2.2 Nutrient Requirements in Plants

Classification of Plant Nutrients

Plants require nutrients for growth, development and reproduction.

Group	Meaning and examples
Macronutrients	Elements or minerals required in large quantities: nitrogen, phosphorus, potassium, magnesium, calcium, sulphur, oxygen, carbon and hydrogen.
Micronutrients	Elements or minerals required in small quantities: boron, manganese, molybdenum, copper, iron and zinc.

Functions of Macronutrients

Nitrogen: synthesises protein, chlorophyll, enzymes and nucleic acids; promotes growth of branches and leaves; increases fruit and seed production.
Phosphorus: synthesises protein and nucleic acids; promotes cell division for flower and fruit production.
Potassium: synthesises protein; promotes plant cell division; strengthens resistance to infectious diseases.
Magnesium: forms an important component of chlorophyll.
Calcium: synthesises spindle fibres during cell division; promotes meristem growth at shoot and root tips.
Sulphur: synthesises protein and chlorophyll; promotes fruit development and plant cell division.
Oxygen: involved in plant cell respiration to produce energy.
Carbon: builds sugar or starch during photosynthesis.
Hydrogen: builds cellulose.

Functions of Micronutrients

Boron: synthesises chlorophyll and cell walls; promotes formation of fruits and seeds.
Iron: promotes growth of young parts of a tree and helps cell respiration.
Copper: involved in photosynthesis and respiration.
Molybdenum: synthesises protein and promotes plant growth.
Manganese: involved in photosynthesis and respiration; promotes formation of amino acids.
Zinc: promotes protein synthesis, chlorophyll formation and leaf formation.

Effects of Nitrogen, Phosphorus and Potassium Deficiency

Deficiency	Effects on plant growth
Nitrogen	Stunted growth; weak stem; top leaves are smaller and fall easily; bottom leaves are pale green or yellow due to lack of chlorophyll (chlorosis); flower and fruit production is stunted; root growth deteriorates.
Phosphorus	Stunted growth and slow maturation; weak stem; top leaves are smaller and purplish; bottom leaves are bluish green, curled and have brown edges; flower and fruit production stops; root growth is reduced.
Potassium	Stunted growth and death before maturity; weak stem; top leaves have brown spots, curled ends and chlorosis; bottom leaves wilt and have dead tissues around the edges; flower production is reduced, fruit production stops and root growth is reduced.

Experiment 2.2: Effects of Macronutrient Deficiency

Aim: Study the effects of nitrogen, phosphorus and potassium deficiency on plant growth.

Problem statement: What are the effects of nitrogen, phosphorus and potassium deficiency on plant growth?

Hypothesis: Deficiency of nitrogen, phosphorus and potassium stunts plant growth.

Variables:

Manipulated: type of culture solution.
Responding: growth of plant.
Constant: volume of culture solution, size and type of seedlings, light and temperature.

Materials: Distilled water, complete culture solution, culture solutions without nitrogen, phosphorus or potassium, maize seedlings, black paper and cotton wool.

Apparatus: Boiling tubes, connecting tubes, air pump and corks.

Set-up: Place one maize seedling in each boiling tube containing complete culture solution or a culture solution lacking nitrogen, phosphorus or potassium. Wrap each tube with black paper and connect it to an air pump.

Procedure:

Place all set-ups in a bright place with sunlight.
Pump air into each culture solution for 5 minutes every day.
Replace each solution once a week with the same type of culture solution.
After two weeks, compare plant size, leaf colour and root growth.

Functions of the controls:

The complete culture solution is the control set-up for comparison.
Black paper prevents light from reaching the culture solution and prevents algal growth.
Aeration supplies oxygen for root respiration.
Weekly replacement maintains the nutrient concentration.

Conclusion: Plants in complete culture solution grow normally; deficiency of nitrogen, phosphorus or potassium stunts growth and produces characteristic changes in leaves, stems, flowers, fruits and roots.