Photosynthesis and Plant Nutrition
4. Mineral Nutrition
Learning outcomes
- I can identify essential mineral nutrients required by plants.
- I can explain the functions of nitrogen, phosphorus, potassium, and magnesium.
- I can describe symptoms of common nutrient deficiencies.
- I can explain how plants obtain mineral ions from the soil.
- I can relate mineral nutrition to plant growth and health.
Introduction
Like all living organisms, plants need more than just water, carbon dioxide, and sunlight to survive. They also require a variety of mineral nutrients that are absorbed from the soil. These minerals are needed in small amounts, but they are essential for building cells, producing enzymes, carrying out photosynthesis, and supporting healthy growth.
Different minerals have different roles within the plant. If even one essential mineral is lacking, the plant may grow poorly or develop characteristic deficiency symptoms such as yellow leaves, weak stems, or poor flowering. Understanding mineral nutrition helps farmers, gardeners, and scientists improve crop production and maintain healthy ecosystems.
What Are Mineral Nutrients?
Mineral nutrients are chemical elements that plants absorb from the soil as mineral ions dissolved in water.
These minerals are needed for:
- Growth
- Photosynthesis
- Protein production
- Energy transfer
- Cell division
- Flower and fruit development
Plants obtain these minerals mainly through their roots.
Figure 1. Plants absorb dissolved mineral ions from the soil through their roots.
How Plants Obtain Mineral Ions
Water enters root hairs mainly by osmosis, while many mineral ions enter by active transport.
Active Transport
Active transport:
- Requires energy.
- Allows plants to absorb mineral ions even when their concentration is lower in the soil than inside the root cells.
After entering the roots:
- Mineral ions move into the xylem.
- They are transported upward to the stem and leaves.
This ensures that all parts of the plant receive the nutrients they need.
Essential Mineral Nutrients
Plants require many different mineral elements.
Some of the most important are:
- Nitrogen
- Phosphorus
- Potassium
- Magnesium
- Calcium
- Iron
Each mineral has a different function.
Figure 2. Plants require several essential mineral nutrients for healthy growth and development.
Nitrogen (N)
Nitrogen is one of the most important mineral nutrients.
Plants use nitrogen to make:
- Amino acids
- Proteins
- DNA
- Chlorophyll
Nitrogen promotes:
- Rapid growth.
- Healthy green leaves.
- Stem and leaf development.
Nitrogen Deficiency
Symptoms include:
- Yellow older leaves (chlorosis).
- Slow growth.
- Small leaves.
- Weak stems.
Because nitrogen is mobile within the plant, it is often moved from older leaves to younger ones, causing the older leaves to yellow first.
Phosphorus (P)
Phosphorus is needed for:
- ATP (energy transfer).
- DNA.
- Cell division.
- Root development.
- Flower and seed production.
Phosphorus Deficiency
Symptoms include:
- Poor root growth.
- Slow development.
- Small plants.
- Dark green or purplish leaves in some species.
- Reduced flowering.
Potassium (K)
Potassium helps regulate:
- Enzyme activity.
- Water balance.
- Stomatal opening and closing.
- Disease resistance.
- Fruit development.
Potassium Deficiency
Symptoms include:
- Yellow or brown leaf edges.
- Weak stems.
- Poor fruit production.
- Increased susceptibility to disease.
Figure 3. Different mineral deficiencies produce characteristic symptoms that help identify missing nutrients.
Magnesium (Mg)
Magnesium is the central atom in every chlorophyll molecule.
It is essential for:
- Photosynthesis.
- Chlorophyll production.
- Enzyme activity.
Magnesium Deficiency
Symptoms include:
- Yellowing between the veins (interveinal chlorosis) of older leaves.
- Reduced photosynthesis.
- Slow growth.
Without sufficient magnesium, plants cannot produce enough chlorophyll.
Other Important Minerals
Calcium (Ca)
Functions:
- Strengthens cell walls.
- Supports cell division.
- Promotes healthy root and shoot growth.
Deficiency symptoms:
- Poor growth of young leaves.
- Weak growing tips.
- Deformed new leaves.
Iron (Fe)
Functions:
- Needed to produce chlorophyll.
