Science Behind the Scenes: Searching for Microplastics in Garden Soils

Fig. 1: Anna Perry viewing microplastic particles under the microscope

Hello gardeners! Anna Perry here, the newest graduate member of the Garden Ecology Lab. You may recognize my name, as I previously worked in the lab as an undergraduate student. As an undergraduate I was fortunate to be able to both work on my own independent research project (monitoring environmental conditions on a downtown Portland building-integrated-agriculture array), as well as assist Nina Emond Miller with her field work and pinning insects. I am so thrilled to be starting my master’s degree in Horticulture with the Garden Ecology Lab this Fall! For my master’s research project, I will be exploring microplastic pollution in the soils of urban community gardens. In this post, I will be giving you a peek into the very early stages of my research process.

What are microplastics?

Before we get too into the weeds (pun intended), it is important to define the terms we’ll be using. You may have heard the term “microplastics” (MP) before, as they have been a buzzing topic in the news for the past few years. MP are essentially tiny pieces of plastic, which seem to be anywhere and everywhere we look. From remote alpine lakes¹, Antarctic snow², seafood³, and even inside human fetal tissue⁴, microplastics apparently turn up wherever researchers look. 

So just how small are MP? Unfortunately, there is no universal size definition for MP. I’ll be using the definition provided by the US Environmental Protection Agency, which defines MP as plastic particles smaller than 5 mm, but larger than 1 nm⁵. Plastic particles that are smaller than MP are called “nanoplastics”, and those that are larger than MP are called “mesoplastics” and “macroplastics”. 

What started my interest in soil-borne microplastics?

I first began to think about MP in soil when I started gardening at a local community garden here in Corvallis, OR, in 2022. At this garden, us gardeners are provided with access to municipal compost. The first time I worked with this compost, I noticed what seemed to be a lot of plastic. Some of the plastic was what you might expect, such as produce stickers. I was also surprised to find a litany of unusual, sometimes even a bit gross, pieces of plastic. These included: beverage straws, snack wrappers, dog poop bags, and even an acrylic (faux) fingernail! 

After reading an article titled Plastic in compost: Prevalence and potential input into agricultural and horticultural soils⁶, I learned that my municipal compost wasn’t unique. The authors of this article tested composts from regional municipal composts as well as store-bought bagged compost. They found MP in every compost they sampled from. 

I was fascinated. How much plastic was there in my plot’s soil, from compost or other sources? There seemed to be news each week announcing microplastic discoveries in remote bodies of water and human/animal tissue, but what about soil? What does it all mean for the ecological function of soils? The more I read, the more questions I had. 

I didn’t know it at the time, but just a few years after my compost discoveries I would have the opportunity to research the very topic that had so intensely piqued my interest. During one of our meetings in my final year as an undergrad, Dr. Gail Langellotto mentioned that members of the Garden Ecology League had expressed interest in research related to microplastics in garden soils. I could hardly contain my excitement, and quickly rattled off so many of the questions that I would like to explore relating to this hot topic. Fortunately my enthusiasm was shared, and before I knew it I was submitting my application to graduate school, with the intent of researching the endlessly fascinating world of microplastics in soil. 

How do microplastics get into soil?

While most research on MP pollution has been conducted in aquatic environments, up to 23 times more MP are released into soils, than oceans, each year⁷. There are many different ways that MP can enter soils. In fact, there is still much to be discovered regarding all of the different ways that MP can find their way into soils, or any environment. 

Some of the ways that plastics might find their way into garden soils include (but are not limited to):

1. From the breakdown of plastic pots and plant tags
2. From plastic mulches, such as woven weed barrier and plastic film
3. From gardening gloves, which are often coated in nitrile or another type of plastic
4. From synthetic fibers shed from gardener’s clothing
5. From compost or other soil amendments

Why do we care about plastic in soil?

MP contamination in soils can change their physical, chemical, and biological properties⁸. All of these aspects of soil affect its ecological function, and its ability to support plants and other life. Simply put, the effects that MP have on soil are complex, and there is much still to be discovered.

