Gardeners know compost is the best thing for their soil, but the science behind the benefits of compost is generally not well understood. To shed light on how composting works and why compost enriches soil, my guest this week is chemist, author, and Master Gardener Robert Pavlis.
Robert is an expert on busting gardening myths using science. His background and training are in chemistry and biochemistry, and he’s also made laboratory software. He now applies his professional history to his gardening hobby and has written several books that demonstrate how the two intersect. His latest is “Compost Science for Gardeners: Simple Methods for Nutrient-Rich Soil,” an easy-reading, practical guide to the science behind a healthy soil ecosystem and thriving plants. This book is right down my strike zone, and if you’re a compost fanatic like me, you’ll feel the same way.
Robert is one of the most frequent guests on the podcast, and for good reason. He’s been here to discuss houseplant myths, garden products you don’t need, and plant science. Robert is no-nonsense and he doesn’t mince words, which I love. In his appearances, on his website, GardenMyths.com, and in his series of books on garden science, Robert gives straightforward answers to challenging gardening questions. He believes that when gardeners learn a little more about science, they’ll understand what they’re doing and will be able to better interpret what they see and hear.
Robert has a 6-acre botanical garden known as Aspen Grove Gardens in southern Ontario, Canada, packed with more than 3,000 species of plants, trees and shrubs. It’s located in zone 5 and is mostly dedicated to ornamentals with a focus on really interesting plants, though he also has a small vegetable garden.
Robert will also be part of a series of conversations that will be included in my new Online Gardening Academy™ premium course, Organic Vegetable Gardening. I’ll be speaking with top experts in their fields about specific content. Sign up for the waitlist here.
At the end of this month, I’m offering free, live, online training sessions at my Organic Vegetable Gardening Summit. The summit runs from March 28–31 from noon to 1 p.m. Eastern each day and will cover topics such as organic gardening fundamentals, the importance of planning ahead, and setting up your garden to conquer the many challenges that Mother Nature throws our way. Get the details and register.
And while you’re here, I also want to remind you that I have a new book out, “The Vegetable Gardening Book: Your complete guide to growing an edible organic garden from seed to harvest.” It’s chock full of insider tips and new-to-you information that will help you step up your gardening game and tackle challenges.
Composted organic material sure looks like soil, but finished compost and soil are, in fact, quite different. Soil is 50% a mix of sand, silt and clay with a little organic matter mixed in, and the other half is air and water, while compost doesn’t contain any sand, silt or clay.
“Although good soil and compost kind of look the same — they’re both black, they’re fluffy, they’re soft — they’re very, very different on a chemical basis,” Robert says. “The one is essentially ground-up rock, and the other one is all organic matter.”
Robert says explaining compost is quite complicated and most gardeners have a misunderstanding of what it is. In fact, he doesn’t like the term “finished compost” because it is a misnomer, though he acknowledges it is a handy term.
“‘Finished compost’ to a gardener means that it’s composted enough that we can put it in our garden and it’s ready to do some good for the plants and it’s not going to harm anything,” he says. “That’s ‘finished compost,’ but it is nowhere near finished.”
Another definition is compost is that it starts with things we recognize, such as banana peels and grass clippings, gets mixed up and goes through the process of decomposition until it reaches a point where we don’t recognize it as the original inputs anymore.
“We see this black stuff,” Robert says. “The problem is, our eyes aren’t very good at looking at this stuff. I mean we can’t see it on a molecular basis.”
Though we may consider the compost to be “finished” at this point, in actuality, the composting process is just starting, and most won’t happen until after it’s been added to our gardens.
In making compost, we start with large, recognizable things like a banana peel, Robert says. It has lots of plant cells and a lot of structure to it, but it’s absolutely useless to plants. Plants can’t use any of it until it’s been adequately degraded.
“We have to break the cells apart,” he explains. “We have to break the structure apart, and fungi is the most important part of that process. Then we break it down into small cells, and then the cells have to be broken apart, and inside the cells is all kinds of large molecules. And again, even at that stage it’s pretty much useless for plants.”
