Solarization and the Soil Microbiome

Grace Smith, Undergraduate in Molecular and Cellular Biology

Sonja Birthisel, PhD Student in Ecology and Environmental Sciences

Eric R. Gallandt, Professor of Weed Ecology and Management

A soil microbiome consists of tiny organisms such as bacteria, archaea, fungi, and protists that impact plant life. Beneficial microbes decompose organic molecules, rendering them usable by plants and protect against harmful microbes. Conversely, pathogenic microbes can have major detrimental effects on crops.


In June through August  of 2016, we expanded our study of solarization (see previous blog posts about solarization for weed control) to examine the effect of solarization on soil respiration and specific populations of beneficial microorganisms: general bacteria, general fungi, Bacilli, and fluorescent pseudomonads.

A picture of a rose bengal agar plate which was used to select for the growth of general fungi in our experiment.


The Experiment:

Solarization was performed for two and four weeks in a field and closed hoop house at Umaine Greens, located on the campus of the University of Maine, Orono.

Plots were rototilled and irrigated prior to application of previously used clear polyethylene mulch. Temperature was recorded throughout and soil samples were collected at the beginning of the experiment, at plastic removal, and 5 & 14 days after plastic removal for microbial analyses.


Solarization caused average temperature increases of 4 and 7℉ in the field  and  hoop house, respectively; furthermore, maximum temperatures increased by 10 and 15℉. The maximum temperature increase is of interest because prior research indicates that maximum temperature may be more important than average temperature in pathogen control. The dip in soil temperature between July 6th and 13th (labeled “A” in the figure below) corresponds with cool air temperatures during those days (Bangor International Airport, NOAA).


Temperatures over the course of four weeks of treatment in the field and hoop house. CON = control ; SOL = solarized.


Soil Respiration:

Soil respiration was measured to serve as an estimate for total microbial biomass, an indicator of soil health. We found that solarization decreased soil respiration to a minor extent in the field, and more significantly in the hoop house. We originally predicted that soil respiration would be reduced while plastic was in place, but would bounce back to normal levels by two weeks after plastic removal. Since this was not the case, it would be valuable in the future to test how long it takes for soil respiration to fully return to control levels. 


Soil respiration in the field and greenhouse at treatment termination (time of plastic removal) and 14 days after termination. * = significant difference.


Populations of Specific Beneficial Microbes:

In this experiment, we measured populations of four beneficial microbe groups: general bacteria, general fungi, and rhizobacteria Bacilli and fluorescent pseudomonads.  Many general bacteria and fungi decompose large indigestible organic molecules into smaller, plant-useable nutrients. Fungi increase soil water holding capacity by growing hyphae: long, threadlike filaments. Some Bacilli convert atmospheric nitrogen into ammonia making it available to plants, and some fluorescent pseudomonads release antibiotics that decrease populations of plant pathogens.

The good news first: field solarization did not harm any of these four groups of beneficial microbes we were able to grow in the lab.  Under the hotter temperatures in the hoop house, there was a slight decrease in these microbes overall due to a decrease in fluorescent pseudomonads; the other groups of microbes were not significantly impacted.


Number of soil microbe colonies grown from soil collected 5 days after treatment termination in the field and hoop house. * = significant difference.


Literature Review of Expected Pathogen Response to Solarization:

Measuring the effects of solarization on plant pathogens was beyond what we could accomplish in this experiment.  However, to get an idea whether pathogen control with solarization is theoretically possible in Maine, we reviewed papers of known pathogen responses to temperature, and compared this to the maximum temperatures measured in our experiments.  Nearly half of the pathogens we investigated are predicted to decrease in number under temperatures we measured in our field, and over three-quarters are predicted to decrease with temperatures achieved in our hoop house. The only included pathogen that we predicted might increase in response to solarization is noble rot, also known as gray mold, a fungus that affects grapes and other horticultural crops.  These theoretical results need to be backed up with real-world experiments in Maine, but provide a preliminary indication that solarization could contribute to not only weed management (see past blog posts), but pathogen control as well.



