Optical sensing signals nitrogen needs.

Fertilizer recommendations are based in part on yield projections. And as growing conditions influence yield potential, the yields you based your fertilization program on may have been significantly different than actual yields. According to Dr. Bill Raun, plant scientist with Oklahoma State University (OSU), the biggest problem with most fertilization programs is that growers apply all the N they think the crop will need either at seeding or preplant. “The most inefficient use of N fertilizer is preplant,” says Raun.

In fact, Raun says, only 33 per cent of the N applied each year to agricultural crops is actually used by those crops. Based on 2008 fertilizer prices, it’s estimated growers worldwide could save over $25 billion a year by increasing the nitrogen use efficiency (NUE) by just 20 per cent.

Since 2001, Raun and other researchers at OSU have been seeking a better way of determining how much N a crop actually needs. Initially they used an N ramp calibration strip to provide visual comparisons of the growing crop with a check strip that stepped rates of N ranging from 0 to 200 lbs/ac. Comparing the growing crop with various rates of N helped determine the amount of N that needed to be applied in-crop to maximize yield potential.

This research led to the development of the GreenSeeker; an optical sensing device that can measure the biomass produced per day in a growing crop. By comparing the GreenSeeker crop readings with measurements taken in a check nitrogen rich strip placed in the same field, the GreenSeeker can accurately predict the yield potential of a crop at any growth stage and tell the grower exactly how much additional N needs to be applied in-crop to achieve those yields.

Six years ago OSU demonstrated these N management systems on a 120 acre winter wheat field on Brent Rendel’s Oklahoma farm. Rendel applied 50 lbs of N when seeding his winter wheat and, as is customary in his area, expected to top dress another 50 lbs of N in the spring. However, the GreenSeeker sensor indicated no additional N was needed in the spring. Although skeptical, Rendel did not top dress additional N and was very surprised to find this field yielded one bushel above the average of the rest of his 2,000 acres.

He has continued to use the nitrogen rich strip and believes he gets $10 to $15 more net profit per acre than if he was still following the conventional rules of thumb for fertilizer application. “With this system, when wheat does not need additional N, you save money. But you also make more money through higher grain yields when the GreenSeeker indicates higher levels of N are needed. And this additional profit is really easy to capture at very little cost.”