3.1 - Main Inorganic Nutrients

Macronutrients and Micronutrients Required by Plants

Plants require inorganic nutrients to produce organic compounds such as carbohydrates and proteins.
Nutrients are divided according to the quantities required by plants:
- Macronutrients: carbon (C), hydrogen (H), oxygen (O), phosphorus (P), nitrogen (N), sulphur (S), potassium (K), calcium (Ca) and magnesium (Mg).
- Micronutrients: chlorine (Cl), zinc (Zn), iron (Fe), copper (Cu), manganese (Mn), nickel (Ni), boron (B) and molybdenum (Mo).
Carbon, hydrogen and oxygen:
- Are easily obtained from the air, water and soil.
- Form most of the dry mass of plants; deficiencies are rare.
Other nutrients are taken up as mineral salts dissolved in the soil through fertilisation.

Macronutrients

Carbon (C), oxygen (O) and hydrogen (H)
- Functions: important components of the carbon and oxygen cycles; components of all organic compounds in plants; important in sugar synthesis.
- Deficiency: photosynthesis does not take place; less oxygen is released; growth is stunted and may cause death because glucose is absent.
Nitrogen (N)
- Functions: gives plants their green colour through chlorophyll formation; main component of proteins, nucleic acids and enzymes in photosynthesis and respiration.
- Deficiency: chlorosis, mainly in matured leaves; lower leaves fall; protein synthesis is disrupted.
Potassium (K)
- Functions: important in protein synthesis and carbohydrate metabolism; cofactor for some enzymes; maintains plant turgidity.
- Deficiency: stunted growth; protein synthesis is disrupted; leaf edges become yellowish; premature plant death.
Calcium (Ca)
- Function: main component of the middle lamella, cell wall and spindle fibres during cell division.
- Deficiency: stunted growth; distorted and lobed leaves; regions between leaf veins become yellowish.
Magnesium (Mg)
- Functions: main component of the chlorophyll molecule; activates some plant enzymes; involved in carbohydrate metabolism.
- Deficiency: regions between matured leaf veins become yellowish; red spots appear on leaf surfaces; leaves become lobed.
Phosphorus (P)
- Functions: synthesises nucleic acids, adenosine triphosphate (ATP) and phospholipids in the plasma membrane; acts as a coenzyme in photosynthesis and respiration.
- Deficiency: unhealthy root growth; leaves become dark green and dull; red or purple spots appear on older leaves.
Sulphur (S)
- Functions: component of some amino acids; component of vitamin B and some coenzymes.
- Deficiency: leaves or the whole plant turn yellow.

Micronutrients

Chlorine (Cl)
- Functions: important in balancing cell osmotic pressure and in photosynthetic reactions.
- Deficiency: wilting; slow root growth; chlorosis; reduced fruit production.
Iron (Fe)
- Functions: cofactor in chlorophyll synthesis; important for the growth of young plants.
- Deficiency: young leaves turn yellow.
Manganese (Mn)
- Functions: activates photosynthetic enzymes; important for cell respiration and nitrogen metabolism.
- Deficiency: dark green veins against a light green background; light brown or grey spots between leaf veins.
Boron (B)
- Functions: assists roots in calcium-ion uptake and sucrose translocation; involved in carbohydrate metabolism; assists pollen germination.
- Deficiency: terminal buds die and growth becomes abnormal; leaves become thick, rolled and fragile.
Zinc (Zn)
- Functions: important in leaf formation; synthesises auxin (growth hormone); cofactor in carbohydrate metabolism.
- Deficiency: leaf surfaces become spotted with chlorotic parts; stunted growth.
Copper (Cu)
- Functions: involved in nitrogen metabolism and photosynthesis; important for growth, reproduction and flower formation.
- Deficiency: young shoot apexes die; brown spots appear on terminal leaves; stunted growth.
Nickel (Ni)
- Function: component of plant enzymes that break down urea into ammonia usable by plants.
- Deficiency: stunted growth; reduced crop yield; burnt effects at leaf tips due to urea accumulation.
Molybdenum (Mo)
- Function: involved in nitrogen fixation and nitrate reduction during protein synthesis.
- Deficiency: chlorosis between matured leaf veins; pale green leaves; reduced crop yield.

Knop’s Culture Solution

A culture solution is used to study the importance of nutrients for plant growth.
A complete Knop’s culture solution contains all nutrients, including trace elements, required for healthy growth.
Wilhelm Knop prepared the complete culture solution in 1859.
Composition:
- Calcium nitrate, $C a (N O_{3})_{2}$ : 0.8 g
- Potassium nitrate, $K N O_{3}$ : 0.2 g
- Potassium dihydrogen phosphate, $K H_{2} P O_{4}$ : 0.2 g
- Magnesium sulphate, $M g S O_{4}$ : 0.2 g
- Iron(III) phosphate, $F e P O_{4}$ : trace amount
- Distilled water: 1000 cm³
The effects of nitrogen, phosphorus and potassium ratios are investigated by growing equal-sized maize seedlings in:
- Complete Knop’s culture solution.
- A control without mineral nutrients.
- Culture solutions without nitrogen, phosphorus or potassium; replacement salts keep the other mineral nutrients present.
Keep the solution volume constant, wrap culture bottles with black paper to prevent algal growth, supply oxygen to the roots, replace the solutions weekly and place all set-ups in light.
After four weeks, compare leaf colour, plant height, root length and stem strength.