Fertilizers help plants grow faster. Natural fertilizers, such as cow and horse manure, were once the only kinds of fertilizer that farmers used. But in order to grow more food, especially on degraded soil, chemical fertilizers were developed.

When fertilizer is applied to fields, the crops cannot absorb all of it. Some fertilizer runs off the ground and washes into rivers, streams and lakes. The nitrates in the fertilizers increase the growth of algae, polluting the water. When the algae die and decompose, oxygen in the water is used up, which causes fish and other aquatic creatures to die.