Sam WORTMAN & Frank FORCELLA
Grit-Blast Weed Control
Crop Protection 77:157-162.
Air-propelled abrasive grits reduce weed
abundance and increase yields in organic vegetable production
Sam E. Wortman
Two applications of abrasive grits reduced weed density by
Abrasive-weeding reduced weed biomass by 69–97%.
Abrasive-weeding increased tomato yield by up to 44%.
Organic fertilizers were effective when used as abrasive grits.
Abrasive-weeding is a novel weed management tactic with potential
to reduce tillage and hand-weeding in organic agriculture.
However, abrasive-weeding has not been tested in vegetable
cropping systems and growers are interested in the potential for
using organic fertilizers as abrasive grits to control weeds and
supplement crop nutrition in one field pass. A two-year field
study was conducted at the University of Illinois Sustainable
Student Farm to determine the effect of air-propelled abrasive
grit type, including organic fertilizers, and application
frequency on weed density and biomass and crop yield and
marketability in organic tomato (Solanum lycopersicum L.) and
pepper (Capsicum annuum L.) cropping systems. Abrasive-grits,
including granulated walnuts shells and maize cobs, greensand
fertilizer, and soybean meal, were applied via compressed air
between one and four times within planting holes of plastic mulch.
Weed density was quantified 25 or 37 days after the first
application and weed biomass was harvested at the end of the
growing season. Tomatoes and peppers were harvested ripe and
graded for marketability. Two applications of abrasive grits,
regardless of grit type, reduced weed density by 63% and 80% in
tomato and pepper, respectively. Broadleaf weeds were more
susceptible to abrasive-weeding than grass weeds. Abrasive-weeding
reduced final weed biomass by 69–97% compared with the weedy
control, regardless of grit type or application frequency. Total
tomato yield was up to 44% greater in treated plots compared with
the weedy control, whereas total yield gains in pepper (up to 33%)
were only approaching significance (p = 0.09). Yield and the
marketability of fruit was not negatively affected by grit
application, despite minor stem and leaf tissue damage after
applications. Organic fertilizers used as abrasive grits in this
study could contribute between 35 and 105 kg N ha−1, which may
improve the functionality and economic feasibility of
Sam Wortman : Abrasive Grit Weed Control
Why do organic farmers need specialized
technologies to control weeds?
While many conventional farmers can rely on just a handful of
herbicides to control most weeds, there are no comparable “silver
bullets” available to organic farmers. Instead, organic farmers
must develop an ecological approach to weed management that
includes a diverse mix of cultural, mechanical, and physical
tactics to control weeds. Cultural practices like crop rotation,
cover cropping, and delayed planting dates are helpful for
disrupting weed growth, but mechanical tillage is the primary form
of weed control on most organic farms. Fossil fuel use and soil
disturbance associated with intensive tillage have raised
questions about the environmental sustainability of organic
farming. Thus, research is needed to develop novel mechanical and
physical organic weed control tactics that reduce dependence on
tillage without sacrificing crop productivity or farm
Figure 1. Project director Sam Wortman (left), applying
abrasive grits by hand to peppers in a straw-mulch raised-bed.
What methods have organic farmers been utilizing to control
weeds in lieu of synthetic herbicides?
Many organic farmers have tried cover crop mulches (e.g.,
roller-crimped rye), natural herbicides (e.g., citrus oil), and
flame-weeding to reduce tillage. Each of these strategies can be
effective in certain cropping situations, but they also have
limitations. For example, cover crop mulches can harbor insect
pests and interfere with crop emergence and nutrient availability.
Natural herbicides are expensive and not effective against many
weed species. Flame-weeding is effective against a broad range of
weed species, but is fossil fuel-intensive and can cause
significant crop damage and yield loss.
