Fertilizer is any material supplement that is added to soil to supply one or more plant nutrients that are essential to plant growth. These can be organic (i.e. derived from animals, plants, fungi) or inorganic (i.e. minerals), and they can be natural (e.g. compost) or artificial (e.g. ammonium nitrate fertilizers).
Water (as with atmospheric carbon dioxide and oxygen) is classed separately and is not considered a fertilizer. Similarly, non-nutritional soil improvers (e.g. sand for drainage, and lime for pH) are also not considered fertilizer.
Fertilizers typically provide:
- Macro-nutrients: Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, Sulphur
- Micro-nutrients: Boron, Chlorine, Copper, Iron, Manganese, Molybdenum, Zinc
And are typically provided in ionic form - e.g. nitrate and chloride.
Commercial (and especially artificial) fertilizers are often labelled with their NPK content. These are three numbers giving the composition (by weight) of nitrogen (N), Phosphate (P2O), and Potassium Oxide (K2O). Note the discrepancy: Nitrogen is atomic weight (it is typically in NO3 ions) but Phosphor and Potassium are listed by molecular weight. This can lead to confusion, especially as Potassium is often provided in other forms (e.g. Potash / KCl).
Specific plants may like (or react differently to) stronger or weaker concentrations of these nutrients, but good general advice is for a "balanced fertilizer" that provides all nutrients.