Proceedings 2008 |
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CD-AY-332 |
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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. . Larry Bundy Professor Emeritus |