It’s not until other microbes move in, particularly bacteria, that the large molecules are broken up into smaller and smaller and smaller pieces until they release a nitrate molecule. “Only then can plants use it,” Robert says.
“In general, that compost will continue composting for about five years after we put it in the soil, and I think that’s the part that a lot of gardeners don’t really appreciate,” he says. “So they take this compost, they put it in the soil and say, ‘Ah, now I’ve added some food for my plants. It’s helping my plants.’ But it’s really not. It’s a slow process.
“It’s going to help them over a five-year period, and there is some nutrients released right away, and some tomorrow and some the day after, and some next year and some the year after.”
That slow degradation process taking place in the garden is what makes compost so valuable. It’s slow-feeding plants, which is much better for them than a fast feed, Robert says.
“Another critical piece is that nitrogen runs away very quickly in soil with rain,” he adds. “So we have a heavy rain, and that washes all the nitrogen away from the soil and past the root zone. And so plants are now starving, but the compost the next day will start decomposing a bit more and release some nitrogen. And so that’s what’s feeding our plants. They want that long-term slow, steady feed.”
More Benefits of Compost
In addition to feeding plants, compost does so much to improve soil. The organic matter helps to build up soil aggregates out of sand, silt and clay, which improves soil structure so it holds air and drains properly.
“We talk a lot about the nutrients, but for soil building, the carbon is much more important,” Robert says. “And that’s what compost is adding. It’s adding all that carbon and oxygen, and that’s what’s being used by the microbes in the soil to develop better soil structure.”
Compost also has a high cation-exchange capacity, which means it holds nutrients well, like little magnets, he says.
Clay also has a high cation-exchange capacity, but sand and silt don’t. “In sand and silt, those nutrients just run through the soil,” Robert says. “When we put compost in there, the compost now holds onto those nutrients and basically holds them close to the roots until the roots can actually use them.”
Why We Should All Be Composting
Organic material is one of the best things you can ever add to your garden, if not the top thing. So if you aren’t composting, you’re really missing out. There are also numerous environmental concerns.
“The biggest problem with not composting is that you send the material off your site,” Robert says. “It leaves the soil that you have, and that causes a bunch of problems. First of all, we then have to truck it around the city to somewhere else, which is bad for the environment.”
If the city takes that organic material from you and makes compost at a municipal facility, it has to be trucked around to get back to gardeners and farmers.
“So we’re taking this valuable resource and trucking around the city using up gasoline, polluting the air producing CO2, when you could just leave it on your property,” Robert says.
Another problem for gardeners is that removing organic material from your property repeatedly will reduce the amount of organic matter in your soil, degrading the soil quality.
“What you’ve essentially done is taken organic matter out of the soil, turn it into plant growth, then we cut that, and then we put it in a bag and send it to the city,” Robert says.
It makes no sense to do that, he says. You don’t even need to have a compost pile and engage in hot composting to keep organic material on site. For the most part, Robert just cuts and lets the cuttings drop when he trims or deadheads plants. His grass clippings stay on the lawn. He lets most of the leaves stay where they fall, excluding leaves that fall on the lawn and risk smothering it. Even in that case, he just rakes it into garden beds rather than move it into a compost pile.
“Nature compost automatically,” he says. “We don’t even have to do anything.”
Hot composting may be faster, but it’s not necessary for turning organic materials into usable compost.
The Nutrients in Compost
Compost has an NPK ratio of 1:1:1 or 0.5:0.5:0.5. That’s a measure of how much nitrogen, phosphorus and potassium is in a fertilizer, by volume.
“It’s a low amount, but it’s released steadily to the soil,” Robert says.
He also reminds us that because we’ll add compost every year, by year three, four or five, there will be a lot of nutrients in the soil, at a fairly high level. He recommends an inch or two a year, so it will never build up nutrient levels so high that they will cause problems.
Most of us don’t get our soil tested, so we don’t know what deficiencies our soil has. But that doesn’t stop gardeners from applying fertilizer anyway.
