ABSTRACT
Nutrient cycling is the key to nutrient management in sustainable agricultural systems. Cycling can be viewed at several levels. On a field level in a natural system nutrients move from soil into plants and are returned to the soil via residue as plants die. Agricultural systems differ from natural systems because nutrients are removed from the cycle in the harvested product. A large fraction of the nutrients consumed by animals do not leave the cycle because they are returned to the soil in manure. Harvested crops, animals, and animal products leave the farm and are processed before being sold to consumers in the city. Much of the nitrogen in feces and urine is in the ammonia form or is quickly converted to ammonia. Because most of the excreta remains on the soil surface, the potential for ammonia volatilization is high. Soil moisture status can sometimes be modified by management, such as subsurface drainage or irrigation scheduling to minimize saturated conditions.
