It has long been recognized that soil organic matter is the key to soil fertility. As a nutrient store it gradually provides essential elements which the soil cannot retain for long in inorganic form. It buffers growing plants against sudden changes in their chemical environment and preserves moisture in times of drought. It keeps the soil in a friable, easily penetrated physical condition, well-aerated and free draining, providing young seedlings with an excellent medium for growth. But it has another property, the nature and extent of which have been the subject of argu ment and controversy...