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