Updated December 2, 2016:
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Impact of drainage water management and subirrigation on soil and water quality
Xinhua Jia and Thomas Scherer
Department of Agricultural and Biosystems Engineering
North Dakota State University
Fargo, ND 58108

Soybean is very sensitive to soil moisture and salinity conditions in the field. For the last twenty years, it has been difficult to plant or harvest many fields in the Red River Valley due to wet conditions. To solve this problem, tile drainage was quickly established in the region to remove excess water, reduce the soil salinity, and create critical windows of time for planting and harvesting. However, tile drainage water contains high soluble chemicals, including nitrate and dissolved minerals (salts) that can pollute our streams and lakes. In this study, we used six farm fields, four in Clay County, Minnesota, and two in Richland County, North Dakota, to monitor the nutrients and salts coming out from tile drainage outlets.
In our research, we compared four different ways to manage the water and reduce the nitrate loss from a tile-drained field. These included conventional drainage, meaning the drainage water is directly discharged to a surface ditch; controlled drainage, which has a control board at the drainage outlet to control the time and amount of drainage flow; and subirrigation, which adds water back to the tile drained field at the control drainage structure. We also have two check fields, which only have surface drainage and no tile drainage. Our results indicated that nitrate nitrogen concentration in the tile drainage water (28.3 ppm) was 22 times higher than that in the surface drainage ditches (1.3 ppm). With controlled drainage, the nitrate was retained in the field. Phosphorus monitoring showed that both surface and subsurface flows contained phosphorus concentrations that exceeded the water quality standard. A higher amount of salts were found in tile drainage flows, when compared to the surface ditch water. With good water management practices in the field, we can help retain the nitrate and salts in the field in late spring and summer, and improve the water quality in our surface water environment.
Soil salinity is a big concern for many soybean growers because it can cause soybean iron chlorosis and reduce the soybean yield. Tile drainage is the only effective way to permanently leach out the salts, reduce the soil salinity, and keep the land productive. We monitored the soil salinity changes in the field using soil sampling and salinity maps. Our results clearly indicated that soil salinity was reduced with tile drainage. The biggest benefit to the soybean growers and the soybean industry is to understand that through drainage water management, we can achieve a better yield in the field and maintain a clean water for all citizens in the Red River Valley.