AGRONOMIC AND ENVIRONMENTAL BENEFITS OF WATER-TABLE MANAGEMENT

The benefits of water-table management for increasing crop yields, and reducing nitrate pollution were assessed for the St. Lawrence lowlands region of eastern Canada. Soybeans [Glycine max (L.) Merr. cvs. Apache and KG 30] were grown in 0.4-m I.D. field lysimeters in relatively wet (1990) and dry (1991) years, and subjected to controlled water tables of 0.4, 0.6, 0.8, and 1.0 m. The latter was regarded as conventional, free-outlet drainage. Plant parameters, including yield. as well as nitrate, ammonium, and moisture distributions in the soil profile (0.3 m and 0.7 m) were measured. Seed and pod number per plant, and percent nitrogen in the seed were approximately 20% higher for the 0.4-, 0.6-, and 0.8-m water-table treatments. compared to the conventional drainage treatment. No significant differences were observed between the three shallowest water-table depths. Soil nitrate levels were higher in the conventional drainage treatment, compared to the controlled water tables, particularly at the 0.7-m sampling depth. There were no significant differences in nitrate between the three shallower water-table depths. Soil ammonium levels were lowest under conventional drainage and increased throughout the growing season, particularly at the 0.7-m sampling depth. Due to lack of significant differences in yield and soil nitrate for the 0.4-, 0.6-, and 0.8-m water tables, it is recommended that water tables be maintained at 0.6 to 0.8 m. This practice will result in reduced nitrate contamination of water resources and maximum soybean yield.