Figure 2. Project collaborator Dan Humburg (right), working on
the prototype two-row grit applicator for vegetable cropping
What is abrasive weeding, how does it work and what are the
Abrasive-weeding, or “weed blasting”, is a physical form of weed
control that uses air-propelled abrasive grits to kill small weed
seedlings within the crop row. Any small, gritty material can be
used in abrasive-weeding (e.g., granulated walnut shells and corn
cobs), but we are interested in using organic fertilizers as
abrasive grits so that farmers can control in-row weeds and
supplement crop nutrition in one field pass.
Abrasive grits are applied in a 4-6 inch band within the crop row
via compressed air (100 PSI) either by hand (Fig. 1) or by a
prototype grit applicator (Figs. 2 & 3). Abrasive grits
defoliate and kill small weed seedlings (i.e., those with two
leaves or less) if the growing point is above the soil surface.
Thus, abrasive-weeding is most effective on broadleaf weeds and
less effective on grasses because the growing point is often
protected beneath the soil surface.
Like many organic weed control tactics, abrasive-weeding is most
effective when there is a size differential between the crop
(large) and weeds (small); thus, planting or transplanting into a
weed-free seedbed is important for success. Abrasive-weeding can
partially defoliate and slow the growth of larger weeds, but weed
mortality decreases greatly when the weeds have three or more
leaves at the time of grit application.
Are there limitations on the crops that it can be used on?
Thus far, we have demonstrated successful weed control with
abrasive-weeding in corn, soybean, tomato, and pepper. Further
research is being done in broccoli and kale. Abrasive-weeding will
likely be effective in most crops with a vertical growth habit or
those that are vertically trellised. However, the number of
applications and the amount of grit required in a specific
cropping system will determine the economic feasibility of
abrasive-weeding. As a result, this technology may have the
greatest potential in organic vegetable crops because economic
returns per unit area are high, and grit applications can be
paired with a straw or (bio) plastic mulch, which should reduce
the quantity of grits required to achieve season-long weed
Is this technology available to organic farmers now? Where can
they learn about it?
Hand-held application of abrasive-grits is currently available to
organic farmers as it only requires a portable air-compressor and
a hand-held siphon- or gravity-fed grit applicator (Fig. 1), both
of which can be found at your local hardware store. Larger-scale
mechanized abrasive-weeding is still in the research and
development phase, but our research team hopes to publish
blueprints for a four-row (grain crops) and two-row (vegetable
crops) prototype grit applicator within the next two years (Figs.
2 & 3).
January 21, 2016
Weed blasting offers new control method for
Weeds are a major scourge for organic growers, who often must
invest in multiple control methods to protect crop yields. A
relatively new weed control method known as abrasive weeding, or
"weed blasting," could give organic growers another tool. The
method, recently field-tested at the University of Illinois, is
In conjunction with plastic mulch, abrasive weeding reduced final
weed biomass by 69 to 97 percent compared to non-weeded control
plots, said U of I agroecologist Samuel Wortman.
Abrasive weeding involves blasting weed seedlings with tiny
fragments of organic grit, using an air compressor. For the
current study, grit was applied through a hand-held siphon-fed
sand-blasting unit connected to a gas-powered air compressor,
which was hauled down crop rows with a walk-behind tractor. The
study looked at a number of grit sources: walnut shells,
granulated maize cob, greensand, and soybean meal. If applied at
the right plant growth stage, the force of the abrasive grit
severely damages stems and leaves of weed seedlings.
Wortman found no significant differences between the grit types in
terms of efficacy. "When it leaves the nozzle, it's at least Mach
1 [767 mph]," Wortman noted. "The stuff comes out so fast, it
doesn't really matter what the shape of the particle is." Because
ricocheting particles can pose a risk to the applicator, Wortman
advises using protective eyewear.
Blasted grit does not discriminate between weed and crop
seedlings, which makes it important to use this method in
transplanted crops that are substantially larger than weed
seedlings at the time of grit application. Although some visible
damage occurred on stems and leaves of both tomato and pepper
crops, the damage did not affect marketable fruit yield. Studies
are ongoing to determine whether abrasions on crop tissues could
result in increased susceptibility to disease, but early results
show little effect.