Potential effect of solarization on some pathogens of vegetable and horticultural crops in Maine, based on temperatures measured in our experiments and known temperature tolerance of these pathogens. 🠋: pathogens that may decrease in response to solarization; 🠉: pathogens that may increase in response to solarization; Ø: pathogens that are expected to be unaffected by solarization.  



This study suggests that solarization did little harm to beneficial soil microbes in an open field, but in a hoop house soil respiration and populations of the beneficial fluorescent pseudomonads bacteria were significantly reduced, at least in the short term.   Further research is needed to see if these effects  are long lasting and have subsequent  impacts to crop growth. Based on the soil temperatures we measured, it is possible that solarization could contribute to plant pathogen control in Maine, though more research on this topic is needed to confirm this.  



Solarization to Prepare a Stale Seedbed

Sonja Birthisel, PhD Student in Ecology and Environmental Sciences

Eric R. Gallandt, Professor of Weed Ecology and Management

Solarization is the practice of using clear plastic mulches to trap solar energy, heating soils to temperatures lethal to pests including weeds.  Solarization is nothing new; it has been researched and used by growers extensively since the 1970s in warm, sunny places like Israel and California, but the conventional wisdom has been that it is not consistently effective in cool, northern places like Maine.

We expected that two weeks of solarization during May-June in Maine would not achieve temperatures hot enough to kill weeds, but would rather lead to an early flush of increased weed emergence.  After solarization, we thought these weeds could be killed by flaming, resulting in creation of a better stale seedbed than a “control” created with flaming only.


Hypothesis: two weeks of spring solarization will encourage weed seeds to germinate so they can be killed, depleting the seedbank and creating a better stale seedbed. 

We tested this hypothesis through four experiments in May-June of 2015 and 2016.  At the start of each experiment, fields were rototilled, thoroughly irrigated, and solarization plots were covered with salvaged 6-mil clear polyethylene hoophouse plastic.  We secured the plastic edges by clipping them to metal pipe laid in a shallow (4” deep) trench around each plot.  Control plots were left fallow after rototilling and irrigating.  After two weeks, plastic was removed, and all plots (solarized and control) were flamed using a hand-held propane burner to create stale seedbeds.  Two weeks after flaming, we counted the number of weeds that had emerged in each plot.


To our surprise, two weeks of springtime solarization actually suppressed weed emergence, both while plastic was in place and after plastic removal and flaming.  On average, solarization plus flaming resulted in stale seedbeds with 78% fewer weeds than control stale seedbeds created with flaming only.  Soil temperatures were higher in solarized plots, reaching a maximum of 117°F at 2” soil depth, as compared to a maximum of 100°F in controls.

Picture5.pngResults: two weeks after we removed plastic and created stale seedbeds, there were 78% fewer weeds in the solarized treatment than the flamed control.  The “*” indicates a statistically significant difference between solarized and control treatments.  

 The weed suppression following solarization was so visually apparent, we wondered whether flaming after plastic removal was necessary.  To address this question, during one of our experiments we kept half of each plot un-flamed for comparison.  We found that flaming significantly reduced weeds in the control plots, but not the solarized plots.  In short, solarization did a good enough job that flaming afterward was not necessary.

Picture6.pngSolarization with or without flaming created an excellent stale seedbed.  The “*” indicates that flaming significantly reduced weeds in the control treatment, the “ns” indicates that flaming did not have a significant effect in the solarized treatment. 


Overall these results suggest solarization is a very promising strategy for stale seedbed preparation in Maine.  Although laying the plastic is labor-intensive, the weed control benefits may be worth the extra effort, especially prior to planting high value direct seeded crops.  More blog posts about solarization coming soon!




Weed Master Video Update!

We are wrapping up the final reports for our NE SARE-funded project that supported our on-farm trials of the Weed Master.  In addition to inspiring this blog, we used this project as a test case for using video to capture the on-farm trials and experiences of participating growers and our research group.  Tavi Merrill, who recently graduated from the University of Maine Sustainable Agriculture undergraduate program, has done a fine job of directing and editing these videos.

We are continuing our field research on hand tools and scale-appropriate weeding tools for the small farm.  Ben Costanzi is a new M.S. student who will focus on this topic for his thesis work.

