Top Crop Manager

Features Fertilizer Seed & Chemical
End-of-season stalk test adds confidence

Results encourage more use, research.

November 13, 2007  By Top Crop Manager

14aAs the price of inputs continues to rise, many growers are looking for ways
to minimize their reliance on nitrogen-based fertilizers. One of the latest
'tools in the toolbox' to help meet those needs may be an end-of-season stalk
nitrate test first used at Purdue University to detect nitrogen deficiency in
1930. In 1996, the test was reconfigured as a diagnostic tool at Iowa State

The key point that must be stressed is this test is an assessment of the past
growing season, not a predictive tool for the following year. Any literature
on the subject, including that from University of Nebraska and University of
Minnesota, emphasizes this.

The rationale behind the test is quite simple. As the corn plant matures, excessive
nitrate accumulates in the lower portions of the stalk. The test is a quick
and inexpensive method of gaining valuable insight into the crop's progress,
says Shawn Damen, field sales agronomist with Pioneer Hi-Bred. "As energy
prices rise, and with the current environmental focus, N is starting be a nutrient
we're paying more attention to," he explains, adding that he has done considerable
reading into the subject. "A lot of regions in the US seem to put a heavy
emphasis on it as a really good tool that farmers can use to make some significant
changes in the way they manage their nitrogen."


Sampling made simple
Taking samples from the field is a straightforward process to be done sometime
between one and three weeks after black-layer on 80 percent of kernels. Growers
should select three rows from the same area of a field and cut five or more
stalks from each row. Sampling must be done on healthy corn stalks, with no
signs of disease or insect damage and leaf sheaths should be stripped from the
segments. Cuts are made at six inches, and 12 to 14 inches above the ground.
University of Minnesota research suggests samples should include the bottom
node of the plant. The samples from each row should be placed in their own separate
paper bags. Plastic bags are more likely to promote the formation of mould.
Samples also may be refrigerated, but not frozen if not sent immediately to
the laboratory.

The scale for the results is also fairly straightforward: zero to 250ppm is
a low reading, meaning nitrogen was probably deficient during the growing season;
250 to 700ppm is marginal, where nitrogen may have limited yield; 700 to 2000ppm
is optimal and indicates there was likely no shortage of nitrogen, and 2000ppm
and above indicates excessive levels.

Damen likes the simplicity and cost benefits of the end-of-season test and
pledges to continue studying it in 2006. "What I want to do is focus on
growers who are using manure and hopefully prove to them that we can change
things over time," says Damen, adding he is looking for co-operators to
help with the research. "There's probably enough nitrogen there, with that
good soil structure, organic matter and from the 5000 gallons of liquid hog
manure, yet growers are still putting on 150 pounds of N just as insurance.
Until we can prove they don't need it, it's hard to get them to change."

He adds there is the potential for adapting the test to use at quarter milk-line
to help silage growers. "This kind of testing isn't a one year phenomenon.
We need to do this for a few years and get a handle on where we are to adjust
our N rates accordingly."

Know why to test and what they can do
Gaining more understanding through continued testing is fine with Keith Reid.
As soil fertility specialist with the Ontario Ministry of Agriculture, Food
and Rural Affairs, he is concerned with the type of information growers are
hoping to get from nitrate tests.

The fact it is called an 'end-of-season' stalk nitrate test indicates the kind
of information it yields. "It's sort of a report card on how you did, but
it's not the part of the report card that people usually pay the most attention
to," he says, conceding most growers still ask 'what was my yield and did
it pay to apply fertilizer?'. "As a tool, it's integrating the nutrient
supply from the soil, the weather conditions that carried that nutrient into
the plant and the ability of the plant to utilize it."

Reid agrees with US university literature which emphasizes the test only provides
a look back at what the crop has used, not a glimpse of what the crop needs
the following year. The only condition is that a grower would need an extensive
history of end-of-season results on which to base the coming year's nutrient

The other weaknesses Reid has found is that some growers take too much time
between gathering samples and sending them to the laboratory, with drying affecting
the results. "Getting the stalks to the lab as soon as possible after they're
harvested is going to get far more consistent results," says Reid, advising
against freezing the samples. "The stalk isn't like a leaf where you can
dry leaf tissues and the nitrate levels will be stable." The other weakness
is an apparent lack of sensitivity in its results. Although it can detect significant
over-supply or under-supply, there is a broad range in the middle where the
test is not as accurate. "It's fine-tuning probably within 50 pounds over
or 50 pounds under, but not where you're five pounds over or under," explains

Still supportive
However, he does believe the test has its benefits. Reid agrees with Damen that
livestock farmers who traditionally apply extra fertilizer on top of manure
applications can more-accurately gauge their success at season's end. "From
that perspective, it's hugely valuable. It'll give someone the confidence that
they can start shaving those fertilizer rates the next year," says Reid.



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