguish them.
How Plants Absorb Mineral Ions
- Mineral ions are dissolved in soil water at low concentration
- Root hair cells absorb them by active transport — requires ATP (energy from respiration)
- This moves ions from low concentration (soil) to high concentration (root cells) — against the concentration gradient
- If soil is waterlogged or anaerobic, root cells cannot respire aerobically → less ATP → less active transport → deficiency symptoms even if minerals are present in soil
Quick Check: A plant has yellow leaves but is otherwise apparently healthy — no spots, no surface fungal growth, no wilting. Suggest two possible mineral deficiencies that could cause this, and explain how you would distinguish between them.
Two possible deficiencies:
1. Magnesium deficiency — magnesium is needed to make chlorophyll. Without it, chlorophyll cannot be synthesised so leaves turn yellow (chlorosis). Yellowing would appear across leaves of all ages as the plant cannot make new chlorophyll in any leaf.
2. Nitrate deficiency — nitrate is needed to make amino acids and proteins, including those associated with chlorophyll production and green pigmentation. Yellowing would begin on older leaves first, as the plant moves remaining nitrate towards younger growing leaves.
How to distinguish: Look at which leaves yellow first. If older/lower leaves yellow first → nitrate deficiency. If yellowing appears across all leaf ages fairly evenly → magnesium deficiency. Nitrate deficiency also causes noticeably stunted growth (the plant cannot make proteins for cell division), while magnesium deficiency initially affects colour more than overall size. A definitive test would be to add nitrate fertiliser to one group and magnesium sulfate to another, then observe which treatment restores green colour.
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