You can view the most recent videos below, or go to our YouTube Channel:


Prologue to the Weed Master Project


Weed Master Field Trial:  Fisher Farm


Weed Master Field Trial:  Fail Better Farm

Weed Seedbanks: 2009 On-farm Sampling

“Credits” to the weed seedbank occur when weeds shed seed, i.e., seed rain, and “debits” when seeds are removed from the soil by germination, predation, or decay/death, in this declining order of importance.
Seedbanks on the Beech Grove, New Leaf and Peacemeal Farms
During the 2009 field season we visited Eric and Anne Nordell of the Beech Grove Farm in Trout Run, PA, Dave and Christine Colson of New Leaf Farm in Durham, ME, and Mark Guzzie and Marcia Ferry of Peacemeal Farm in Dixmont, ME, to collect soil samples from selected fields.  At each sampled location on each farm we collected ten soil cores (6.5 cm diam. x 10 cm deep) from a 25 square meter area.  Soil samples were sieved to remove stones and spread over a layer of vermiculite and placed in a greenhouse.  Weed seedlings were identified, recorded and removed weekly.  The soil was allowed to dry and was then mixed and replaced in the flats once a month for four months.
Weed communities were comprised of an average 8 to 10 species on each farm (see Table, below).  The three most abundant species at the Peacemeal Farm are troublesome in most vegetable crops and are a widespread problem among northeastern vegetable growers.  At the New Leaf Farm, smooth crabgrass was the top-ranked species, primarily because of a large infestation in a field where pigs had been pastured.  Low cudweed and corn spurry, while a problem in salad mix, are not particularly troublesome in most other vegetable crops.  At the Beech Grove Farm, typically pernicious summer annual weeds were rare and not among the top ranked species.

Typical samples are shown below.  These photos were taken after approximately 4 weeks in the greenhouse.  They offer a visual representation of the “low,” “medium,” and “high” seedbanks measured on these farms.

Seedbank Management

Managing weeds with a focus on the seedbank looks at the farming system with an eye first towards opportunities for preempting seed rain with short-season cash or cover crops that are harvested or terminated before troublesome weed species produce mature seeds.  Next, opportunities for shallow soil disturbance, strategically-timed to be coincident with weed species-specific peak emergence potential, will encourage germination.  Subsequent disturbance events can kill these “flushes” of weeds.  When seed rain occurs, opportunities for predation are greater if seeds remain on the soil surface.  This hypothesis, that fall tillage should be delayed to encourage seed predation is the focus of current field studies.

For more information see:

Managing weeds with crop rotation

and these eXtension articles:

Manage the Weed Seed Bank—Minimize “Deposits” and Maximize “Withdrawals”

Manipulating Weed Seed Banks to Promote their Decline

A Look Back at Hand Weeding Tools

In preparation for some fall meetings I spent some time recently looking for other examples of hand weeding tools and, thanks to the scanning efforts of Google, happened upon some fantastic advertisements from the 1919 issues of The Garden Magazine. It would seem that we could have initiated the Weed Master Project nearly a century ago!

From:  The Garden Magazine, 1919

From: The Garden Magazine, 1919

"On-farm evaluation of innovative cultivation tools"  The Garden Magzine, 1919

"On-farm evaluation of innovative cultivation tools" The Garden Magzine, 1919

I have started a collection of images of these tools for a future article, so if you have any good examples please send them our way.

"Eliminating the backbreaking drudgery of weeding."  from The Garden Magazine, 1919

"Eliminating the backbreaking drudgery of weeding." from The Garden Magazine, 1919

Rain, rain, rain…

Tuesday, July 7, 2009

Wolf Pine Farm, Alfred, Maine

On the way to the York County Farmers’ Network July meeting at Wildroot Farm in Kennebunk, we stopped by Wolf Pine Farm so Amy Sprague and crew could field test the Weed Master cultivation tools.  Unfortunately, the rain started soon after we completed assembly, but with a coarse textured site in mind, and the seemingly unending days of rain we have had in recent weeks, we headed to the field.

