Determining the optimum rate for those applications is no longer a matter of an old rule of thumb such as “1 pound of N per expected bushel of corn.” Today’s rate tools are getting away from yield-based recommendations and taking into account soil characteristics, plant physiology, the price of grain and the cost of fertilizer.

That makes “the right rate” a much better reflection of what will be most environmentally sound or generate the optimal return on investment — but it makes it a lot harder to pin down “right” from year to year.

For instance, optimum nitrogen rates for corn at Purdue’s research site near West Lafayette, Ind., from 2006-2012 have been calculated at 130, 182, 179, 165, 186, 186 and 221 pounds of N per acre, respectively. That’s a massive variation, and a vivid illustration of the fluctuations in soil N levels, fertilizer N loss, crop
health, plant N efficiency and weather that can occur from year to year on a single farm.

Purdue researchers have also noted that the agronomic optimum N rate — the AONR — varies significantly in a given year based on soil characteristics. The AONR for common Indiana soils can range from 180 pounds of N per acre in better-drained, fine-textured soils in west-central, north-central and northwest areas of the state to 216 pounds of N per acre in fine-textured, poorly drained soils in eastern areas.

Factoring in the price of N and the value of grain, the researchers calculate the economic optimum N rate — the EONR — which is lower than the AONR, and fluctuates with markets.

Thinking in terms of how to put every N molecule to work in the crop is a new and savvy way to approach fertilizer management.