Developing methods for sampling and extracting microplastics from garden soil

In preparation to begin my research project, I combed through nearly all of the published literature I could find. I quickly learned that isolating plastic particles from soil was far more complicated than extracting them from water or animal tissue. This is due to the complexity of the soil matrix, which is comprised of a “mineral” fraction and an “organic” fraction. In simple terms, the mineral fraction contains some combination of sand, silt, and clay particles, in varying proportions depending on the type of soil. The organic fraction consists of living organisms and “organic matter” (OM), material that originates from plants, animals, or other organisms, in varying stages of decay. It is the organic fraction that seems to pose the largest issue when it comes to microplastic extraction. This is because OM particles share many of the same characteristics as MP particles, such as electrostatic properties and density⁹. In fact, as readers with a background in organic chemistry may know, plastics are technically organic compounds. I knew this could pose a challenge for me, as garden soils tend to be relatively high in OM, compared to the values recommended for agricultural soils¹⁰. Most of the papers I was able to find described research conducted using agricultural soils or river sediments, both of which are pretty different from garden soils. 

For this project I will be recruiting gardeners who tend community garden plots in Oregon. I’ll sample soil from each participating gardener’s plot, which I will then divide into three subsamples, as illustrated in figure 2. One of the subsamples will be used for microplastic extraction and identification. Another subsample will be used for microbial DNA extraction. The final subsample will be sent to the Soil Health Lab here at OSU, for analyses which will give me information on the soil’s chemical, physical, and biological properties. 

Fig. 2: Flowchart of sampling protocol

To get a start on developing my methods, I elected to take some soil samples from my own community garden plot. For this, I used a soil probe (figure 3), along with a random sampling method, to get a “representative composite sample”. As is pictured in figure 2, this sampling method entails taking several samples in a random pattern, across the area of interest (in my case, my garden plot). All of these small samples are then combined and thoroughly mixed up. What you are left with is a “composite sample”, which should give a pretty good idea of the conditions in the soil throughout my garden plot. I chose this method of sampling not only because it allows you to get a pretty good picture of conditions across an entire plot, but also because I figured most gardeners would not take too kindly to me leaving a large hole in their beloved garden. 

Fig. 3: Anna Perry holding a soil probe

After I collected my composite soil sample, it was time to head back to the lab to try out what I felt could be the most promising method for extracting microplastics from garden soils, the “Density Separation Technique”. Before embarking on my density separation adventure, I sieved the soil and picked out any large pieces of plastic (figure 4).

Fig. 4: Suspected macro- and meso-plastics, obtained from sieving composite soil sample

The density separation technique

The density separation technique exploits the simple fact that most plastics are less dense than the mineral fraction of soils. For this method, soil is added to a prepared, denser-than-water, solution. For this experiment I used table salt to prepare my density separation solution. The density of salt water is greater than many, but not all, types of plastic. The soil/saltwater mixture is thoroughly stirred and then allowed to settle. In my case, it took about 12 hours for the mixture to completely settle. What you are left with is: mineral soil at the bottom, and OM and MP floating at the top (figure 5).

Fig. 5: Settled soil/saltwater mixture

After the mixture settled, it was time to decant the floating OM and MP from the top and into a funnel lined with filter paper. As this was my first try, I wasn’t quite able to avoid getting some of the sediment from the bottom in my filter paper (figure 6). Science is nothing if not an endless learning experience!

Fig. 6: OM and MP on filter paper

After drying and scraping the OM and MP off of the filter paper, it was time to deal with the organic matter. 

Removing the organic matter

In my readings I came across a few options for removing OM. After careful consideration and discussion with Adam Fund, the manager of OSU’s Soil Health Lab, I settled on using 30% hydrogen peroxide (H₂O₂) to oxidize, or digest, the OM. This is a procedure they regularly use in the Soil Health Lab, to remove OM from soil samples prior to texture analysis. 

Fig. 7: OM and MP post-density separation, day one of H₂O₂ treatment

Fig. 8: OM and MP post-density separation, day nine of H₂O₂ treatment

I brought in my OM/MP sample from my try at density separation, as well as some additional soil from my composite sample, to the Soil Health Lab (which is very conveniently located just one floor below the Garden Ecology Lab). I wanted to see if it was better to remove OM before, or after the density separation (both ways are described in the available literature). Over the course of nearly two weeks we slowly added a total of 16 mL of hydrogen peroxide to the OM/MP mixture (figures 7 and 8), and 50 mL each to two beakers of soil. We needed to add the hydrogen peroxide slowly, in 2 mL and 5 mL increments, to avoid excessive bubbling. 

Fig. 9: Bubbles in soil sample after H₂O₂ treatment

After this process was complete, I brought my samples back to the Garden Ecology Lab. I filtered and rinsed my OM/MP mixture, dried it, and eagerly started poking through it under the microscope, searching for MP. As you may be able to see in figures 10 and 11, the hydrogen peroxide treatments were not able to completely remove the OM. 