Although the finger weeders were quickly clogged, we were able to use the disk hillers in leeks despite the rain (although it was raining too hard at this point to get the camera out!).

Disk-hilling leeks

Disk-hilling leeks

We then headed to the nearby Wildroot Farm where the York County Farmers’ Network was holding a potluck and meeting.  We retreated indoors to assemble the Weed Master, examine the components, and discuss the experiences from our field tests conducted to date.

Unloading the Weed Master at Wildroot Farm in Kennebunk.  July 7, 2009

Unloading the Weed Master at Wildroot Farm in Kennebunk. July 7, 2009

Farm Training Project Workshop

June 16, 2009

Peacemeal Farm, Dixmont, Maine

There was an impressive turnout Tuesday evening for the Weed Management workshop at the Peacemeal farm.  Part of the Farm Training Project Workshop program organized for farm apprentices, and others, by MOFGA, the Maine Organic Farmers and Gardeners Association, the presentations, demonstrations and field tour for this session were focused on weeds.  After some introductory comments related to the ecology of weeds in organic farming systems, the seventy five participants moved to the field for a demonstration of hand tools, including the Weed Master.

Clayton Carter, Fail Better Farm, Montville, Maine, demonstrates the Weed Master to MOFGA Apprentices at the June 16 evening Farm Training Project Workshop on weeds.  Photo Credit: Mike Mardosa, University of Maine

Clayton Carter, Fail Better Farm, Montville, Maine, demonstrates the Weed Master to MOFGA Apprentices at the June 16 evening Farm Training Project Workshop on weeds. Photo Credit: Mike Mardosa, University of Maine

Mark Guzzie, Peacemeal Farm, Dixmont, Maine, offers Russell Libby, Executive Director of MOFGA a turn at the helm of the Weed Master.  Photo Credit: Mike Mardosa, University of Maine.

Mark Guzzie, Peacemeal Farm, Dixmont, Maine, offers Russell Libby, Executive Director of MOFGA a turn at the helm of the Weed Master. Photo Credit: Mike Mardosa, University of Maine.

Weed Master vs. “The Pig” vs. “Scuffles”

13 May 2009

It was a very exciting day comparing the Weed Master to tried-and-true cultivation tools at the Peacemeal Farm.  Depending on conditions, the crew would use a favorite old steel-wheeled hoe with a wide stirrup, “The Pig,” and/or scuffle hoes.  We decided to compare the Weed Master, with sweeps or finger weeders, to these typical tools, looking at both efficacy (proportion of weed seedlings killed), and working speed.

Mike "The Pig," vs. Mark "The Weed Master," vs. Tim "Scuffles," May 13, 2009

Mike "The Pig," vs. Mark "The Weed Master," vs. Tim "Scuffles," May 13, 2009

One bed of onions each in a timed event with pre- and post-emergence efficacy measured in four randomly located 1/16th square meter quadrats.  The contoured, 216 ft. long beds contained three rows of transplanted onions on 15 in. centers; onions were 4 to 6 in. tall at the time of the trial.  Post-cultivation censuses of weeds was performed the following day.

The stirrup hoe was a 7 in. wide model commonly available through garden suppliers (e.g., Johnny’s Selected Seeds).  The operator, Tim, is a highly-regarded by farm members as an expert with the stirrup or “scuffel” hoe, and has several seasons of experience.

Scuffle hoeing transplanted onions at Peacemeal Farm.  May 2009

Scuffle hoeing transplanted onions at Peacemeal Farm. May 2009

“The Pig,” as it is affectionately known on the Peacemeal Farm, is a wheel hoe that is no longer commercially available.  A copy of the Planet Jr No. 17 that was reproduced by Red Pig tools, now in Oregon.  Although not presently available, Rita Denman of Red Pig Tools indicated that they have plans to reintroduce “Red Pig No. 1,” which they hope will be available in the spring of 2010.  Two notable features of this tool offer superior performance to currently-available options.  First, the steel wheel does not bounce over stones or other obstructions, a problem with the pneumatic tires of, for example, the Glaser® wheel hoe.  Secondly, the Peacemeal team likes the heavy cast frame assembly of The Pig.

