Fertilizer (Civ5)/Civilopedia

Fertilizer is a substance that feeds and speeds the growth of plants. Fertilizer has been around for as long as human civilization, dating back to the time when the first farmer realized that grass grew taller where the oxen had pooped. From that point on farmers have been collecting animal by-products and applying them to the soil, increasing the crop yield, especially from fields that have been farmed continuously for generations and thus have been stripped of most nutrients.
Fertilizers generally contain nitrogen, phosphorous, and potassium as their active ingredients. While historically most of the fertilizers have been organic and animal-based, many modern fertilizers are actually mined from beneath the earth's surface or chemically manufactured in vast factories.
As the world's population has increased, so has the need for ever more potent fertilizers, and today's farms are far more productive than at any other time in history. However, this productivity has come at a price: rainwater from farms carries the fertilizer into streams and rivers, causing explosive growth in certain microorganisms which grow so fast they almost literally choke the life out of the waterway. Also, many modern fertilizers are created at least in part from petrochemicals, and their price can fluctuate dramatically along with the price of oil. In short, fertilizers are extremely useful and can greatly increase the world's food supply, but care must be taken to ensure that they don't do more harm to the environment than good.