Crops with an extensive root system can help clean up excess nitrogen after shallow rooted crops. A 10-year study conducted by Agricultural Research Service scientists evaluated and compared potential management strategies for reducing nitrogen and nitrate nitrogen levels in soil and groundwater. The research focused on irrigated cropping systems in the Arkansas River Valley, an agricultural region of Colorado. What was found is that corn was a good residual nitrogen scavenger crop.
The cause is, in part, due to heavy application of nitrogen fertilizer and the prevalence of shallow-rooted crops such as onions. Onions used only about 12 to 15% of the fertilizer nitrogen applied to the crop. Much of the remainder stayed in the top six feet of soil. The next year corn recovered about 24% of the fertilizer nitrogen that had been applied to the onion crop.
Following that study, the scientists grew alfalfa on the land for five years, followed with a watermelon crop and then a corn crop. In the first year that corn was grown, an unfertilized control plot yielded about 250 bushels of corn. By comparison, a plot fertilized with 250 pounds of nitrogen per acre yielded about 260 bushels.