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.  




Comparing Solarization & Occultation

Comparing Solarization & Occultation

Grace Smith, Undergraduate Student

Sonja Birthisel, PhD Student in Ecology and Environmental Sciences

Eric R. Gallandt, Professor of Weed Ecology and Management


We have seen increased interest in using black plastic silage tarps for stale seedbed preparation, a technique known as occultation or tarping.  Our prior research suggests that solarization using clear plastic can also be used to create an excellent stale seedbed. We conducted two experiments to compare solarization to occultation, and during our second experiment measured the impacts of these practices on the soil microbiome.


BlogPostFig1.jpgPhoto showing a summer 2016 field experiment comparing solarization and occultation,

 Rogers Farm, Old Town, ME


Experiment 1

In the first experiment, we rototilled, irrigated the soil, and applied solarization and occultation (securing plastic edges tightly by clipping them to metal pipe) for seven weeks starting in April of 2016.  Two weeks after plastic removal in early June, we counted emerged weeds.  There was significant weed emergence in control plots, but zero weed emergence following both solarization and occultation.


BlogPostFig2.jpgWeed emergence measured two weeks after plastic removal in control, occultation and solarization treatments


The field in which this experiment took place had been amended with compost before the trial began in April and was very high in organic matter (17.9%).  This meant that we were actually performing “biolsolarization” – that is, coupling solarization with addition of organic amendments.  Past studies suggest that biosolarization is often more effective at controlling pests than regular solarization, so this may have contributed to the excellent weed control we measured.


Experiment 2

We were curious how solarization and occultation compared when applied for shorter treatment periods, so we conducted a second experiment starting in early July of 2016. As before, we rototilled, irrigated, and applied plastic.  This time we included seven treatments: solarization for two, four, and six weeks; occultation for two, four, and six weeks; and a control.  Organic matter in this field was 3.7%, which is more typical than in our first experiment.  We measured weed emergence two weeks after plastic removal for each treatment, and found that solarization created a better (less weedy) stale seedbed than occultation for all treatment durations.

BlogPostFig3.jpgSolarization resulted in an effective stale seedbed at all treatment durations; occultation suppressed weeds at two and six weed durations only


Surprisingly, more weeds emerged after four weeks of occultation than two weeks.  We think this was because overall 2015 was a very dry summer, but there was abundant rainfall during the two weeks between removing plastic and counting weeds in the four week treatments, which likely encouraged weed germination.  The fact that the solarization plots did NOT respond the same way to rain suggests that solarization may have impacted weed seeds in the seedbank more dramatically than occultation, likely through a combination of direct seed death, fatal germination, and forcing seeds into dormancy.


Soil temperatures were consistently highest in the solarized plots, intermediate in the occultation plots, and lowest in the controls.  We expected this trend based on the physics underlying these soil heating techniques: clear plastic allows energy from the sun’s rays to directly heat water in the soil underneath, while black plastic absorbs the sun’s energy and re-radiates some heat to the soil below, but the energy transfer is not as efficient.

BlogPostFig4.jpgSoil temperatures, measured at 2 inches depth during six weeks of treatment, were highest under solarization, intermediate under occultation, and lowest in the control


To explore the impacts of solarization and occultation on the soil microbiome, we measured soil biological activity before, during, and after four weeks of treatment.  A soil microbiome consists of tiny organisms such as bacteria, archaea, fungi, and protists that are essential to plant life. Beneficial microorganisms convert nitrogen from the air into ammonium in order to render it useable by plants; protect against harmful microorganisms; and simply crowd out harmful microorganisms. Soil biological activity is essentially a measure of how much the soil is breathing, and is considered to be an indicator of good soil health.


BlogPostFig5.jpgSoil biological activity was not significantly impacted by solarization or occulataion during treatment, but two weeks after plastic removal, soil biological activity was reduced in the solarized plots.  Shown here are data from 0-1 inch soil depth.


Treatment did not significantly affect soil biological activity during treatment, but soil biological activity was reduced in solarized plots two weeks after plastic removal.  Soil depth (0-1 inch, 1-2 inches, and 2-4 inches) did not impact these results.  It is possible that during solarization, natural selection changed the composition of microorganisms present in the soil, favoring heat-loving species.  After plastic was removed, the heat loving microorganisms could have been at a disadvantage and declined in population faster than they were replaced by other moderate-temperature-loving species, so the overall soil biological activity decreased temporarily as a result.  We expect that if we had been able to measure again a month or two after plastic removal, we might have seen soil biological activity return to pre-treatment levels.


Beyond stale seedbed creation, occultation and solarization could be useful small-scale methods for terminating no-till cover crops.  New research at UNH is looking into this application, with early results suggesting that occultation may have an edge when it comes to subsequent weed suppression.



Overall, these two experiments suggested that solarization and occultation can both create effective stale seedbeds, but solarization appears to be more consistently effective than occultation at shorter treatment durations.  Solarization may negatively impact soil biological activity, though we expect effects to be temporary; we’ll be posting results soon describing two more experiments that take a closer look at the impacts of solarization on soil microbes.

This work was made possible by a grant from the Maine Food and Agriculture Center

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!




A simple tool to explore alternative weed management strategies

Bryan Brown, Ph.D. Candidate

Eric Gallandt, Professor of Weed Ecology and Management


Weed management philosophies and employed strategies have inherent tradeoffs.  A surprising result from our field studies conducted in organic onion was the impressive performance of zero seed rain and mulch-based strategies, which performed better than expected even in the first year of use.


Below is a screenshot from the Excel-based decision aid, which can be downloaded here.

