The green color of corn leaves offers a quick check on the most economical level of nitrogen (N) fertilizer to apply to a growing crop, a University of Missouri soil scientist says.

"Leaf color measurements are more closely related to the most profitable nitrogen fertilizer rates than any soil test," says Peter Scharf, who cooperated in a fertility study with scientists in seven Corn Belt states.

Results of the leaf color study are in the current issue of Agronomy Journal, a major scientific publication. The scientists report on 66 N experiments on corn.

"Traditionally corn producers have applied generous rates of nitrogen fertilizer because it was relatively inexpensive," Scharf says. "Corn is so responsive to nitrogen that being caught short can be a disaster."

Sharp increases in the price of N fertilizer are causing corn producers to re-think this strategy.

The soil scientists measured yields at each N rate, which allowed them to calculate the most profitable rate of fertilizer in each of the 66 fields. They used a wide range of soil N tests in each field, along with the in-season corn-leaf color measurements.

The scientists used a Minolta chlorophyll meter to measure leaf color. They found readings from this meter were more closely related to the most profitable rate than any of the traditional soil tests.

"This suggests that corn color may be one of the best ways to predict how much nitrogen fertilizer to apply," Scharf says. Good predictions were achieved at the seven-leaf stage of corn growth, and as early as the five-leaf stage.

Both the most profitable N rate and the likely yield response to N could be predicted from the meter readings. "Predictions were best if they were based on comparison to a 'standard' reading taken from plants that received a high rate of nitrogen.

"Management decisions tailored to a specific field can increase the profitability, and the efficiency, of the fertilizer input," Scharf says.

Most corn producers in the North Central states do not apply in-season N. That may be changing.

"The potential to fine-tune nitrogen rates and save fertilizer without shaving yields may lead some farmers to consider this practice. Corn producers in southeastern states, where early-season nitrogen loss potential is higher, often sidedress corn," Scharf says. "Meter-assisted decisions could be adopted fairly easily in that region."

Hand-held meters were used in the experiments. "That may be too labor intensive for managing large corn fields," Scharf says. "However, the next generation of vehicle-mounted corn-color sensors, or aerial imagery, promise easier nitrogen-management decisions."

Related research suggests that diagnosing and precisely meeting crop N needs will benefit water quality, as well as improve economic response.

The field experiments were conducted in Illinois, Kansas, Michigan, Minnesota, Missouri, Nebraska and Wisconsin.