Fertilizers are organic or inorganic substances containing chemical elements that improve plant growth and fertility of the soil. Organic fertilizers (manure) are derived from animal products and plant residues containing organic nutrients such as nitrogen. However, the percentage content of nutrients in organic fertilizers is relatively low. In inorganic or mineral fertilizers, the nutrients are inorganic salts, obtained by extraction or physical and chemical processes.
Inorganic fertilizers are most commonly a blend of three primary plant nutrients in varying proportions:
N - nitrogen,
P - phosphorus,
K - potassium.
Source: Glossary of Environment Statistics, Studies in Methods, Series F, No. 67, UN, 1997