Weed Suppressive Soils

I had an opportunity to attend the annual meeting of the Weed Science
Society of America, held in Norfolk, Vir., earlier this week.  A wide range
of papers, sessions, and posters addressed the current state of the art in
chemical weed control; I organized a workshop on measurement of adoption of
Integrated Weed/Pest Management Systems, as they impact pesticide
use/reliance; several sessions addressed the new herbicide reistant
varieties and weed management systems.  I was most intested in sessions on
weed biology and ecology.  There was also an excellent afternoon symposium
on sustainable ag, featuring several large, successful organic/sus ag
producers, who discussed their approaches to weed management.
        The farmer presentations, coupled with the papers/psoters, left me
and others with a strong sense that it is time to much more formally address
the concept of weed suppressive soils, with the goal of figuring out
mechanisms of action and how farmers can alter management systems to enhance
these mechanisms.
        A poster by Dr. Robert Kremer, USDA-ARS, Univ. of Missouri was
titled "Biologically Active Metabolites of Rhizobacteria With Potential
Applications in Weed Management".  The poster assessed a number of bacteria
isolated from the roots of common weeds and crop plants that, under certain
circumstances, could be pathogenic to weeds, through the production of
auxins and/or hydrogen cyanide.  Just as rhizobacteria, and soil microboial
activity in general, plays a key role in nutrient cycling and micobial
bioconrol of plant pathogens, might it be that the same mechanisms produce
auxins and/or trigger the plant to produce weed suppressive root exudates
that are pathogenic to weeds?  It would make sense for nature to equip
plants with this capacity, for the same reasons root exudates also play a
critical role in nutrient cycling and other areas of pest management.
        Dick and Sharon Thompson of Boone Iowa made a fascinating
presentation, including much information about weed succession as a function
of tillage and herbicide use.  They spoke of their experiences with foxtail
in their ridge till system.  They want to see a good flush of foxtail prior
to planting with the Buffaloe planter.  The planter takes care of these
weeds, and leaves the ground looking nearly black, since the soil from the
top of the ridges is kicked into the center of the rows.  Sharon Thompson
made the perpelxing statement during the talk -- "the little weeds somehow
tell the other weeds not to bother.." or some such comment.  At the time it
made little sense, but with further thought, a possible mechanism occured to
me.  This ground was also treated with manure before planting, clealy adding
additional weed seeds -- yet the Thompson's reported little trouble with
weeds for several weeks after the planting operation.  Their point is that
sometimes the best thing to do is just leave the ground alone.  What might
the mechanism be?   Their experience undermines the conventional wisdom that
there is an unavoidable trade-off between tillage and dependence on
herbicides.  In effective sus ag systems, it is equally important NOT to
till at several key points of the season, while also using mechanical
cultivation tools -- minimally evasive -- at just the right times and right
ways.  Tillage is good and bad, depending on when and how it is deployed.
There is lots of data that shows that tilling at the worng time causes weed
problems to worsen and dependence on herbicides to increase.  The
conventional wisdom is that this is because tillage stirs up to the surface
buried weed seeds; but tillage also dramatically depresses microbial
activity, which might also provide weed seedlings a window of opportunity.  
        In the Thompson ridge-till system, the conditions are ripe for a
very high level of microbial activity in the weeks after planting.  It is
possible the combination of manure, residues and other factors stimulates
rhizobacteria indigenous to the region, that have always lived with (and
attacked) the weeds of the region.  But the reliance on mechanical
cultivation and use of the ridge till planter early in the year would knock
back the young weed seedlings, killing growth of their roots.  But the
decaying roots would remain an excellent food source for the
weed-suppressive rhizobacteria present in the soil, which would then also
benefit from spring heat and moisture, and the animal manure.  The bacteria
would undoubtedly go through a flush, despite the lack of weed roots to
attack.  In the days and weeks that follow, any weed seedling that
germinates would be overwhelmed by the bacteria in the soil.  Even if this
flush and weed suppressive state lasted only 2 weeks or so, that would
create the window during which the crop would get far enough ahead of
later-germinating weeds, assuring that mechanical cultivation would be easy
and effective.  It is intriguing to note that one reason this mechanism
could work at high enough a level to control weeds is that a first flush of
weeds is allowed and able to germinate, so that bactria get going on their
roots, prior to cultivation.  In a filed treated with a pre-plant herbicide,
their would be no weed roots for the bacteria to colonize, and hence no
microbes to trigger a flush if and when the conditions were ripe.  
        If researchers and farmers notice parts of fields with unusually low
weed pressure when all the conditions seem ripe for high pressure, it might
be worthwhile to explore this possible explanation.  It would be interesting
to do some experiments on land coming out of the CRP, which will have very
high weed seedbanks, high organic matter and high microbial activity.   It
might be possible to manage CRP lands returning to production in a way that
maximizes biological breakdown of weed seeds in the seedbank, and controls
those seeds that germinate.  Treating such land with hot fertilizers and/or
soil insecticides will surely depress this mechanism of weed suppressive.
        Weed scientists are now exploring the notion that seed predation,
and microbial mechanisms that limit seedling viability, may explain observed
differences between weed model predictions and actual yield outcomes -- in
many fields, the linkage between weed seeds and crop yields, as impacted by
control measures, does not seem to make sense, especially in reduced and
no-herbicide plots/farms.  Something other than cultivation is helping to
control the weeds on some farms. It may be possible that what we have been
attributing to alleopathy is actually microbial biocontrol in action.
        I would be interested in people's thoughts and observations on this
topic.  If anyone knows Dr. Kremer at Missouri, please share this post with
him, and perhaps he would offer his thoughts/reactions.