Managing N Losses to Water from Agricultural Fields

Leaching is a pathway of N loss of a high concern for water quality. Soil particles do not hold on to nitrate very well because both are negatively charged. As a result, nitrate easily moves with water in the soil. The rate of leaching depends on soil drainage, rainfall, amount of nitrate present in the soil, and crop uptake. The EPA has set the maximum contaminant level for drinking water at 10 ppm N in the form of nitrate (US EPA, 2005; www.epa.gov/safewater/mcl.html#mcls).

In general, well-drained soils, unexpected low crop yield, high N inputs (especially outside of the growing season) and high rainfall are all conditions that increase the potential for nitrate leaching from the soil. In addition, fields with a thin layer of soil, regardless of drainage class, over fractured bedrock and coarse textured soil over aquifers have an elevated potential for leaching losses to groundwater. Improving N efficiency in field crop management can significantly reduce the loss of nitrate to ground- and surfacewaters. Strategies described in the previous pages of the Crop and Soil N Management section and summarized, below, will improve N use efficiency in field crop production. The following best management practices to reduce N leaching losses are based on research done by, among others, Sogbedji and coworkers (2000) and Van Es and coworkers (2002) and can be found in the New York Nitrate Leaching Index Manual (Czymmek et al., 2003).

• Unless the New York Phosphorus Index identifies the need for P based fertility management, manure and fertilizer application rates should be based on Cornell guidelines for meeting crop N needs.
• For corn, pre-plant (other than starter fertilizer) and early post plant broadcast applications of commercial nitrogen without the use of nitrification inhibitors are not recommended.
• Sidedress applications should be made after the corn has at least four true leaves.
• If starter N must be broadcast (e.g., for small grains or new seedings of grass), apply fertilizer as close to expected planting date as possible (ideally within 3 days).
• For row and cereal crops, including corn, maintain starter fertilizer N rates below 50 lbs/acre actual N under normal conditions.
• Manure and fertilizer applications should be adjusted based on information provided in “Nitrogen Recommendations for Field Crops in New York”, (Ketterings et al., 2003b).
• Evaluate the need for sidedress N applications based on PSNT or other soil nitrate-nitrogen tests.
• Sod crops should not be incorporated in the fall. Chemical sod killing may be carried out when the soil temperature at the four-inch depth is approaching 45^oF. Depending on location, this will not likely take place until early October.
• Minimize fall and/or winter manure application on good grass and/or legume sod fields that are to be rotated the following spring.
• Appropriate ammonia conservation is encouraged. Losses can either be reduced by immediately incorporating manure or eliminated by directly injecting manure as a sidedress application to growing crops.
• Plant winter hardy cover crops whenever possible, regardless of, but especially when, fall manure is applied (e.g., rye, winter wheat, or interseed ryegrass in summer).
• Manure may be applied in the fall where there is a growing crop. Judicious amounts of manure can be applied to or in conjunction with perennial crops or winter hardy cover crops.
• Applications should generally not exceed the greater of 50 lbs/ace of first year available N or 50% of the expected N requirement of next year’s crop.
• Frost incorporation/injection is acceptable when soil conditions are suitable but winter applications should be made in accordance with the New York Phosphorus Index.
• Manure N application on legumes is acceptable to satisfy agronomic requirements when legumes represent less than 50% of the stand. When legumes represent more than 50% of the stand, manure may be applied at a rate not exceeding 150 lbs of available N/acre.

Other findings and case studies for reducing N losses from fields to ground- and surfacewaters are found in the articles, below.

Manure and Groundwater Protection Guidelines

• By K.J. Czymmek, H.M. Van Es, and L.D. Geohring. 2004 [Online]. Available at http://nmsp.css.cornell.edu/publications/groundwater.pdf (accessed 16 September 2005; verified 16 September 2005). Cornell University, Ithaca, NY.

Supplemental Manure Spreading Guidelines to Reduce Water Contamination Risk During Adverse Weather Conditions

• By K.J. Czymmek, L.D. Geohring, Q.M. Ketterings, P.E. Wright, and A. Eaton. 2005. What’s Cropping Up? 15(3): 1-3.

Impact of dairy farming on well water nitrate level and soil content of phosphorus and potassium.

• By S.J. Wang, D.G. Fox, D.J.R. Cherney, S.D. Klausner, and D.R. Bouldin. 1999. J Dairy Sci. 82:2164-2169.

Hutson, J. L., R. E. Pitt, R. K. Koelsch, J. B. Houser, and R. J. Wagenet. 1998. Improving dairy farm sustainability II: Environmental losses and nutrient flows. J Prod Agric. 11(2):233-239.

Sogbedji, J.M., H.M. van Es, C.L. Yang, L.D. Geohring, and F.R. Magdoff. 2000. Nitrate leaching and N budget as affected by maize N fertilizer rate and soil type. J. Environm. Qual.
29:1813-1820.

Van Es, H.M., K.J. Czymmek, and Q.M. Ketterings. 2002. Management Effects on N leaching and Guidelines for an N Leaching Index in New York. J. Soil Water Conserv. 57(6): 499-504.