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"The Illinois Soil N Test is a
new type of soil test that measures the amount of organic nitrogen
available for mineralization, the process that generates inorganic
nitrogen for crop uptake," said Richard Mulvaney, U of I 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 said.
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
percent 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," said Khan, co-developer of the
Illinois Soil N Test and U of I research specialist in agriculture.
However, more than 50 percent
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," explained Khan.
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"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," said Mulvaney. "If this increase has taken place
over time, we are very interested in knowing more about what has
caused it."
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," said 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 percent perfect yet, and it will probably never be 100 percent
reliable," he said.
The failure rate so far has
been 10 percent in detecting non-responsive sites and about 20
percent in detecting responsive sites.
"The latter 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," said Mulvaney.
To
date, the funding organization of this research has been the
Fertilizer Research and Education Council, but the researchers are
seeking alternative funding sources for the next phase, which is a
field calibration study.
[University
of Illinois news release]
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