Grit-Blast Weed Control
Crop Protection 77:157-162.
DOI: 10.1016/j.cropro.2015.08.001

Air-propelled abrasive grits reduce weed abundance and increase yields in organic vegetable production
Sam E. Wortman

Two applications of abrasive grts reduced weed density by 63%–80%.
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 abrasive-weeding.

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 profitability.

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 systems.

What is abrasive weeding, how does it work and what are the benefits?

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 suppression.

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 organic farmers

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 surprisingly effective.

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 control plots.

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," he said.

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 testing.
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:


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 specialty crops.
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 weeds.

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.

High-Velocity Fertilizer

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 weeds.”

Pigweed Slayer

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.”
Organic Broadcaster

Research shows ‘sandblasting’ works to control weeds

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 weeds.

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 through abrasion.

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 months.

“It was an impressive feat in my eyes,” said Forcella. “Cory did a fantastic job.”

They developed PAGMan “for selective, post-emergence, in-row weed control in corn, soybean, and other crops that are grown in widely spaced rows.”

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 system.

“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 available.

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 Service.

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 farmer.

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.