Picture1.pngRequires Microsoft Excel. Runs with macros enabled or disabled.  Note:  This decision aid is for educational purposes only. Results should be interpreted with an understanding that each farm is unique and this decision aid may not accurately represent the conditions present at each farm. Downloading the decision aid represents an acceptance of these terms.

A Comparison of Organic Weed Management Strategies in Onions

Bryan Brown, Ph.D. Student, and Eric Gallandt, Associate Professor

 What’s your strategy for managing weeds? Cultivate until the crop is large enough to tolerate late-emerging weeds, sometimes returning to harvest from a dense patch of weeds? Cultivate season long and pull any mature weeds as part of a longer-term strategy to prevent weed seed rain and make weeding easier and less costly over time? Intensively mulch to prevent weeds, perhaps improve soil quality, and reduce labor demands for weeding later in the season?

There are successful organic farmers who rely on each of these strategies, some using different strategies for different crops, others with a singular focus. Clearly there is no “best” strategy, but rather, trade-offs and compromises associated with each. Our aim with this field study comparing weed management systems is to quantify multiple dimensions of each system so farmers can evaluate and choose a strategy that is best aligned with their own philosophy, priorities and infrastructure constraints.

which strategy

Using yellow storage onions as our test crop (planted with two onions per hole, spaced 6” apart within rows and 3 rows per bed) on a field with a moderate weed seedbank at the University of Maine Rogers Farm, we implemented several prominent strategies:

1) Critical Period Weed Control (CPWC) – Control weeds only during the crop’s sensitive adolescent stage. This is the minimum amount of weeding you can do and still get a viable crop. However, it allows late-season weeds to go to seed.

2) Zero Seed Rain (ZSR) – Frequent cultivation with the goal of not letting any weeds set seed so that none “rain” to the ground. A strategy expected to be initially costly, but with decreasing cost over time as weed pressure declines.

3) Black Plastic Mulch (BPM) – Suppresses weeds and warms soil. Requires cultivation for the paths.

4) Black Plastic Mulch with Straw-Mulched Paths (BPMSP) – Suppresses weeds in the path as well; added organic matter in the paths.

5) Straw Mulch – Suppresses weeds and adds organic matter to the soil. Applied by hand in June after soil has warmed and onions are approximately the diameter of a pencil.

6) Junk Hay Mulch – Similar to straw but less expensive.

  Aside from primary and secondary tillage and application of plastic mulch, all activities were done by hand. Cultivation was achieved by wheel hoeing the paths, scuffle hoeing the shoulders and between rows, and using short-handled hoes for within rows. Drip irrigation was used to keep soil moisture levels optimum for each plot.

labor by activity

Labor by Activity from our 2014 field season (above) demonstrates that CPWC plots required the least amount of labor. Although they were weeded clean through early July, by the end of the season these plots were a weedy mess, and resultantly, had the longest harvest times. In the spring of 2015 we will collect soil samples to see how much weed seed was added to the seedbank.

In ZSR plots, weeding events took place about every ten days in the early- and mid-season, depending on weather, and less often later in the season as weed germination slowed.

Plastic-mulched plots required three hand-weedings to control the crabgrass coming through the planting holes, suggesting that plastic might be better suited to crops with wider spacing. Also, transplanting by hand took longer in plastic mulch. Soil temperatures under the black plastic were consistently 5-10 degrees Fahrenheit higher than the others. Onions under black plastic matured several weeks earlier, which may have contributed to the decreased yield. The BPMSP required the lowest amount of weeding labor of all strategies.

We used high quality oat straw mulch that didn’t have any weed seed but it did bring in a lot of oat seed (112 seeds/lb) that germinated within the mulch and forced us to hand pull twice. The straw mulch was much easier to apply than the junk hay, which stuck together. The junk hay mulch brought in a lot of weed seed (170 seeds/lb) but few weeds emerged through the mulch.

Not surprisingly, at the end of the season, mulched plots had less compacted soil and better water infiltration than unmulched plots. Plots with organic mulch had more earthworms than the others.

After harvest, the onions were cured in a greenhouse and weighed to measure marketable yield. Insect damage and disease were minimal for all strategies.

Breakdown of Net Income

In the Breakdown of Net Income (above), labor costs were set at $10/hour. Materials costs included fertility, mulches, tractor use, and an estimate of curing, packing, and shipping costs. Sales were calculated by assuming that 90% of the cured marketable yield could be sold at the organic wholesale price of $0.75/lb. Net income was determined as the difference between sales and costs of labor and materials.

It was surprising that the strategies typically used for long-term aims of reducing the weed seedbank (ZSR) or building soil quality (organic mulches), were the most profitable in the first year of implementation. That these more expensive strategies were more profitable than the others demonstrates the importance of high yields.

The CPWC was the lowest yielding strategy, indicating that the weed control period was not long enough. Based on growing-degree-days we expect that the onions should have been weeded through late July to avoid yield loss. This highlights the sensitivity of onions to competition.

In 2015, soil samples will be collected to determine the effects of the contrasting weed management strategies on the weed seedbank and soil organic matter. The onion experiment will also be repeated on a new field to show which results are consistent and which results are subject to yearly variation, so stay tuned for this season’s results!

Innovations in Electric Weeders!

After years of searching for innovations in hand tools, my students informed me of two tools I had not seen previously.  We are hoping to acquire these for 2014 field testing, comparing efficacy and working rates to Glaser wheel hoes and the Weed Master.

First, from France, the Pellenc Cultivator.  A battery-powered scuffle hoe, that looks very intriguing:

The second, from the U.S., Tillie and Solus, the Electric Wheel Hoe, from Carts & Tools:


And, this is what I’m hoping to add to the Weed Master!