In response to my video, “You Just Composted WHAT?”, I decided to look for a soil scientist to hear their thoughts on some controversial “compostable” items that are often written about: waxed cardboard, condoms, eggs contaminated with salmonella, etc.
Who stood out most to me was Dr. Michael Mulvaney. His blog, named “The Dirt Dude”, has lots of excellent material about soil and agriculture, and I could tell he would be able to provide some great feedback for my wonderful readers. So here we go:
TW: When did you decide you were so interested in soil and pursue a career? Was it an early childhood fascination for you? Tell me a bit about how you got to where you are now. What are your goals as an agronomist, and what would you like to see change in today’s agricultural practices?
MM: I used to help my mother garden when I was a child, but never realized that I could make a career out of it until I was a Peace Corps Volunteer in rural Bolivia. It was then that I realized that the majority of the world’s population lives and dies by subsistence agriculture. Unfortunately, colonial histories have made them forget how to farm sustainably, and now face serious land degradation issues. There are several simple things that they could be doing to sustain their resources, such as crop rotation, or plowing on the contour, but they just don’t have the education to implement them. My goal as an agronomist is to improve the livelihood of subsistence farmers around the world.
TW: What are some of the main “critters” in a compost pile on a microbial level that do the dirty work? Are they always present in soil, and if not how do they “find” your pile?
MM: The microorganisms (MO’s) in compost are divided into categories based on their ability to withstand heat. A fresh compost pile will activate mesophilic MO’s, which thrive at moderate temperatures. They remain active for a few days and “eat” (metabolize) the easy stuff: soluble compounds, sugars, starches, etc.
Then the compost really starts to heat up (due to MO biological activity), and the thermophilic MO’s take over. This is the critical temp at which pathogens are destroyed (thermal kill). That’s why organic producers must take the temp of their compost and make sure that the temp is >55C for several days. Keep in mind that compost is defined as having manure in it, otherwise it’s called mulch. You want the temp to remain between 55-65C. Above 65C, the microbes themselves can become limited, so aerate (mix) your compost to keep it below this temp. This phase breaks down more complicated molecules: proteins, fats, complex carbohydrates, etc.
After the thermophiles have done the heavy lifting, the mesophiles take over again. This is the finishing of the compost, and can take several months. You’ll see the temp slowly decline during this period.
Specific MO’s that are involved in the process include bacteria (about 85% of the total MOs present), actinomycetes (which are bacteria but grow like fungi, and give that familiar earthy smell), fungi, and to a lesser extent rotifers and protozoa. Soil microbiologists often say, “Everything is everywhere, and nature selects.” That is, all MOs are everywhere, and it’s the microclimate that selects for them. That’s why I think activators are bunk, and also why I believe there’s life on Europa!
TW: I’m going to name a number of different products that are frequently asked about with regards to their compostability. For your replies, assume the composting efforts are performed at home, but if you feel there’s a different answer if the material is sent to an industrial-sized composting facility (forced air currents, high heat, constant monitoring), please respond accordingly.
Paper and cardboard constantly come up with contention as they use a variety of ingredients such as chlorine, ammonia, chloroforms, hydrochloric acid and sulfuric acid in their creation. Do you find any of these to be dangerous for your compost?
MM: Those compounds are in such small quantities that I certainly don’t worry about them for home composting. Besides, the MOs will take care of them in time. In fact, ammonia is good for MOs: they use it for building proteins within their cellular structure. Although it’s not good if your compost smells like ammonia: that’s indicative of anaerobic activity. If that’s the case, aerate and mix, and add more carbon (sawdust).
TW: Composting laundry lint is a handy thing to do… is there any risk from traces of detergent?
MM: No, not from detergent. Again, it’s present in such small quantities that MOs can handle it easily. One thing about lint, though: if it contains non-natural fibers (nylon, polyester, etc.), those won’t break down. But again, it’ll be such a small amount that some people might not worry about it. Of course, it’s nice to keep all plastics out of the environment, so unless you use only natural fibers, consider just throwing your lint in the garbage.
TW: I began my experiment involving latex condoms, as I’ve read several articles promoting them as compostable items. Would spermicide cause a problem, and wouldn’t latex take quite some time to break down?
MM: I don’t know how latex will act in a compost. Let me know your results! I’m not concerned about nonoxynol-9. It has a long chain of ethers, which I think can be easily munched on by MOs at high temps. The benzene ring is probably more stable, but will degrade in time as well. The amount present is so small, that I think the MOs can handle it pretty easily. You’d have to be one busy dude to build up the nonoxynol-9 levels to toxic quantities!
TW: HA! There’s been a lot of salmonella outbreaks over the years, and everyone loves to compost their egg shells. Would you consider this to be a contamination risk?
MM: Thermal kill. Get your temp above 55C for a couple days, and you’re good to go. Don’t add more pathogens after that time, though.
TW: What contaminants would you rank highest to be concerned about in your compost pile? Feel free to scare me.
MM: Things that break down VERY slowly, although they’re not contaminants by any means: citrus peels, egg shells, leather, hair. Contaminants: plastics (like those little stickers you find on fruit these days), glass, metal, etc. Those last two are terrible because as you lovingly stick your hand in your newly finished compost, you can slash yourself on that bit of glass in there. Most chemicals in minor to moderate quantities are easily metabolized by MOs. Even oil is metabolized by MOs! It’s a great way to negate the negative impact that those chemicals might otherwise have. We call it “bio-remediation”, and we use it all the time.
TW: I would like to thank Mike for his excellent input, and hope that he will write for the page in the future. One thing I forgot to ask him about was the crappy Sun Chips bag. I have a feeling that falls into the “breaks down very slowly” category. We’ll see!