an essay by Peter A. Ensminger
This passage is an excerpt from an essay by Peter A. Ensminger,
“Control of Weeds by Plowing at Night,” from his collection of
essays Life Under The Sun. ©2001 by Peter A. Ensminger and
Yale University Press.
Many millennia before the invention of herbicides,
farmers simply plowed their felds to control weeds. Even
today, plowing can constitute a valuable part of an integrated
weed-management program. Although plowing kills
standing weeds, farmers have long known that it ofen
leads to the emergence of new weed seedlings in a few
weeks.
Ecologists have shown that a farmer’s feld can have
50,000 or more weed seeds per square meter buried
beneath the soil surface. Plant physiologists have shown
that seeds buried more than about one centimeter below
the soil surface do not receive enough light to germinate.
Do the blades of a plow, which can reach more than a foot
beneath the soil surface, bring some of these buried seeds
to the surface where their germination is induced by
exposure to sunlight?
Two ecologists, Jonathan Sauer and Gwendolyn Struik,
began to study this question in the 1960s. In a relatively
simple experiment, they went to ten different habitats in
Wisconsin during the night and collected pairs of soil
samples. Tey stirred up the soil in one sample of each
pair in the light and stirred up the other sample of each
pair in the dark. Tey then exposed all ten pairs to natural
sunlight in a greenhouse. For nine of the ten pairs of soil
samples, weed growth was greater in the samples stirred
up in light. Tey concluded that soil disturbance gives
weed seeds a “light break,” and this stimulates their
germination.
More recently, Karl Hartmann of Erlangen University
in Germany reasoned that when farmers plowed their
felds during the day, the buried weed seeds are briefy
exposed to sunlight as the soil is turned over, and that
this stimulates their germination. Although the light
exposures from plowing may be less than one millisecond,
that can be enough to induce seed germination. Tus the
germination of weed seeds would be minimized if farmers
simply plowed their felds during the night, when the
photon fuence rate (the rate at which photons hit the
surface) is below 1015 photons per square meter per
second. Although even under these conditions hundreds
of millions of photons strike each square millimeter of
ground each second, this illumination is below the
threshold needed to stimulate the germination of most
seeds.
Hartmann says that he was very skeptical when he
frst came up with this idea because he assumed that such
a simple method of weed control as plowing at nighttime
must be ineffective or it would have been discovered long
ago. But the subsequent experiments, frst presented at a
1989 scientifc meeting in Freiburg, Germany, clearly
demonstrated that the method can be effective.
Hartmann tested his idea by plowing two agricultural
strips near Altershausen, Germany. Te farmer Karl
Seydel cultivated one strip, repeated threefold, at around
midday and the other strip at night. No crops were
planted in these pilot experiments, to avoid possible
competition with the emerging weeds. Te results were
dramatic. More than 80 percent of the surface of the feld
plowed in daylight was covered by weeds, whereas only
about 2 percent of the feld plowed at night was covered
by weeds.
Tis method of weed control is currently being used
by several farmers in Germany. Because many of the
same weed species that invade farmers’ felds in Germany
also invade felds elsewhere in the world, this method
should be successful elsewhere. In fact, recent studies at
universities in Nebraska, Oregon, Minnesota, Denmark,
Sweden, and Argentina support this idea.
Number of emerged seedlings
in soil disturbed in
Sample Source of soil light darkness
Number of Emerged Seedlings in Soil Samples
One Month afer Soil Was Disturbed
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