With nitrogen (N) costs in the $400/ton range, getting the most out of your N is a must, especially when 200 units or pounds or more are needed to make a corn crop. The benefits of residual N — the free kind — are becoming more important.

Arkansas' Grady Keith knows that. So does Kearney, NE, grower Dick Prascher. Both count on local N recommendations. Records from soil tests are considered, but yield results are the biggest reason these growers carefully plan their fertilizer needs.

A corn-following-corn rotation usually provides the highest potential for significant residual N build up. This is often due to the amount of fertilizer N added to the previous corn crop. Manure additions are also often made to continuous cornfields. When soybeans are grown previously, much less residual N is usually found because beans will use residual N in the soil in addition to the N that it symbiotically fixed within its nodules.

Keith has only been growing corn for four years on his Southeastern Arkansas farm near Lake Village. He has yet to face a drought situation with a corn crop, but when it happens he realizes that even though yields could be hurt, residual N left behind from a low-yielding crop could help save fertilizer costs the following year.

“We rotate corn with cotton and usually put down 200 units of nitrogen,” says Keith, who also grows soybeans. “There is no preplant application. The N is knifed in with a coulter after the corn develops a good stand. That sidedressing is usually about the first week of April.”

Previous soil tests normally indicate the need for a 200-lb. application each year. Keith sticks with that level and averages 135 bu. on dryland corn and 175 bu. on irrigated corn.

“Corn is new in the rotation so we haven't had any corn grown twice on the same field,” he says. “I know I'll eventually see a dry year in which corn yields are low, especially on the dryland. That could enable us to decrease our N applications the following year, whether we come back with cotton or corn. We don't apply any N to our soybeans because of the residual that remains from the previous year.”

Prascher's corn and soybean rotation is similar to many in the Midwest. He's been at it more than three decades. Residual N from corn means he must apply only about 25 units of N to soybeans. Soil tests have shown about 40 lbs. of N remains in the soil after soybeans.

“Soil tests have always indicated we needed about 220 units of N for our corn,” says Prascher. “After we pencil in the 40 units left over from the beans, we apply 175-180 units for corn.”

Those applications are split in his ridge-till operation. “We apply about 40 units with our phosphorous, sulfur and zinc on top of the row with the ridge-till,” says Prascher. “The remaining application is made through an anhydrous ammonia system. We apply what we need to meet soil test specifications.”

Parts of Missouri faced a drought in 2003. University of Missouri agronomist Peter Scharf tested previously parched acres to determine residual N.

“Corn yields were low for most fields in this area (northwest and central area), meaning that nitrogen uptake also was low,” he says. “The crop wasn't able to effectively use the nitrogen fertilizer that had been applied.”

He and his research team sampled soil to a depth of 3 ft. in five fields where corn was grown in 2003. They found the average soil N content to be 105 lbs./acre.

“Typically, we expect to find about 50 lbs./acre of nitrogen in a field that hasn't been fertilized, so our soil test results were about 55 lbs./acre higher than normal,” he says. “This suggests that if these fields are returned to corn, nitrogen rates could be cut back without hurting yield.”

The amount of residual N in the soil depends on the rainfall received the previous year. In contrast to years following drought, when fields will have significant residual N, relatively wet years will produce little residual N for the next crop.

In Minnesota, Extension agronomists are looking at better testing to determine residual N and other nutrients. A five-step process is suggested to determine N recommendations:

  1. Determine N recommendations based on yield goals, the previous crop and the soil's organic matter content.

  2. Determine whether conditions are such that residual N may be appreciable.

  3. If soil N testing is recommended, collect a preplant, 0-2 ft. soil sample and send to a laboratory for nitrate-N analysis.

  4. Determine residual N credit based on the measured soil nitrate-N concentrations.

  5. Calculate the final N recommendation by subtracting the residual N credit from the previously determined N recommendation.

Growers should watch out for over application. Besides adding to unwanted costs of production, over application can cause environmental concerns.

The best approach may be the one Prascher takes year after year. “We don't use any more N that we have to,” he says.