Zeroing In On Optimum Nitrogen Rates

New soil test measures the amount of organic nitrogen instead of basing recommendations on long-term averages. Compiled by staff

Published on: Mar 11, 2004

In the past, farmers have determined nitrogen application by using long-term averages. Now a new type of soil test promises to be more scientific and accurate.

"The Illinois Soil N Test measures the amount of organic nitrogen available for mineralization, the process that generates inorganic nitrogen for crop uptake," says Richard Mulvaney, University of Illinois professor of soil fertility. "By contrast, current nitrogen recommendations according to the Illinois Agronomy Handbook are based on long-term averages, a kind of one-size-fits-all approach that seldom works in the real world. These recommendations were never intended for individual fields in a particular growing season, and yet, year after year, this is how they have been used."

Mulvaney says the usual outcome is either under- or overfertilization, mostly at the farmer's expense. But he says that's all that can realistically be expected of a system that ignores soil nitrogen availability. "The fact is that the soil supplies the majority of nitrogen taken up by the corn crop, even with heavy fertilization," he says.

Corrects overfertilization problems

When the Illinois Soil N Test was compared with the current yield-based system, some interesting differences emerged.

"One major difference that stands out is the ability of the test to predict sites that will not respond to nitrogen. Remarkably, over six growing seasons, it was 90% effective in predicting non-responsive sites. By comparison, only six of these sites would have been detectable by the Illinois Agronomy Handbook method due, in each case, to the application of manure above accepted rates," says Khan, co-developer of the Illinois Soil N Test and U of I research specialist in agriculture.

However, more than 50% of the remaining non-responsive sites were cropped to continuous corn or corn after soybeans, which points out the problem with the current system.

"A major problem with the present system is that nitrogen credits for manure or a previous crop of soybeans does not apply beyond one year. This leads farmers to overfertilization. Even in the case of continuous corn, where nitrogen rates can exceed 200 pounds per acre, there is no nitrogen credit for unused nitrogen," explains Khan.

"Also, the widespread occurrence of sites non-responsive to nitrogen in our study suggests the possibility of an increase in the nitrogen-supplying power of Illinois soils," says Mulvaney. "If this increase has taken place over time, we are very interested in knowing more about what has caused it."

How the Illinois Soil N Test works

The Illinois Soil N Test is relatively simple. First, a soil sample is treated with strongly alkaline solution. Then the sample is heated on a griddle for five hours, converting amino sugar nitrogen to gaseous ammonia. The ammonia is collected in an acidic trapping solution. The amount of ammonia is determined by titration, which estimates the soil's nitrogen supplying power.

Efforts are under way to make the test even simpler so it can be done by any soil testing lab or even by a farmer himself.

Despite the advances made, many questions remain about such issues as the optimal sampling depth, the best time for sampling, and especially the number of samples needed per acre. "It just takes time to answer these questions because of the need for an extensive database large enough to ensure reliability," says Khan.

The test has not yet been officially released to soil testing facilities or the public. Mulvaney wants to improve on the failure rate first. "We're not claiming the test is 100% perfect yet, and it will probably never be 100% reliable," he says.

The failure rate so far has been 10% in detecting non-responsive sites and about 20% in detecting responsive sites. "The later failures are the biggest concern, but we think most of these were related to field variability and that they could have been avoided by improving the sampling strategies," says Mulvaney.