6.3 Mineral Nutrition

Plants make their own food through photosynthesis — they use sunlight, water, and carbon dioxide to produce sugar. However, sugar alone is not enough. Plants also need mineral ions to stay healthy and carry out important life processes.

Mineral ions are tiny, dissolved particles that plants absorb from the soil through their roots. Think of them like vitamins that plants must get from their environment. Two of the most important mineral ions are nitrate ions and magnesium ions.

Nitrate ions (written as NO₃⁻) are particles found in the soil that contain the element nitrogen. Plants absorb nitrate ions dissolved in water through their root hair cells.

Nitrogen is a key ingredient in amino acids. Amino acids are the small building blocks that join together to make proteins.

Here is the step-by-step chain:

Inside the plant, sugar is made during photosynthesis. This sugar contains the elements carbon, hydrogen, and oxygen — but not nitrogen. The plant combines the sugar with nitrate ions (which supply the nitrogen) to build amino acids.

An amino acid is a small molecule that contains nitrogen. It is the basic unit — or "building block" — from which proteins are made.

Once amino acids are made, the plant links them together in long chains to form proteins. Proteins are large, complex molecules that the plant uses for many vital jobs, including:

• Building and repairing cells
• Making enzymes (special proteins that speed up chemical reactions inside the plant)
• Helping the plant grow

If the soil is low in nitrate ions, the plant cannot make enough amino acids, and therefore cannot make enough proteins. The result is:

• Stunted growth — the plant stays small because it cannot build new cells properly
• Yellow leaves — especially the older, lower leaves turn yellow first. This yellowing is called chlorosis (when leaves lose their green colour). Without enough protein to maintain healthy cells, the leaves break down.