Importantly, plots with plastic mulch and one or more blasting
treatment achieved the same fruit yields seen in hand-weeded
plots, and 33 to 44 percent greater yields than in non-weeded
An additional benefit of weed blasting is the potential for
growers to use organic fertilizers, such as soybean meal, as
blasting material. "We expect that abrasive weeding could
contribute between 35 and 105 kg nitrogen per hectare [31 - 94 lbs
per acre] to soil fertility." The idea that a grower could both
fertilize and kill weeds in a single pass is appealing, but it is
still unknown whether the fertilizer would be available for plant
uptake within critical windows.
According to Wortman's research, weed blasting does affect some
weeds more than others. Essentially, the smaller the seedling, the
better. Also, seedlings whose growing points are aboveground
(annual broadleaf species) are more susceptible to blasting than
seedlings whose growing tips are located belowground (grasses and
broadleaf perennials). Finally, Wortman noted that the presence of
plastic mulch seemed to factor strongly into the equation. Weed
blasting alone "is not a silver bullet, but it is an improvement,"
The method is now being tested in different horticultural crops,
including broccoli and kale, with and without additional weed
control methods. Early results suggest that the presence of
polyethylene mulch or biodegradable plastic mulch strongly
enhances the success of weed blasting, as compared with straw
mulch and bare soil. Wortman and his collaborators have also
developed a mechanized grit applicator, which they are currently
Weed Technology 28(1):243-252. 2014
Integrating Weed and Vegetable Crop
Management with Multifunctional Air-Propelled Abrasive Grits
Sam E. Wortman
Assistant Professor, Department of Crop Sciences, University of
Illinois, Urbana, IL 61801.
Corresponding author's E-mail: firstname.lastname@example.org
Abrasive weed control is a novel weed management tactic that has
great potential to increase the profitability and sustainability
of organic vegetable cropping systems. The objective of this study
was to determine the effect of air-propelled organic abrasive
grits (e.g., organic fertilizers) on weed seedling emergence and
growth and vegetable crop growth. A series of thirteen greenhouse
trials were conducted to determine the susceptibility of weeds to
abrasive weed control with one of six organic materials including:
corn cob grits, corn gluten meal, greensand fertilizer, walnut
shell grits, soybean meal, and bone meal fertilizer. In addition,
crop injury was quantified to determine the potential utility of
each organic material as abrasive grits in tomato and pepper
cropping systems. Of the six organic materials, corn gluten meal,
greensand fertilizer, walnut shell grits, and soybean meal
provided the broadest range of POST weed control. For example, one
blast of corn gluten meal and greensand fertilizer reduced Palmer
amaranth (one-leaf stage) seedling biomass by 95 and 100% and
green foxtail (one-leaf stage) biomass by 94 and 87%,
respectively. None of the organic materials suppressed weed
seedling emergence when applied to the soil surface, suggesting
that residual weed control with abrasive grits is unlikely. Tomato
and pepper stems were relatively tolerant of abrasive grit
applications, though blasting with select materials did increase
stem curvature in tomato and reduced biomass (corn cob grit) and
relative growth rate (corn gluten meal and greensand) in pepper.
Results suggest that organic fertilizers can be effectively used
as abrasive grits in vegetable crops, simultaneously providing
weed suppression and supplemental crop nutrition. Field studies
are needed to identify cultural practices that will increase the
profitability of multifunctional abrasive weed control in organic
December 3, 2015
Blasting Weeds to Oblivion
Frank Forcella was surrounded with apricot pits – a seemingly
useless collection of fruit stones saved in an endless line of
5-gallon buckets. Apricot trees don’t fare well in the cold of
Minnesota, but Forcella scratched out a hobby harvest each year,
and hit a bumper crop in 2007. More apricots; many varieties; and
a massive jump in his pit bucket mother lode. What to do with a
never-ending pit collection?