Fig. 10: Filter paper containing MP, bleached woody OM, and some sediment

Fig. 11: Anna Perry holding a dish containing filter paper with MP, bleached OM, and some sediment

Though I wasn’t able to remove 100% of the OM with the H₂O₂, I was still able to identify some MP under the microscope from this sample. The woody material left over was quite bleached, and for the most part I could tell what was (and what wasn’t) MP. I’ve included a few microscope images from this process, of suspected MP (figures 12-16).  

Fig. 12: Clear film MP, with some attached sediment

Fig. 13: Red fiber-shaped MP

Fig. 14: Clear film MP, with some attached sediment

Fig. 15: Clear film MP, with some attached sediment

Fig. 16: Suspected MP, irregular shape and highly weathered 

What’s next?

The next step for me is to continue practicing the density separation technique. Like any skill, practice makes perfect! First, I will try the density separation technique on the soil we treated with hydrogen peroxide. After I feel more comfortable with the technique, I will conduct what are called “validation experiments”. Validation experiments are used to determine both how successful my methods are in recovering MP, and the degree with which my procedures contaminate my samples with MP. To accomplish this, I will conduct what are called “spike” and “blank” experiments. Stay tuned for a future blog post, where I will share more about these processes!

Have you ever found plastics in your soil? In your compost? Comment to share your experiences!

Citations

1 – Negrete Velasco, A. de J., Rard, L., Blois, W., Lebrun, D., Lebrun, F., Pothe, F., & Stoll, S. (2020). Microplastic and Fibre Contamination in a Remote Mountain Lake in Switzerland. Water, 12(9), 2410. https://doi.org/10.3390/w12092410

2 – Aves, A. R., Revell, L. E., Gaw, S., Ruffell, H., Schuddeboom, A., Wotherspoon, N. E., LaRue, M., & McDonald, A. J. (2022). First evidence of microplastics in Antarctic snow. The Cryosphere, 16(6), 2127–2145. https://doi.org/10.5194/tc-16-2127-2022

3 – Traylor, S. D., Granek, E. F., Duncan, M., & Brander, S. M. (2024). From the ocean to our kitchen table: Anthropogenic particles in the edible tissue of U.S. West Coast seafood species. Frontiers in Toxicology, 6. https://doi.org/10.3389/ftox.2024.1469995

4 – Sun, H., Su, X., Mao, J., Liu, Y., Li, G., & Du, Q. (2024). Microplastics in maternal blood, fetal appendages, and umbilical vein blood. Ecotoxicology and Environmental Safety, 287, 117300. https://doi.org/10.1016/j.ecoenv.2024.117300

5 – US EPA. (2022, April 22). Microplastics Research [Overviews and Factsheets]. https://www.epa.gov/water-research/microplastics-research

6 – Braun, M., Mail, M., Heyse, R., & Amelung, W. (2021). Plastic in compost: Prevalence and potential input into agricultural and horticultural soils. Science of The Total Environment, 760, 143335. https://doi.org/10.1016/j.scitotenv.2020.143335

7 – Qiu, Y., Zhou, S., Zhang, C., Zhou, Y., & Qin, W. (2022). Soil microplastic characteristics and the effects on soil properties and biota: A systematic review and meta-analysis. Environmental Pollution, 313, 120183. https://doi.org/10.1016/j.envpol.2022.120183

8 – de Souza Machado, A. A., Lau, C. W., Kloas, W., Bergmann, J., Bachelier, J. B., Faltin, E., Becker, R., Görlich, A. S., & Rillig, M. C. (2019). Microplastics Can Change Soil Properties and Affect Plant Performance. Environmental Science & Technology, 53(10), 6044–6052. https://doi.org/10.1021/acs.est.9b01339

9 – Möller, J. N., Löder, M. G. J., & Laforsch, C. (2020). Finding Microplastics in Soils: A Review of Analytical Methods. Environmental Science & Technology, 54(4), 2078–2090. https://doi.org/10.1021/acs.est.9b0461810 – Nelson, M., Mhuireach, G., & Langellotto, G. (2022). Excess fertility in residential‐scale urban agriculture soils in two western Oregon cities, USA. Urban Agriculture & Regional Food Systems, 7. https://doi.org/10.1002/uar2.20027