Fertilisers are soil amendments applied to promote plant growth; the main nutrients present in fertiliser are nitrogen (N), phosphorus (P), and potassium (K) (the macronutrients). Other nutrients (micronutrient) are sometimes added in smaller amounts. There are inorganic fertilisers (mined or synthetically produced) or organic fertilisers. Due to a growing world population, today, the mineral resources of the main components of fertiliser (e.g. P) are depleting leading to exaggerate prices and the difficulty of small-farmer to provide them with sufficient fertiliser for food production. In sustainable sanitation systems, nutrients, initially contained in food, is recycled from wastewater products (e.g. urine, faeces etc.) providing an affordable and environmentally sound source of fertiliser at a local level.