Fertilizers can greatly increase crop yields, but they can also cause environmental pollution and damage the balance of nutrients in the soil. Excessive fertilizer runoff can cause eutrophy in streams and bodies of water, creating such an overabundance of nutrients that algae and other microorganisms spread out of control. Overuse of fertilizer can also saturate the land with inorganic chemicals that may work their way into the water table.
Many synthetic fertilizers rely heavily on petroleum, a non-renewable resource. The byproducts of these fertilizers can also run off into streams and rivers, where they can decay into harmful substances and raise ammonia and methane levels. A 2007 study of salmon in rivers near Portland, Ore. found dozens of synthetic chemicals in the tissues and organs of the fish.
Organic fertilizers can also be dangerous if overused. The presence of too many nutrients in the water table encourages the growth of algae, which can use up the natural oxygen in waterways. Depleting the oxygen levels can kill fish and wildlife, and some varieties of algae even release toxins into the water that can be harmful to humans and larger animals. In August 2014, a toxic algae bloom directly over the municipal water intake for Toledo, Ohio rendered the entire city's water supply unsafe to drink.