Agricultural Drainage

Bulletin 871-98

Minnesota

Rate and Timing of Nitrogen Application

Results of several studies in Minnesota reveal that fertilizer N management, particularly the rate and time of application, plays a large role in the loss of nitrate-N to surface waters. Nitrogen is often applied as a fertilizer for corn in the form of anhydrous ammonia. Fall application for corn soon after soybean harvest has been a desired time of application for many farmers. A problem with fall application is the length of time between application and the use of nitrogen by plant uptake the next summer. High discharge volumes in late winter and early spring can result in the loss of N to surface waters.

Producers apply N in the fall for several reasons: (1) more time is available after harvesting soybean, and prior to or during corn harvesting; (2) fall N application frees up time in the spring to enhance the timeliness of planting (crucial to yields); (3) the soil is usually in better condition to receive N in the fall, especially following soybean where primary tillage is not required; and (4) less soil compaction occurs in the fall, and surface compaction that does occur can be relieved by freezing and thawing over winter. However, there are agronomic, economic, and environmental advantages to spring application of N.

In a 3-year study beginning in 1976, nitrate-N concentrations in drainage waters, loss from drainage systems, and accumulation in soil profiles, were determined following annual application rates of 18, 100, 200, and 400 lbs N/acre to continuous corn. There was relatively little increased nitrate-N accumulation in the soil profile or loss from drainage systems at the recommended application rate of 100 lbs N/acre compared to the lower rate of treatment (18 lbs N/acre). The total loss of nitrate-N to drainage systems over the 3-year study is shown in the graph (at right). Nitrate-N losses occurred when available N exceeded plant requirements.

Total loss of nitrate-N to subsurface drainage water related to field
application rates (3-year study period). The recommended rate based on
soil testing was 100 lbs N/acre.

A study was begun in 1986 at Waseca to address the economic and environmental concerns of fall application of N compared to spring application. The study focused on the effects of application timing on nitrate-N losses to drainage systems, and on yield and N uptake by continuous corn. The 5-year annual loss of nitrate-N to drainage water and the annual average corn yield are shown in the chart below for three different rates of N application (0, 120, and 180 lbs/acre). Nitrogen was applied as ammonium sulfate near November 1 for fall application and May 1 for spring application. The results show that nitrate-N losses to subsurface drainage are greatly influenced by rate of N application and moderately influenced by time of N application.