What Do Plants Need To Feed On?

Plants take carbon, hydrogen and oxygen from the air and soil water to manufacture starch and sugars. All they need from the soil is a number of simple chemicals with which to create all the amino acids, proteins, vitamins, enzymes, etc.
Soils have these simple plant nutrients. They come from the mineral part of the soil and from the humus.
To feed a plant you need to give it the same simple things. Nutrients in a liquid fertilizer containing inorganic salts will be immediately available to the roots. Nutrients in a coarsely-ground organic form may wait months for release.
When ground is cultivated and plants grown, the balance is upset. Essential elements in the soil diminish more rapidly than can be replaced naturally. The most serious loss concerns: Nitrogen, Phosphorus and Potassium. These are the major nutrients, required in volume. They must be applied regularly. Some will come from compost or manure, but fertilizers are the main supply.
A fertilizer is a material which provides appreciable quantities of one or more of the major nutrients without adding significantly to the humus content of the soil.
The rule is to feed plants regularly but no more than the package recommends. If undecided, be guided by their vigour.