Purdue report: Applying too much nitrogen is wasteful, risky

WEST LAFAYETTE, Ind. - Using more than the recommended amount of nitrogen-based fertilizers on a corn field is a waste of money and could pose environmental risks, two Purdue Extension agronomists say in an updated report.

“Applying more than enough nitrogen is no longer the cheap insurance it once was many years ago,” said Jim Camberato, soil fertility specialist and co-author of Nitrogen Management Guidelines for Corn in Indiana. “High nitrogen fertilizer costs and environmental impacts should encourage growers to critically evaluate their nitrogen management program, including application rate, fertilizer material and timing.”

Nitrogen is the most expensive nutrient used in corn production. If applied properly, it makes individual plants stronger and increases yield.

“Beyond some level of applied nitrogen, grain yield stops increasing with more additions,” said co-author Bob Nielsen, Extension corn specialist. “Consequently, applying more nitrogen than the crop requires is dollar wasteful and environmentally distasteful.”

Plants are able to use only a certain amount of nitrogen, depending on the soil type, weather conditions and other factors. Excess nitrogen can be lost by leaching or runoff or by passing off as vapor through volatilization, potentially polluting the air or water systems.

In the report, Camberato and Nielsen provide updated, region-specific guidelines for nitrogen use based on field trials throughout Indiana. For corn grown after soybean, the economic optimum nitrogen rate, or EONR - based on corn grain at $3.50 per bushel and nitrogen fertilizer at 40 cents per pound - varied considerably across the state.

The authors conclude the EONR for the medium and fine-textured soils commonly found in the southern and west-central regions of the state is 176 pounds of nitrogen per acre. The EONR for the northwest and north-central regions is 183 pounds, 201 pounds for the central region, 220 pounds for the east-central region, and 223 pounds for the northeast region. Non-irrigated sandy soils have an EONR of 191 pounds per acre. Any previously applied spring or at-planting nitrogen or credits for manure or a leguminous cover crop should be subtracted from the guideline. Corn grown after corn requires about 40 to 50 pounds more nitrogen per acre than these guidelines. EONR for other grain and nitrogen prices are listed in tables at the end of the report.

Timing the application is also important, Camberato said. Applying fertilizer in the fall or early spring increases the chances of nitrogen loss. The most efficient application method is to inject the nitrogen-based fertilizer when plants are in the V6 growth stage, or about 18 inches tall with six leaves.

“The bottom line on nitrogen use in corn is that it is part of a complex biological system that interacts with everything under the sun, including the sun,” Nielsen said. “We cannot accurately predict the weather. We cannot accurately predict the soil nitrogen supply throughout the year. Yet we cannot afford financially or environmentally to simply apply ‘more than enough’ nitrogen.”

The report is available on Nielsen’s Web site, the Chat ‘n Chew Café, at http://www.kingcorn.org/news/timeless/NitrogenMgmt.pdf. 

Writer: Darrin Pack, 765-494-8415, dpack@purdue.edu 

Sources: Jim Camberato, 765-496-9338, jcamera@purdue.edu

Agricultural Communications: (765) 494-2722;
Keith Robinson, robins89@purdue.edu
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