Both organic and inorganic fertilizers are used to keep croplands productive. A. Animal manures are effective organic fertilizers, but their use in the United States has decreased because of changes in modern farming. B. Many economically important crops, including corn, tobacco, and cotton, can only be grown in the same fields year after year if large amounts of inorganic fertilizers are added to the soil. C. Since 1950, farmers worldwide have begun to replace organic fertilizers with inorganic commercial ones, because the latter helps soil to retain oxygen. D. Compost, a humuslike material that is rich in organic material, is often used together with green manure, since the combination of the two helps to prevent soil erosion. E. Green manures, compost, mushroom spores, and crop rotation, if used correctly, all have positive impacts on growing conditions for crops that go beyond simply providing nutrients. F. Commercial inorganic fertilizers play a key role in feeding the world's population, but they can also cause serious environmental damage.