A fertilizer may be an organic or inorganic material added to the soil to replace or increase plant nutrients.

Organic fertilizers – including animal and green manure, fish and bone meal, guano (seabird excrement), and compost – are decomposed by soil microorganisms, and their elements are freed for plant use.

Most inorganic or chemical fertilizers contain the major nutrients (nitrogen, phosphorus and potassium) in proportions required by the crop.

Properly used, fertilizers increase crop yields; they do not affect a crop's nutritive properties unless specifically intended to do so.