A nutrient is any substance that organisms require for growth, sustaining life or reproduction.
Nutrients can be classified as both elements and compounds, depending on their nature and their function within ecosystems. Examples of the former include carbon, nitrogen, phosphorus and potassium. The latter can be either inorganic compounds, such as carbon dioxide for carbon and nitrates for nitrogen, and organic compounds, such as carbohydrates, proteins and vitamins.
Nutrients can also be categorized into macronutrients and micronutrients. The former are required in larger quantities and include nitrogen, phosphorous, potassium, calcium, magnesium and sulfur. Among the micronutrients are iron, copper, zinc, manganese and boron.
Nutrient cycles, also called biogeochemical cycles, are the absorption, transferring and release back into the environment of nutrients, and then their eventual reabsorption among cells, organisms, and ecosystems, including both their living and non-living components. Nutrients consumed by plants and animals are returned to the environment by excretions (e.g., manure and fallen leaves) and after death and decomposition.