Nitrogen Rate and Water Quality Following BMP’s

Jerome Lensing and George Rehm – Discovery Farms – Minnesota

The 4R concept is an excellent guide for nutrient use in Minnesota. This concept is usually thought of as affecting production; but, there are water quality implications as well. As harvest wraps up, thoughts turn to RATE of N to be used for the grain crop in the rotation next year. This discussion will focus on the importance of testing in determining RATE of N to use.

To illustrate the impact of RATE of N used, let’s consider the edge of field monitoring at a farm in Blue Earth County, where both surface flow and subsurface flow were measured for nitrogen. Manure from hogs in confinement is usually applied before the corn crop in a corn/soybean rotation. Soils have a silty clay or silty clay loam texture.

RATE of manure used is adjusted for the analysis of the manure which has remained relatively constant from year to year. Potential loss of N is minimized because manure is fall applied and injected. Based on the manure analysis, RATE of manure used is considered to be adequate for optimum yields which have been in the range of 180 to 220 bu/acre.

Data from the 2015 water year (Oct. 1, 2014 through Sept. 2015) are used to illustrate nitrogen movement in the landscape. Most of the rainfall (30.8 inches) occurred during the spring and summer. Nitrogen from surface flow was 3.4 lb. N per acre. For subsurface flow (tile lines), N movement was 23.0 lb. N/acre. Soil samples following corn harvest showed a total of 34 lb. N/acre to a depth of 2 feet. Considering the high organic matter content (5.0% to 6.0%), this is a low amount and a good indication that an excessive RATE of N was not used.

Use of laboratory analysis is an important management practice for this Minnesota Discovery Farm. Analysis of the manure is a very important piece of information. Analysis of soil samples to a depth of 24 inches for nitrate-N after harvest is an excellent tool to indicate if N RATE has been excessive.  Analysis of soil samples collected to a depth of 24 inches for nitrate-N has been a suggested management practice to determine RATE of N to use for corn for many years. This is a fall practice for the western Corn Belt. Suggested time for sampling shifts to spring in eastern Minnesota and the Eastern Corn Belt. In addition to an asset in making decisions regarding production, analysis of manure and/or soil will continue to be a suggested management practice for protecting water quality.