- Supports enzyme activity.
Deficiency symptoms:
- Yellowing between the veins of young leaves.
- Reduced photosynthesis.
Unlike nitrogen and magnesium, iron is not easily moved within the plant, so deficiency symptoms usually appear in younger leaves first.
Figure 4. Different mineral deficiencies affect different parts of the plant depending on each mineral's role and mobility.
Mineral Deficiencies
Scientists often identify nutrient deficiencies by observing leaf symptoms.
| Mineral | Main Function | Deficiency Symptoms |
|---|---|---|
| Nitrogen | Proteins, chlorophyll | Yellow older leaves, slow growth |
| Phosphorus. | ATP, DNA, roots | Poor root growth, purple leaves |
| Potassium | Enzymes, water balance. | Brown leaf edges, weak stems |
| Magnesium | Chlorophyll | Yellowing between veins of older leaves |
| Calcium | Cell walls | Poor growth of young tissues |
| Iron | Chlorophyll production | Yellowing between veins of young leaves |
Recognising these symptoms helps farmers diagnose problems quickly.
Mineral Nutrition and Plant Health
Healthy mineral nutrition allows plants to:
- Grow rapidly.
- Produce large healthy leaves.
- Carry out photosynthesis efficiently.
- Produce flowers and fruits.
- Resist disease.
- Survive environmental stress.
Poor mineral nutrition reduces crop yields and weakens plants.
For this reason, farmers often analyse soil nutrients before planting crops.
Figure 5. Adequate mineral nutrition supports vigorous growth, while nutrient deficiencies reduce plant health and productivity.
Fertilisers
When soils lack essential minerals, farmers may apply fertilisers.
Fertilisers supply nutrients such as:
- Nitrogen
- Phosphorus
- Potassium
These are often referred to as NPK fertilisers.
Although fertilisers improve crop growth, excessive use can lead to:
- Nutrient runoff.
- Water pollution.
- Eutrophication.
For this reason, fertilisers should be used carefully.
Worked Example
Question
A plant has yellow older leaves but the younger leaves remain green.
Which mineral is most likely deficient?
Solution
Answer: Nitrogen
Explanation:
Nitrogen is mobile within the plant. When supplies are limited, nitrogen is moved from older leaves to younger leaves, causing the older leaves to turn yellow first.
Real-World Connection
Modern farmers often test soil before planting crops to determine which nutrients are lacking. Based on these results, they apply the correct type and amount of fertiliser. This improves crop yields while reducing unnecessary fertiliser use, lowering costs and helping to protect rivers and lakes from nutrient pollution.
Did You Know?
The numbers printed on fertiliser bags, such as 10–10–10 or 20–10–5, represent the percentage of nitrogen (N), phosphorus (P), and potassium (K) they contain. Different crops require different nutrient balances depending on whether they are being grown mainly for their leaves, roots, fruits, or flowers.
Key Terms
Active transport – The movement of substances across a cell membrane using energy.
Chlorosis – Yellowing of leaves caused by a lack of chlorophyll.
Deficiency – A shortage of an essential nutrient required for healthy growth.
Fertiliser – A substance added to soil to provide essential mineral nutrients for plants.
Interveinal chlorosis – Yellowing of the leaf tissue between the veins while the veins remain green.
Magnesium – A mineral required to form chlorophyll.
Mineral ion – A dissolved nutrient absorbed by plant roots from the soil.
NPK fertiliser – A fertiliser containing nitrogen, phosphorus, and potassium.
Root hair – A specialised root cell that absorbs water and mineral ions.
Xylem – Vascular tissue that transports water and dissolved mineral ions throughout the plant.
Key Takeaways
- Plants require essential mineral nutrients in addition to water, carbon dioxide, and sunlight.
- Mineral ions are absorbed through root hairs, mainly by active transport.
- Nitrogen, phosphorus, potassium, and magnesium each have specific roles in plant growth and development.
- Different mineral deficiencies produce characteristic symptoms that help identify which nutrient is lacking.
- Good mineral nutrition supports healthy growth, efficient photosynthesis, disease resistance, and successful flowering and fruit production.