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One of my major concerns
about the transition to organic is my ability to maintain
soil fertility. Conventional grain
farmers routinely apply nitrogen fertilizer as a hedge to
maximize yield, even on fields of high fertility. Organic
grain farmers don't have the luxury of inexpensive, readily
available nitrogen fertilizers. Instead, they must carefully
manage their soils, patiently building and maintaining soil
fertility with legume fallow crops, crop rotations, manures
and composts.
Here in upstate New York, organic farmers often transition former
hay fields since they most easily meet the certification requirements.
In these situations, the initial soil nitrogen status may be
unknown. As fields move through the rotation, moreover, the
decrease or increase in soil nitrogen depends on (among other
factors) the weather over the past few years, each year’s
crop yield and the success of the legume fallow crop. Unlike
with other fertility elements such as phosphorous and potassium,
many factors can cause soil nitrogen loss. Even plowing and
cultivating stimulates nitrification, denitrification and subsequent
nitrogen loss.
Historically, conventional
grain farmers have estimated nitrogen fertility by using the
pre-plant soil nitrate test. Unfortunately,
nitrate is typically the smallest and most variable fraction
of the total supply. The soil nitrate level has been shown
to vary radically with soil temperature, moisture and numerous
other factors. It is also the form of soil nitrogen most susceptible
to leaching and loss.
About 20 years ago, in an effort to eliminate some of the
uncertainty, a pre-sidedress nitrate test was developed for
conventional corn production. This test had the advantage
that application of nitrogen fertilizer could be delayed until
soil nitrate values were higher, so that nitrogen could be
applied only where needed. Unfortunately, the variability
of soil nitrate was still a factor, and unless test results
showed very high levels the usual practice was to apply nitrogen
anyway as a hedge. Obviously this is not practical for an
organic farmer.
A new way to measure soil nitrogen
Soil scientists have long known that the total soil nitrogen
content of even highly depleted soils was many times higher
than the nitrogen available to the crop. The total soil nitrogen
of an acre-furrow of soil is typically 2,000 to 4,000 pounds
per acre, far more than the 200 pounds per acre required by
a corn crop. At any given moment, soil nitrate can account
for only 10 to 30 pounds of soil nitrogen per acre. It is
apparent, then, that there is some component of total soil
nitrogen that acts as a reservoir for the growing crop. The
other obvious form of nitrogen is ammonium. Unfortunately,
the ammonium form is even more volatile and harder to measure
than nitrate.
Soil scientists speculate that this more readily available
fraction of soil nitrogen probably consists of the plant material
from previous crops and soil microbes both living and dead.
In the 1990s, two soil scientists from the University of Illinois,
Richard Mulvaney and Saeed Khan, were trying to explain why
many soils judged low in available nitrogen gave no increased
yield when nitrogen fertilizer was applied. Their
goal was to avoid the unnecessary over-fertilization that
causes water pollution. They showed evidence that there
are two soil nitrogen fractions: first, a readily available
fraction consisting of sugar-like compounds loosely defined
as 'amino-sugars;' second, a less available fraction consisting
of amino acids and protein-derived compounds called the 'amino
acid' fraction.
Both of these fractions contained 200 to 400 pounds per acre
of the nitrogen needed by a growing crop.
When they compared the yield response of a wide variety of
soils, they found that the amino-sugar fraction correlated
well with the resulting yield response. They observed
that if the soil amino-sugar content was over 245 parts per
million (ppm), there was no yield increase to applied nitrogen.
And in addition, if the soil amino-sugar content was less
than this 245 ppm level, the yield response was inversely
related to the 'amino-sugar' content. The lower the amino-sugar
level, the more dramatic the yield response.
They subsequently went on to develop a fairly simple practical
test, which they called the “alkali-labile soil derived”
nitrogen test, to measure this amino-sugar fraction. Since
it is performed in a wide-mouth Mason jar, we refer to it
as the “Mason Jar” soil nitrogen test. Unfortunately,
this information is of little value to conventional farmers
since they either rely heavily on applied fertilizer or, in
the case of large livestock and dairy farms, they have an
excess of manure which they must dispose of in any event.
However, for the organic grain farmer
this new test offers a way to monitor the success of his or
her farming practices and can remove some uncertainly from
decision making. The test is conducted on air-dried
soil samples collected in late winter or early spring, leaving
plenty of time to make decisions before the busy spring season.
Since the components being measured are relatively stable,
no complicated storage and handling conditions are necessary.
The only precaution is that if there has been a recent application
of manure or ammonium fertilizer, the test must be modified
to correct for this with a pre-treatment step.
Trying it for yourself
In order to discover how this new test might be used, we
assembled the necessary materials and equipment and began
testing soils from our own farm. We collect at least 30 core
samples to a depth of 10-12 inches from each field. Samples
are spread out on a large cookie sheet and allowed to air
dry in a warm place (80-90° F) until completely dry. Each
sample is then pulverized and blended before being sub-sampled
for the test.
We have obtained values ranging from 200 to 350 ppm depending
on the field and the stage in the soil fertility building
process. For the most part, the test
values follow trends anticipated by the cropping history and
yield potential. Two situations gave somewhat surprising
results, however. The first was a lower than expected value
following one year of clover fallow. It may be that one year
of fallow is not enough to accumulate and replenish this soil
nitrogen fraction. The second was an old grass hay meadow
from which the hay had been harvested for over 15 years without
any added fertilizer. Despite a very low phosphorous level
and low hay yield, the soil had test values of about 350 ppm.
In a third case, a field tested following three years of
alfalfa-timothy sod and one corn crop gave a Mason jar test
value of greater than 250 ppm. This suggests that a second
corn crop might be feasible in the crop rotation for this
field. As test results accumulate in the future, we should
gain a more complete understanding of their significance.
Richard Glenister has been raising cattle on a small
farm in central New York for many years, and is in the process
of transitioning to certified organic mixed grain and livestock
production. He wishes to acknowledge the generous assistance
of Dr. Mulvaney with this project.
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