“If we don’t know, then I think it’s wrong to just throw a bunch of fertilizer around hoping it’s going to work,” Robert says.
What NPK Ratio to Shoot For
At some point along the way, a balanced fertilizer, such as a 10:10:10, became a standard recommendation for gardens. But that’s not really what plants need.
“I don’t know where the 10, 10 10 came from. I’m pretty sure this goes back a long ways, when we first started making fertilizer and we really didn’t know what plants needed,” Robert says. “So some marketing person thought, well, if we give them some of each, people will be happy.”
He says this makes no sense because no plant is missing all three nutrients in equal proportions, and plants don’t use all three nutrients in the same way.
“I spent some time actually looking at what plants use, and every plant I’ve looked at uses a ratio of 3:1:2, maybe 3:1:2.5,” Robert says.
This means plants need three times as much nitrogen as phosphate, and potassium needs are somewhere in the middle of the other two.
“Our job as gardeners is to replace the nutrients that are missing in soil, not to feed the plants, and I think that’s a key point that gets missed in so many discussions,” Robert says. “… So we should be better understanding what’s missing in our soil.”
Because compost is a balanced fertilizer, it adds more phosphate than plants need. While the nitrogen is used up or runs away quickly and the potassium runs away fairly quickly, the phosphate stays put.
“Next year I come along and I add another layer, 1:1:1, same thing happens,” Robert says. “So over time, I’m slowly building up the phosphate, and that is a real problem in gardens.”
It’s a worse problem when using manure that contains even more phosphate than compost, he adds.
Phosphate in synthetic fertilizers is in a form that is readily available to plants right away, but when there is too much readily available phosphate present, natural processes will convert it into a form that is unavailable — like rock phosphate.
Other Benefits of Compost
Because of its high cation-exchange capacity, compost buffers pH. It absorbs ions, helping to raise the pH of acidic soil and to lower the pH of alkaline soil, bringing them closer to neutral.
Compost can also potentially remove toxins and disable pesticides. Robert attributes this to the fact that compost is great for encouraging microbial activity. Those microbes will degrade the organic compounds that make up herbicides.
“The active ingredient in Roundup is glyphosate,” Robert points out. “There are actually bacteria that eat glyphosate. So if we get our bacterial level higher and we put on some Roundup, it will disappear faster because of the microbes.”
Heavy metals are a different story.
“When we’re looking at heavy metals, they never disappear,” Robert says. “So something like a lead molecule won’t disappear. It’s there forever. We can move it around from place to place, but we can’t get rid of it. And all our soil has heavy metals, by the way, which a lot of people don’t realize. Even good soil has heavy metals. It’s just that the levels are low enough that we’re not too concerned about it.”
Heavy metals will stick to compost the same way nutrients stick to compost via cation exchange. This essentially deactivates the heavy metals, making them unavailable to plants.
How to Apply Compost
Some gardeners, like English horticulturist Charles Dowding, use compost as the mulch layer on their garden beds. Then there are gardeners like me who put leaf mulch over the compost, to protect it.
For gardeners in a warm climate, it makes sense to protect the living things in compost from the baking sun. In cooler climates, this will be less of a concern.
Robert points out that in vegetable gardens, working compost into the soil is easy, while in a herbaceous garden, where gardeners don’t want to disturb the roots, just laying compost on the soil surface makes the most sense.
When you topdress with compost, worms and other soil organisms will come up, grab the compost, and take it down, so before long it’s in the soil anyway, he says.
Working compost into the soil of a new garden bed to get good soil fast also makes good sense, but Robert says for an established garden that already has good soil, it’s not really necessary, and topdressing is the way to go.
Why Gardeners Shouldn’t Go Overboard with Compost
If a little compost is a good thing for the garden, why not use a lot more? I get asked this question a lot, and there are a number of reasons not to go overboard with compost.
As mentioned above, excessive compost will lead to a buildup of phosphate in the soil, and that’s bad for plant roots.
“If you use too much compost, you can end up with toxic soil,” Robert says.