Commercial fruit processors often grind a portion of shell pits
for use in commercial sand blasting as a soft grit. As an ARS
research agronomist, Forcella believed organic grit might be an
effective weed killer. His determination to break from convention
has resulted in a four-row grit blaster capable of obliterating
Initially, Forcella bought a cheap, handheld sandblaster and
hooked it to an air compressor at the North Central Soil
Conservation Research Lab in Morris, Minn. He tested grit on
greenhouse seedlings and found a split-second blast destroyed the
weeds. Next, he purchased a bigger blaster, mounted it on an ATV,
and switched to corn cob grit. Driving down rows and manually
gunning down weeds, Forcella knew the system had potential. He
wrote a funding proposal for a larger machine and was approved for
a Sustainable Agriculture Research and Education (SARE) grant.
Dan Humburg, an ag engineer at South Dakota State University, and
grad student Corey Lanoue built the tractor-hauled grit
applicator. “Typically, when you’re sandblasting, operations are
very power hungry. Trying to find the optimum balance between the
application rate and still satisfy the farmer was difficult. We
tried to maximize efficiency and ultimately that’s turning
compressed air into abrasive velocity,” says Lanoue, currently a
design engineer with Vermeer.
With four rows and eight nozzles firing grit at 100 lbs. per
square inch, Forcella runs the system through corn field plots
twice every season, each nozzle blasting a 4” band. (First, when
corn is 4-6” high at the 1- to 3-leaf stage. Second, when corn is
1’ tall at the 5-leaf stage.) His field trials have shown 80% to
90% season-long weed control. In addition, Forcella has
successfully tested a weed-and-feed method for organic growers,
using cottonseed and canola seed meal. Essentially, he’s killing
weeds by spraying high-velocity fertilizer.
What about crop damage? “One of the things we were worried about
was disease. Make a hole in a crop plant, and open a door to crop
disease. However, there are no problems because the crops aren’t
hurt at all. The grit is aimed at the base of the corn plants and
nails the small weeds. Weeds must be below a couple of inches or
the system won’t work,” Forcella notes.
When a broadleaf weed – such as Palmer amaranth – is hit with
grit, the stem is severed and the root cannot regrow. The root
system withers due to a lack of photosynthesis. However, when
grasses germinate, the growing point is still below the surface.
Blasting grit will eliminate the leaves, but the grass continues
growing. Specific grasses must be hit at least twice to kill them,
according to Forcella. “We’re going over twice anyway to
compensate for early-germinating and late-germinating broadleaf
Forcella’s grit applicator has gained attention from Sam Wortman,
a weed scientist with the University of Illinois Department of
Crop Sciences. Wortman is working with South Dakota State
University to build a device specifically for vegetable crops on a
commercial scale. Manuel Perez, with the Department of
Agroforestry Engineering at the University of Seville in Spain, is
designing a grit-blasting system with robotics for application in
olive orchards and vineyards.
Costs to run the grit applicator system range from $50-$100 per
acre due to diesel and grit expenses. “A conventional grower
doesn’t want to spend over $50 per acre for weed control, but
that’s pretty good for an organic grower,” Forcella says. However,
he notes changes in agriculture may open the conventional door to
the weed blaster, particularly the spread of herbicide-resistant
Palmer amaranth. “Palmer continues to creep upwards. When I see
pictures from the South of workers chopping Palmer and throwing it
in carts, it looks like images from 100 years back. In situations
where there are no reliable alternatives, a grit applicator could
work for a conventional farmer.”
From apricot pit to grit applicator, Forcella has found success.
“As a weed scientist, I wondered if you could kill weeds with
grit. On its face, it’s certainly a crazy idea, but I couldn’t
shake the thought. This is still small-scale, but it works.”
