Effect of controlled drainage/sub-irrigation on tile drainage water quality and crop yields at the field scale

Controlled drainage and subirrigation have been recommended as a sustainable agricultural management practice. Controlled drainage regulates tile discharge to provide storage of rainfall and minimize tile discharge losses of nitrogen and other agricultural chemicals. Stored water and nitrate can also be used by the crop during dry periods in the growing season which would otherwise leach from the crop root zone. Controlled drainage combined with subirrigation have been shown to improve corn and soybean yields. The system however needs to be evaluated at the farm scale for its impacts on sustainable agriculture and environmental quality. A large farm scale evaluation of this new water management system on producer land raises farmer awareness and acceptance of these innovative technologies. In Southwestern Ontario, three field scale 'onfarm' demonstration sites were established in cooperation with three producers. A controlled drainage (CD) system was established on two clay loam sites in a paired watershed (no- tillage versus conventional tillage), whereas a controlled drainage and subirrigation (CDS) system was established on a sandy loam site. The objectives of the study were to provide on-farm demonstrations of controlled drainage and subirrigation systems, and to determine their effect on crop yields and environmental benefits. On two clay loam sites, soil structure, organic matter content and water storage in the soil profile were improved with no-tillage (NT) compared to conventional tillage (CT). No-tillage also increased earthworm populations. No-tillage was found to have higher tile drainage volume and nitrate loss which were attributed to an increase in soil macropores from earthworm activity. The controlled drainage system (CD) reduced nitrate loss in tile drainage water by 14% on CT site and 29 % on NT site compared to the corresponding free drainage system (DR) from May, 1995 to December, 1996. No-tillage farming practices are definitely enhanced by using a controlled drainage system for preventing excessive nitrate leaching through tile drainage. Average soybean yields for CT site were about 12 to 14% greater than the NT site in 1995 and 1996. However, both CT and NT treatments had little effect on soybean yields, between free drainage and controlled drainage systems in 1995 and 1996 due to extremely dry growing seasons. On a sandy loam site, the controlled drainage and subirrigation (CDS) system reduced flow weighted mean nitrate concentration in tile drainage water by 31% and total nitrate loss by 24% compared to the free drainage (DR) system from May, 1995 to December, 1996. The controlled drainage and subirrigation system increased marketable tomato yields by 11% in 1995. The average marketable tomato yields were 58.4 t ha-1 for DR system and 64.9 t ha-1 for CDS system. The CDS system also increased corn yields by 64% in 1996. The average corn yields were 6.7 t ha-1 for DR system and 11.0 t ha-1 for CDS system. Thus, the CDS system effectively reduced total nitrate loss and improved yields of both processing tomatoes and grain corn on a sandy loam soil.