By Robert Battel
Existing subsurface (tile) drainage systems can be modified to provide crops with water during dry periods, and decrease the amount of nutrients exiting the system.
Crops grown on fine-textured and poorly drained soils require artificial drainage for optimum crop production. A dual-purpose subsurface drainage system can drain away excess water in wet periods and hold water for crop use in dry periods. Additionally, holding water in the soil profile can help keep some nutrients from escaping through the tile system.
With controlled drainage, water control structures installed on the main pipe allow the water in the field to be raised or lowered. Boards, or stoplogs, in water control structures are removed (water table is lowered) one to two weeks prior to planting, so the field dries for spring field operations. The boards (stoplogs) are re-installed (water table is raised) as the crop grows during the growing season to create a potential storage of water for the crop, especially important if there is limited rainfall throughout the summer. The boards are again removed about a week prior to harvest and remain out during fall field operations. The boards are again installed after fall field operations, and remain in the structure (water table is raised) throughout late fall and winter.
When the water level is kept high in the field through controlled drainage, water and the nutrients carried with it are forced through the preferred path of the soil via seepage. A high water table can also promote denitrification, thus reducing the loss of nitrate through the water. Although controlled drainage has also been documented to reduce phosphorus transport from of the field, research for dissolved reactive phosphorus (highly bioavailable for organisms) reduction with controlled drainage is scarce.
For this purpose, Michigan State University, led by Ehsan Ghane, is investigating the effectiveness of controlled drainage in Lenawee County at two on-farm sites with varying soil types. This research will determine the effectiveness of this practice in reducing nutrient load from the field.
For controlled drainage to work well, field topography must be nearly flat, 0.5 – 1.0 percent, typically. Operating a controlled drainage system also requires initial installation costs and a moderate level of management. However, if a landowner or operator has flat fields and is willing to meet the cost and management obligations, controlled drainage can help meet production and environmental protection objectives.