In 1800, more than 80 per cent of the American population worked and/or lived on a farm. In the modern world, by contrast, a mere 2 per cent of the U.S. population are farmers -- and yet that 2 per cent are able to feed far more people than were alive two centuries ago. The difference is due in large part to industrial agriculture.
Plants get energy from sunlight and carbon dioxide (CO2) from the air, but the important nutrients they also need come from the soil -- nitrogen, phosphorus and potassium being the three most prominent examples. Ordinarily, when plants die and decay, decomposers recycle their remains. When crops are harvested and eaten, however, the nutrients the plants contained become part of human or animal biomass and are not returned to the soil. Various natural processes replace these nutrients over time but are slow acting.
In the past, farmers used to let their fields lie fallow one year in every three, giving soil microorganisms more time to replenish nitrogen and allowing natural processes to replace nutrients the farmers had removed. Alternatively, they would rotate crops, planting legumes in certain years to enrich the soil. These methods were relatively inefficient, however, and fertilisers make it possible to produce more food on the same amount of land by sustaining a higher rate of plant growth. Since farmers do not need to leave their fields fallow, they can produce crops on each field every year rather than just some years for certain fields and other years for others.
Pests and Weeds
Pests devour one-third of the world's gross food production, according to Duke University. Many of the most destructive pests are insects, although bacterial pathogens and fungal diseases like fusarium wilt also take a heavy toll. Undesirable plants or "weeds" may also be able to outcompete the species farmers are trying to grow. The weeds do not need to expend any energy on producing large fruits or calorie-rich seeds like the plants people have bred for their own use; consequently, in a competition between weeds and crops, the weeds may have the advantage.
Armed with pesticides, farmers can kill off insects that might otherwise devastate their yields -- and prevent weeds from taking advantage of the fertilisers meant for crop plants. Unfortunately, some pesticides are toxic to humans or other animals and may have environmental effects associated with their use. Moreover, pests can sometimes develop resistance to pesticides over time, forcing farmers to turn to newer or even more environmentally unfriendly agents. Consequently, while pesticides do a great deal to increase yield, it's important to use them wisely.