Purdue University | Indiana CCA

Proceedings 2008

Indiana Certified Crop Adviser Conference


Making Sense of Nitrogen Credits for Corn Production

Increasing nitrogen (N) fertilizer costs make accurate assessment of corn N needs more important than ever. Accurate determination of N fertilizer needs in corn production requires information about the amount of N necessary to optimize yield and estimates of how various factors influence the amounts of N available from non-fertilizer sources. Crediting or accounting for N from previous legume crops and manures is essential where these organic N sources are available. Established forage legumes can contribute most or all of the N needed by a following corn crop, and legume cover crops can provide smaller, but significant amounts of N. Depending on nutrient availability and application rate, manures can also provide the entire corn N requirement. Previously, the N contributions from manures and legumes were often discounted or viewed as “insurance” nutrient inputs; however, current economics makes accurate crediting critical for profitable corn production. Previous crop soybean effects on corn N needs are due to enhanced net N mineralization in soils rather than a direct N contribution from the soybean crop. However, corn following soybean usually requires less fertilizer N than corn following corn. The soybean N effect can best be accounted for using N rate recommendations based on N response trials conducted with corn following soybean and usually averages 30 to 40 lb N/acre. Residue management including tillage and residue removal can affect N availability to corn, but this influence depends on the previous crop. Soybean residue removal or tillage has little or no effect on the soybean N contribution. Where corn follows corn, residue removal and tillage influence soil temperature which, in turn, affects net soil N mineralization. The most difficult N contribution to assess is the amount of available N supplied by the soil (N supplying capability, NSC). This can be a significant component of the corn N supply with more than 50% of the crop N need coming from this source. The emerging maximum return to N (MRTN) method of making corn N rate recommendations based on N response data and the economics of N and corn prices partially accounts for soil NSC since the average NSC is reflected in the corn N response data base used to develop the recommendations. However, the need remains to develop a site-specific technique to predict the amount of available N that the soil will furnish. Various diagnostic tests can also be useful for predicting corn N needs or assessing non-fertilizer N contributions. The presidedress soil nitrate test (PSNT) has been found useful for predicting corn N needs particularly where contributions from legume forages and/or manures are expected. The PSNT critical value (20-25 ppm nitrate-N) in the top foot of soil when corn plants are 6 to 12 inches tall is applicable across a wide geographic area, but the test is influenced by early season soil temperature in northern production areas. Once the amount of fertilizer N needed to optimize yields has been determined, management techniques such as source, timing, and placement of the fertilizer N can influence the effectiveness of the applied N. For example fall-applied N has an average effectiveness 10-15% less than the same amount of N applied in spring, and surface applications of urea-containing fertilizers are subject to losses through ammonia volatilization that can range for 0-25%. Minimizing potential losses though effective use of management practices is key to optimizing economic returns from fertilizer N. .


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Larry Bundy Professor Emeritus 
University of Wisconsin-Madison

Larry Bundy is a Professor Emeritus at the University of Wisconsin-Madison. He was a Professor and Extension Soil Scientist for 25 years until retirement in 2007. He conducted research and educational programs on understanding and managing soil and fertilizer nutrients in crop production while minimizing nutrient losses to the environment. His work has emphasized nitrogen and phosphorus management for agronomic and environmental benefits, and he has authored over 65 research publications and over 200 Extension articles. He has received numerous research and extension awards, and he is a fellow of the American Society of Agronomy and the Soil Science Society of America. Before working at the University of Wisconsin, Bundy received B.S. and M.S. degrees form the University of Illinois and a Ph.D. degree in soil chemistry from Iowa State University. Subsequently, he worked for 8 years as a research scientist in the food processing industry.