Mike and "The Pig," a favorite wheel hoe at the Peacemeal Farm

Time. Considering only the working time, the Weed Master was the clear winner.  With the finger weeders it took 7 min. 10 sec. to weed the three rows of onions in the 216 ft. bed; with sweeps, 10 min. 20 sec.  This compares to 13 min. 58 sec. for The Pig and 29 min. 6 sec. for the scuffle hoe.

However, the Weed Master can only weed one row at a time and we could not set up the parallel linkages in one location on the tool bar that would be suitable for both the center and the outside rows.  Mark started with the linkages adjusted for the center row, but at the end of this pass he had to move them to the side to cultivate the outside rows.  Thus, if we add this “fiddling time” with the working time and considering total time, the Weed Master w/finger weeders barely inched out a victory over The Pig: 13 min. 16 sec. total for the Weed Master w/finger weeders vs. 13 min. 58 sec. for The Pig.  The scuffle hoe and The Pig had no need for such adjustments.  Total time for the Weed Master w/sweeps was similar at 14 min. 30 sec.

Efficacy. Were the tools similarly effective at killing weeds?  Generally yes.  We measured the proportion of weeds killed (mortality, from 0, none killed, to 1, all killed), both between the crop rows and within the crop rows.  As expected, each of the tools did a fine job killing between-row weeds, with mortality ranging from 80 to nearly 100% (see figure, below).  The means and variation within the treatments were remarkable similar; the overall mean of all treatments was 89%.

Mortality (proportion of weeds killed) between onion rows

Mortality (proportion of weeds killed) between onion rows

Control of the within-row weeds was more consistent with The Pig, which was unexpected.  I would have considered the flexibility of the scuffle hoe and the consistent working action of the finger weeders to have offered more consistent results.   Overall, control of in-row weeds was, as expected, less than control of between-row weeds; overall mortality of in-row weeds averaged 50%.  There is some evidence that The Pig outperformed the other tools based on this criterion.  Mean proportion of weeds controlled and variation were similar for the other tools.  Note that this field evaluation, although multiple samples were measured for each treatment, they represent subsamples, not true replication.

Mortality of weeds within the onion rows

Mortality of weeds within the onion rows

Tim, “Scuffles,” bed was quite attractive in it complete lack of foot prints within the bed.  In using The Pig, Mike used a technique to minimize the number of foot prints between the rows while maximizing speed and control:  “large step, lunge with body and arms.”

Mike's long-stride, lunging technique with The Pig.

Mike's long-stride, lunging technique with The Pig.

Despite the slow working time of the scuffle hoe the quality of this weeding job was outstanding.  The scuffle hoe, in the hands of an experienced user, is unparalleled in ability to immediately adjust to variations in soil conditions, crop, or weed size (e.g., to make passes more or less aggressive as needed).

Overall I am most impressed to date with the finger weeders.  Early in the day, when soil conditions were moist their working action was not impressive.  However, later in the day, as the soil dried to nearly perfect cultivating conditions, I adjusted the yellow fingers to a 1 in. gap and did a final bed, first the center row, and then adjusting the linkages to and pushing handle to the side for the outside rows.  Keeping up a rather quick walking speed, the fingers did a very thorough job of disturbing or throwing soil in the row, between the onion plants.  The working width of fingers was about 6 in. on either side of the row, so the passes on the outside rows mostly obliterated the footprints from my center row pass, and the working speed, although not timed, was quite fast.

Weed Master

The pushing handle section of the Weed Master has proven difficult to keep in a lowered adjustment.  There does not seem to be sufficient friction with the carriage bolts to hold the handle against the force of the user pushing.  The handle then slowly moves up to the maximal height.  This will be a problem for uses who wish to have the handle lower.  Perhaps a pin/hole design could index the handle into a range of positions.

Under challenging conditions, e.g., up hill or on uneven ground, pushing the Weed Master is physically demanding, perhaps due the weight of the tool, and the effort to keep it centered on the row being cultivated.

Mike celebrates "The Pig's" victory over the Weed Master w/sweeps.

Mike celebrates "The Pig's" victory over the Weed Master w/sweeps.