2.21 understand that plants require mineral ions for growth and that magnesium ions are needed for chlorophyll and nitrate ions are needed for amino acids

In addition to water, sunlight and carbon dioxide, plants also require specific minerals. Different mineral ions do different things, for example...

Nitrate – used to make amino acids for use in plant proteins
Magnesium – forms part of the chlorophyll molecule
Potassium - essential for cell membranes
Phosphate - essential part of DNA and cell membranes