Organic farming is an alternative agricultural system which originated early in the 20th Century in reaction to rapidly changing farming practices. Organic agriculture continues to be developed by various Organic Agriculture organizations today. It relies on fertilizers of organic origin such as compost, manure, green manure, and bone meal and places emphasis on techniques such as crop rotation, companion planting. Biological pest control, mixed cropping and the fostering of insect predators are encouraged. Generally, although there are exceptions, organic standards are designed to allow the use of naturally occurring substances while prohibiting or strictly limiting synthetic substances.