Organic farming is a production system that sustains agricultural productivity while avoiding or largely excluding synthetic fertilizers and pesticides. Whenever possible, external resources, such as commercially purchased chemicals and fuels, are replaced by resources found on or near the farm. These internal resources include solar or wind energy, biological pest controls, and biologically fixed nitrogen and other nutrients released from organic matter or from soil reserves. Thus organic farmers rely heavily on the use of crop rotations, crop residues, animal manures, compost, legumes, green manures, off-farm organic wastes, mechanical cultivation, mineral-bearing rocks, and aspects of biological pest control to maintain soil productivity and tilth, to supply plant nutrients, and to control insect pests, weeds, and diseases. In essence, organic farming aims to promote soil health as the key to sustaining productivity, and most organic practices are designed to improve the ability of the soil to support...