February
7, 2005: Maintaining adequate soil nitrogen
levels is the first concern of many farmers considering
transitioning to organic production. But by growing
legume cover crops, soil nitrogen levels can be increased
to optimize organic production in short order, even
on heavily demanding crops such as corn.
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"We have a production
system that will produce nitrogen, control erosion,
uses energy efficiently, helps control weeds and
insects and solves farmer economic problems. It’s
free and [it's] called rotation."
- John Vogelsberg, Researcher |
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Another option for raising soil nitrogen rapidly is the
addition of compost and/or manure to supply the rate of
nitrogen needed for the target crop. For those that have
a hay market, alfalfa/grass hay production is particularly
suitable for building soil nitrogen during the transition
to organic production.
In The Rodale Institute Farming Systems Trial®,
now in its 25th year, we have increased soil nitrogen
from 0.29 percent to 0.34 percent by utilizing mixed
organic rotations and cover cropping. Control plots
managed in a conventional corn-soybean system have seen
soil nitrogen fall slightly, from 0.29 percent to 0.28
percent. Normally, as soil organic matter increases
under organic management, soil nitrogen limitations
are resolved.
In terms of building soil nitrogen, it's hard to beat
hairy vetch, which can fix over 200 pounds of nitrogen
per acre under favorable conditions. We plant hairy
vetch in August after small grain harvest and turn the
green manure under in early May before planting corn.
We get our nitrogen for the cost of growing and turning
under the legume.
In addition, we like to mix oats with hairy vetch.
The oats give early fall weed control and reduces the
seed cost on the legume. Stands that are a 50-50 mix
of oats and hairy vetch can produce the same levels
of nitrogen as hairy vetch alone.
Speaking of weeds, two-thirds of farmers we surveyed
recently say weeds are their number one production challenge.
Our experience supports that view, and we are working
with the Pennsylvania State University and the USDA-ARS
Sustainable Agriculture Laboratory on this challenge.
In the last two years, we have shown that some corn
and soybean varieties tolerate weeds better than others:
With the same weed pressure, yield losses ranged from
less than 10 percent up to 60 percent. We have also
seen that if weeds are less than 1000 pounds per acre
in dry matter, significant crop yield losses are minimal
even with susceptible soybean varieties.
As with soil nitrogen, cover crops are a key to organic
weed management. Using rye as a cover (with up to 8,000
pounds per acre rye dry matter), weeds are greatly reduced
in both susceptible and weed tolerant soybean varieties.
Organic systems are not something that can be created
over night. McSorley (2002) argues that a central focus
of the transition period should be lowering pest populations
and achieving a biological balance that will maintain
a high level of plant health.
In Maryland, researcher Ray Weil is demonstrating the
ability of oil radish and rape to produce a biological
fumigation and sub-soiling effect through the generation
of natural cyanide and roots powerful enough to fracture
the clay hard pan.
Although transitioning to organic may seem fraught
with pitfalls, you can see that we have a wide array
of effective biological strategies to meet the principal
challenges. John Vogelsberg (1987) summarized it this
way: “We have a production system that will produce
nitrogen, control erosion, uses energy efficiently,
helps control weeds and insects and solves farmer economic
problems. It’s free and [it's] called rotation.”
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