He sees this most often in raised beds that gardeners fill with really good soil and lots of compost.
“You’ve made your soil too good,” Robert says. “We have this idea that compost is great, so more is better — and that’s simply not true. Plants grow much better when most of the stuff is soil, that sand, silt and clay. And good soil only has about 5% organic matter, and that organic matter includes not only the compost, but all of the other stuff that dies in there, the old dead plant roots and so on. So our goal is 5% organic matter. That’s it.”
Compost Myth Busting
There are a number of myths surrounding composting, so I thought I’d ask the garden myth-buster himself to separate facts from fiction.
Myth #1: Composting Is Too Difficult
“Composting can be as easy or as hard as you want,” Robert says. “It can be a lot of work and a lot of fun. Or it can be really, really simple.”
When you decide to compost, you need to pick the method that’s most suitable to you, he advises. It will depend on the amount of material you have, the space you have and the amount of time you want to dedicate.
“There is a method that is zero work if you want it,” he says.
The cut-and-drop method he explained above creates no extra work. In fact, it’s less work than bagging up cuttings or grass clippings for disposal.
Myth #2: Composting Attract Rodents
“We don’t see the rodents in our garden very often,” Robert says. “They’re really good at hiding, and they come out at night, and then suddenly you build this pile and you put a bunch of goodies in there and you see them and you go, ‘Oh no, we have mice, we have rats, we have raccoons, we have — well, you have those anyways.”
A compost pile may feed the mice or rats you already have, but it won’t attract rodents that weren’t there before, according to Robert.
It could also be that you are putting things into your compost that you shouldn’t be anyway: fats, greases, dairy, meat, bones, etc. from your kitchen.
And if an open pile is a nuisance, you can choose a closed system, such as a compost tumbler.
Myth #3: Composting Smells
“If a compost pile smells, you’re doing it wrong, bottom line,” Robert says, “and the odors that you get can actually tell you what the problems are. If you do it correctly, there’s virtually no smell whatsoever.”
Compost can smell if there is an imbalance of green (nitrogen-rich) inputs and brown (carbon-rich) inputs. When is too much green material by volume, add browns such as leaves, newspaper and cardboard to fix the imbalance. The compost may also smell if the conditions are anaerobic, meaning there is a lack of oxygen. Mix up the compost to aerate it and refrain from adding too much water, so oxygen-loving bacteria can move in.
And when you do hot composting right, it will neutralize weed seeds and pathogens. Robert adds that by fall all plants are covered in fungus, so he says not to worry too much about composting diseased plant material.
Myth #4: You Need to Buy Compost Activator
Compost activators come in two types. The first contains microbes, the idea being that your compost pile is missing microbes so you need to add them in, like adding yeast to dough to make bread, Robert says. However, he says compost inputs are covered in microbes already.
“Everything you put into that pile is covered in microbes,” he says. “So those products are not needed.”
The second type of activator is a source of nitrogen, which Roberts says can be useful when starting a compost pile in fall, when gardeners have lots of “browns,” like leaves, but not as many “green” inputs.
“We have too much carbon and not enough nitrogen, and so the pile never gets going because of low nitrogen levels. Then adding extra nitrogen can help,” he says.
Compost starters are probably the most expensive nitrogen you can buy, while any nitrogen fertilizer will work, he says. Organic options include blood meal, but anything that has nitrogen in it will get a pile going.
Myth #4: You Must Get the Ratios Right
The browns and greens don’t matter, Robert says. If you pile organic material up, it will make compost.
“The browns and greens are only important if you want to make fast compost,” he says.
Getting the ratios correct will speed up the process, but failing to get the ratios right is fine too. The composting process will be slower, but it will still happen,
In fact, Robert hates the terms “browns” and “greens.” He finds the terms to not be all that accurate. For example, manure and coffee grounds are brown, but actually “greens.” And grass clippings are green, but if they’re too old, they’re “browns.”
Coffee grounds have a ratio of about 30 carbon to 1 nitrogen, so they are almost perfect the way they are, Robert says, in terms of making hot compost. A hot compost pile should also be at that ratio. He explains all the calculations for doing this properly in “Compost Science for Gardeners.”
