Nitrogen Application Rates for Municipal Biosolids and Industrial Wastewater

Richard Wolkowski, Carrie Laboski, and Larry Bundy

Department of Soil Science

University of Wisconsin-Madison

Tuesday, May 16, 2006

The University of Wisconsin has recently adopted the Maximum Return to Nitrogen (MRTN) approach for corn N rate guidelines.  The MRTN approach considers the N:corn price ratio and is most applicable where purchased N fertilizer is used, but can be used to guide manure or other N-containing organic material application rates.  The MRTN approach provides greater flexibility to fine-tune N management based on economic factors and site-specific field conditions. 

How do these guidelines affect the selection of the N rate for municipal biosolids (sewage sludge) or other wastewater applications where significant N contribution is expected?  As previously stated the MRTN approach is most appropriate where purchased N fertilizer is used.  If N sources are provided at no or low cost then there are few economic considerations.  Crop advisors should select a rate that falls in the 0.05 N:corn price range.  Using this price ratio the MRTN rate for a high yield potential soil would be 165 lb N/a, and could range between 135 and 190 lb N/a.  Recognize that the MRTN is the total N applied, including starter.  The MRTN rate guidelines to use for municipal biosolids and other wastewater residual applications are shown in Tables 1.  Forage legume credits (alfalfa, red clover, etc.) have not changed and should be subtracted from the selected MRTN rate as they have been in the past.

Biosolid and wastewater application rates should not be significantly altered from previous N management practices using the MRTN approach.  Attention must still be given to P management, as the MRTN approach will not reduce the excess application of P commonly found with many materials applied to meet crop N need.

Table 1.  MRTN rate guidelines for corn grain where municipal biosolids and other N-containing wastewater residuals.                         

¹ Rate is the N rate that provides the maximum return to N (MRTN)

² Range is the range of profitable N rates that provide an economic return to N within $1/a of the MRTN

³ These rates are for total N applied including N in starter fertilizer and N used for herbicide applications

⁴ Subtract N credits for forage legumes, leguminous vegetables, green manures, and animal manures.  Do not subtract N credits for leguminous vegetables on sands and loamy sands.

⁵ Subtract N credits for animal manures and second year forage legumes.