The main objectives of field drainage are to promote favorable soil' water-air relations. A distinction can be made between soil drainage and crop drainage. Soil drainage is required to maintain soil structure and soil nitrogen supply in states favorable to plant growth. It is also required to maintain soil traffic and workability; i.e. plowing, cultivating, and decrease puddling by livestock. For these reasons the benefits of soil drainage have an indirect but important effect on the productivity of the land. Crop drainage is required to obtain a well aerated root zone in which the plant can grow and flourish to produce maximum yield.