Another thing to take into account is moisture levels in compost inputs. Robert came up with a new way for doing that using measurements in volume rather than weight and three piles: one high in carbon, one high in nitrogen and one that’s more balanced. He can add from the first two piles to the third pile to keep it in balance.
As an avid composter, I feel pretty proud of the compost that I make, and it doesn’t take that long. And I’m very aware of the ratio issues and the carbon and nitrogen and all that you should know on a geekier level. And yet when push comes to shove and I’m in the real world, I’m out there just taking all my organic matter from inside and outside and putting it in, mixing it up periodically and spraying it with a hose — and I have awesome compost in three months. And so for those that are just a little bit intimidated by carbon and nitrogen do not be. It’s going to happen anyway — it’s just a matter of how quickly it’s going to happen.
Things to Compost, or Not Compost
“We can compost everything,” Robert says. “If something dies out in the woods, it composts. Nature takes care of it.”
Still, there are some things we may not want to compost, and on the other side of the coin, things people are concerned about but shouldn’t be.
Meat can attract rodents and is slow to compost, so Robert does not recommend composting it. Grease is also slow to compost and will slow down the process for the whole pile.
“There’s not much point in putting bones in because they’re going to take a hundred years to decompose,” Robert says.
He does throw eggshells into his compost, but he says he knows they won’t decompose quickly either. He doesn’t put paper in his compost because it takes a long time to decompose. However, I shred paper in a $150 confetti shredder, and those tiny paper pieces will decompose within two weeks. Newspapers, magazines and corrugated cardboard are all fine to compost, according to Robert, but he says to avoid paperboard, like cereal boxes, which are printed with special ink. Pizza boxes can be greasy and in the past were commonly coated with chemicals to stop them from absorbing oils. The most notorious chemicals have been largely phased out, so you can compost pizza boxes if you wish and they are not too greasy.
Robert doesn’t mind composting pet waste. He figures that you live with your pets, so you’re already exposed to their waste anyway and including it in your compost won’t expose you any further to diseases.
Some people compost “humanure,” i.e., human manure. Robert says there are some potential disease issues, but they’re a pretty small concern. And human urine is good for compost, according to Robert, because it has high nitrogen levels.
How Long Does Compost Last?
If a pile of “finished” compost just sits there without being added to a garden, it will continue composting.
“The microbes are still active in there,” Robert says. “They’re still degrading things. Nutrients are being released. Now, most of those nutrients will wash away. So if we have this pile in the open and it rains, it’s going to wash the nutrients out and into the soil below the compost pile. So, we don’t lose them, but they may go in an area of the soil you don’t really care about.”
Because water is a charged molecule, rain will pull some nutrients off the compost and wash it away.
The best thing to do is to use compost up right away, Robert says. Alternatively, the compost can be dried up to stop the microbial activity. Because the bacteria and fungi live within a water layer on the compost, in the absence of moisture, the microbes die or hibernate. Either way, they stop breaking down the compost.
Some microbes will go into a state of suspended animation and become spores. “They can last quite a long time that way,” Robert says. Then when moisture is reintroduced, they will reactivate. And when the compost is added to soil, the microbes present in soil already will use the material.
“Think of compost more as a food for microbes rather than an important source of microbes,” he says. “If we take compost and let’s say we sterilize it — kill everything that’s living in there — it still has all the value there. And the minute I take that and put it on soil, the microbes that are in soil will start eating it and start multiplying.”
What Happens to Compost in Winter
How active your compost pile is in winter will depend largely on your climate. Where Robert is in zone 5, his compost pile is frozen solid and doing basically nothing.
“The microbes are not active. There’s no heat produced. It just sits there. So during the winter, we don’t compost, even if the pile is sitting there.”
Compost piles in cold climates will benefit from being sited in a sunny area. The more sun the pile gets, the sooner it will start the composting process again as winter gives way to spring.