Research shows ‘sandblasting’ works to
By Kelli Boylen
The unlikely hobby of growing apricots in Minnesota may have led a
research agronomist to a new way to control in-row weeds on
organic farms: blasting them away.
Frank Forcella works for the USDA North Central Soil Conservation
Research Laboratory in Morris, Minn. He and his wife, Jessica,
have been growing apricots since 1991. In 2007 they had a very
productive year, and Forcella wanted to find a way to use the pits
left over from making jams and dried fruit instead of letting them
go to waste.
A little online research taught him that ground shells of apricot
pits are used in “sandblasting.” In fact, many modern sandblasting
techniques use plant-based products. He was discussing this with
Dean Peterson when both men had the same idea of sandblasting
He and Peterson spent about $50 on a handheld sandblaster,
connected it to an air compressor and started testing it in the
greenhouse. “We’d have corn and weeds growing in a pot and we
learned right away that we could knock down the weeds and the corn
would be okay. We repeated the experiment many times and had
reasonably good success,” he said. When aimed at the bases of crop
plants growing in rows, the propelled grit kills weed seedlings
To test the method on corn growing in the field, they bought a
bigger sandblasting unit, and hauled the air compressor around on
an ATV. “We learned if we made two or three passes at the right
times we could get full-season weed control,” Forcella recalled.
Researcher Frank Forcella applies abrasive grit to corn rows using
an air compresser hauled by a graduate student on an ATV. Forcella
found that just two to three passes at strategic times provided
full-season weed control.
Forcella admits he is not “mechanically astute.” So when it came
time to build a prototype of a larger unit he needed help. He
applied for and received a grant from the North Central
Sustainable Agriculture Research and Education (NC-SARE) for an
“Air-Propelled Abrasive Grit Management” (PAGMan) system.
Forcella said it took a little while to find someone who was an
expert ag engineer who worked with steel, but they were fortunate
to find Dan Humberg, a professor of ag engineering at South Dakota
State University (SDSU).
SDSU graduate student Cory Lanoue took an interest in the project
and started working on it with CAD (computer aided design)
software. Then, using the design he developed over nearly a year,
he and fellow grad students, under the supervision of Humberg, put
together their abrasive grit applicator machine in about two
“It was an impressive feat in my eyes,” said Forcella. “Cory did a
They developed PAGMan “for selective, post-emergence, in-row weed
control in corn, soybean, and other crops that are grown in widely
The 2013 and 2014 growing seasons were their first using the
four-row, eight-nozzle unit. Forcella admitted that the unit may
seem small to many of today’s large-scale farmers, but the size is
realistic for a family organic farm.
The field tests were performed on organically certified land at
the West Central Research and Outreach Center of the University of
Minnesota. These experiments are being led by SDSU graduate
student Mauricio Erazo-Barradas under the direction of Sharon
Clay, a weed scientist.
The idea is that farmers will clear weeds from between crop rows
like they traditionally have for years with inter-row cultivation,
flaming or mowing, but they will use the sandblasting method to
kill the weeds next to the corn plants. The field tests include
all of these treatments.
Ground corn cobs make an effective grit for “sandblasting” weeds.
Any gritty material that is about 0.5mm works with the PAGMan
“I think of using abrasive grit for weeds as one more tool in the
toolbox for controlling weeds,” Forcella said. “It’s not the only
tool—you need lot of them—but this definitely can be one more tool
for organic producers to use.”
Through their trials, they learned that the system works best when
the nozzles are about two feet from the plants using air pressure
of 70 to 100 psi. If the nozzles are too far away or the pressure
too light it doesn’t kill the weeds. If the nozzles are too close
or there’s too much pressure, it can damage the corn.
Forcella said they have been focusing on the control of the most
common annual weeds, such as lambsquarters, pigweed, and foxtail
(Chenopodium album, Amaranthus retroflexus, and Setaria viridis),
which are common throughout the nation and especially abundant in
the Corn Belt.
