Residual Soil Nitrate Content and Profitability of Five Cropping Systems in Northwest Iowa
Many communities in the Midwestern United States obtain their drinking water from shallow alluvial wells that are vulnerable to contamination by NO3-N from the surrounding agricultural landscape. The objective of this research was to assess cropping systems with the potential to produce a reasonable return for farmers while simultaneously reducing the risk of NO3-N movement into these shallow aquifers. From 2009 to 2013 we conducted a field-based experiment in Northwest Iowa in which we evaluated five cropping systems for residual (late fall) soil NO3-N content and for profitability. Soil samples were taken annually from the top 30 cm of the soil profile in June and August, and from the top 180 cm in November (late fall). The November samples were divided into 30 cm increments for analysis. Average residual NO3-N content in the top 180 cm of the soil profile following the 2010 to 2013 cropping years was 154 kg ha-1 for continuous maize (Zea mays L.) with a cereal rye (Secale cereale L.) cover crop, 31 kg ha-1 for perennial grass, 74 kg ha-1 for a three year oat (Avena sativa L.)-alfalfa (Medicago sativa L.)-maize rotation. However, residual nitrate in the 90 to 180 cm increment of the soil profile was as low in the oat-alfalfa-maize cropping system as it was in the perennial grass system. For 2010 to 2013, average profit ($ ha-1 yr1 ) was 531 for continuous corn, 347 for soybean-winter wheat-maize, 264 for oat-alfalfa-maize, 140 for oat/red clover-maize, and -384 (loss) for perennial grass. Rotations utilizing tap-rooted perennial species, such as maize-maize-alfalfa-alfalfa or maize-soybean-winter wheat-alfalfa-alfalfa, appear promising from both economic and environmental perspectives.