Compost piles can also be wrapped in blankets or straw bales to hold heat for longer headed into fall. These wraps should be removed in spring to expose the pile to the sun again.
The Working Microbes in Compost
Different microbes have different preferred temperature ranges. Some are happy when a compost pile is new and still cool. Then as the microbial activity creates heat, microbes that enjoy the warmth will proliferate, and the microbes that like it cool will die off or go into hibernation mode. And some microbes will only be active at very high temperatures.
A pile of leaves exclusively tends not to get hot, so much of the degradation is done by fungi rather than bacteria, since fungi like it cool.
If you haven’t already listened to my conversation with Robert Pavlis about compost science for gardeners, you can do so now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.
Has learning about compost science made you a more successful composter and gardener? Let us know in the comments below.
Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
Episode 048: The Simple Science Behind Great Gardening, with Lee Reich
Episode 116: Understanding the Soil Food Web, with Dr. Elaine Ingham
Episode 124: Using Compost the Charles Dowding Way: More Than Just a Great Soil Amendment
Episode 143: Gardening Products You Don’t Need, and Why
Episode 153: The Science Behind Great Soil
Episode 179: Plant Partners: The Science-based Benefits of Companion Planting, with Jessica Walliser
Episode 223: How Soil Microbes Make Good Soil Great
Episode 270: Plant Science for Gardeners, with Robert Pavlis, Part I
Episode 272: Plant Science for Gardeners, Part II, with Robert Pavlis
Episode 283: A Soil Chemistry Primer: How Protons and Electrons Influence Soil Moisture and Fertility
joegardener free online resource: The Complete Guide to Home Composting.
Organic Vegetable Gardening Summit – Attend this free workshop series March 28th – 31st each day from noon to 1 p.m. ET to discover the importance of planning ahead and setting up your garden to conquer the many challenges that Mother Nature throws our way.
joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.
joegardener Online Gardening Academy Master Seed Starting: Everything you need to know to start your own plants from seed — indoors and out.
joegardener Online Gardening Academy Organic Vegetable Gardening: My new premium online course membership opens in 2023. Sign up for the waitlist here.
joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
joegardener Online Gardening Academy Growing Epic Tomatoes: Learn how to grow epic tomatoes with Joe Lamp’l and Craig LeHoullier.
joegardener Online Gardening Academy Master Pests, Diseases & Weeds: Learn the proactive steps to take to manage pests, diseases and weeds for a more successful garden with a lot less frustration. Just $47 for lifetime access!
joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.
Earthbound Expeditions: Great Gardens of Italy & France with Joe Lamp’l
GardenMyths.com: “Too Much Compost – Is It Poisoning Your Garden?” by Robert Pavlis
Garden Fundamentals on YouTube
Garden Fundamentals Facebook group
“Compost Science for Gardeners: Simple Methods for Nutrient-Rich Soil” by Robert Pavlis
“Plant Science for Gardeners: Essentials for Growing Better Plants” “Soil Science for Gardeners: Working with Nature to Build Soil Health” by Robert Pavlis
“Building Natural Ponds: Create a Clean, Algae-free Pond without Pumps, Filters, or Chemicals” by Robert Pavlis
“Best Garden Plants for Connoisseurs: The best plants from my collection of over 3,000 garden gems” by Robert Pavlis
“Garden Myths: Book 1” by Robert Pavlis
“Garden Myths: Book 2” by Robert Pavlis
Proven Winners ColorChoice – Our podcast episode sponsor and Brand Partner of joegardener.com
Earth’s Ally – Our podcast episode sponsor and Brand Partner of joegardener.com
Territorial Seed Company – Our podcast episode sponsor and Brand Partner of joegardener.com
Soil3 – Our podcast episode sponsor and Brand Partner of joegardener.com
Greenhouse Megastore – Our podcast episode sponsor and Brand Partner of joegardener.com – Enter code JOEGARDENER for 15% off your order
Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Corona Tools, Milorganite, Soil3, Greenhouse Megastore, Territorial Seed Company, Earth’s Ally, Proven Winners ColorChoice and Dramm. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.