“Timing is crucial,” Forcella said, and the weeds must be
seedlings no more than two or three inches tall. The two necessary
times to blast the weeds are once at the 1-leaf stage of the corn
(4-6 inches) and again at the 3-leaf to 5-leaf stage (about 8-12
inches) in order to provide season-long weed control. This also
maintains corn yields comparable to those in weed-free crops.
Using sandblasting along with cultivation offers 80 to 90 percent
weed control, which Forcella described as “quite reasonable,
especially for organic production.” Weed control at 80 to 90
percent is enough to prevent yield loss in field corn. A weedy
corn field can easily lose 20 to 50 percent yield.
Although blasting works well with annual broadleaf weeds, it is
not as effective with seedlings of grassy weeds because their
growing points are below ground. When foxtail is the most common
weed, three applications may be needed for good control; at the
1-leaf, 3-leaf, and 5-leaf stages of corn. The system likely will
not be effective on perennials, as these weeds are simply too
tough-stemmed, and they tend to have large root reserves, which
allows them to sprout continuously.
Forcella and his colleagues have tried many different types of
abrasive grit: walnut shells, ground up corn cobs and even plain
sand. “As long as it’s the right size (about 0.5 mm) and gritty,
it will work,” he said. Most of their experiments have been done
with corn cob grit because it is inexpensive and readily
When they presented the idea to organic producers, they quickly
realized it was more likely to be adopted if the applicator could
be doing “double duty” when moving across the field. Luckily,
Forcella said, many nitrogen-rich organic fertilizers have a
gritty texture as well. They started experimenting with corn
gluten meal, alfalfa meal, cotton seed meal, canola seed meal and
even lime to neutralize acidic soil. “All the grits work equally
well to control small weed seedlings,” he said. Their planned
trials include looking at how and when the plants uptake nitrogen
after gritty organic fertilizers are applied to control weeds.
Other upcoming trials will include determining what type of nozzle
and spray pattern work best and looking at how the idea can be
applied to organic vegetable production.
The project has taken on an international angle as well. For
several years Forcella has collaborated with a weed scientist,
Professor Jose Urbano, from the University of Seville (UdS) in
Spain. One of Urbano’s colleagues at UdS, Manuel Perez Ruiz, is an
ag engineer. He was interested in the concept as well and secured
funding from his government to determine how to apply the idea to
olive orchards and vineyards. His group is investigating how to
integrate GPS and robotic technology with the PAGMan system. This
new work is in collaboration with Professor David Slaughter at the
University of California-Davis and the USDA-Agricultural Research
The four-row, eight-nozzle PAGMan provides selective,
post-emergence, in-row weed control for corn, soybean, and other
crops grown in widely spaced rows. Photo by USDA-ARS
One of the drawbacks of the system thus far is that the unit needs
to move at a rather slow speed, about one to one-and-a-half miles
per hour, so it is time-consuming. Forcella explained that
maintaining the diligence needed to drive at a very slow speed and
staying aligned as to not damage the corn plants is difficult. GPS
and robotic technology could help eliminate this drawback for the
Still others are working to help make this project come together.
Sam Wortman at the University of Illinois recently was awarded a
grant from the National Institute of Food and Agriculture’s
Organic Research and Extension Initiative. This new project will
extend the technology to organic vegetable production. The
aforementioned researchers at SDSU and USDA-ARS will collaborate
on this venture.
Forcella said as a public employee working on a project like this
he has no interest in trying to patent the idea. Since the idea
has already been shared with so many others, the overall concept
may not even be patentable at this point, but smaller things such
as the nozzle design could be.
Even though the idea may not be patented in the future, the unit
is not something that could easily be built by someone on their
own, he added. Forcella and his colleagues hope that a larger
equipment company, a few of which have expressed interest, will
decide to build and market the unit. He hopes the unit will be
commercially available within the next five years.
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