The Yin Yang of Compost

I am constantly slaying horticultural snake oil dragons. There is so much misinformation on the web and even within University/Extension publications. In this blog I turn my attention to compost–a subject that is almost universally cherished by gardeners, gardening groups and horticulturists. Unfortunately there are a lot of misnomers about compost.

Compost is dark, earthy, smells good when aerobic is almost finished when it will no longer heat up on turning.

Plants are composed of cellulose and cellulose is a complicated polymer of glucose molecules. Compost is made from the decomposition of organic matter—usually plant debris. The composting process can be fast or slow depending on aeration, mixing and pile size. Composting requires a carbon source and enough nitrogen to allow microbial respiration of the sugars contained in the plant material being decomposed. Since the laws of thermodynamics indicate that no chemical reaction is 100% efficient, some of the energy of respiration is lost as heat. Billions of respiring microbes heat the pile creating a very hot environment where thermophilic organisms propagate quickly. As all the available sugars in leaves and other less woody components of the compost decompose the thermophilic organisms lose temperature and the readily available sugars necessary for growth. Other organisms begin to grow and attack the cellulose in the wood fibers, attacking the more recalcitrant carbon in the pile. Eventually most of the sugar bound in plant residues is attacked and only the difficult to decompose materials are left, these contain lignin and form the basis for humus. When the compost will no longer heat after turning it is beginning to mature. Once all the easily broken down carbon is utilized, the microbes die off or form spores and go into a resting phase. The compost is now screened to remove large undecomposed particles and is ready for use in the garden


I have often heard composting touted as a natural process. It is not.
Composting is a process that is “man made”. The alternative is litter fall and mulching which is a natural process that processes organic matter much more slowly. Composting is a process that requires a specific mass of feedstock, sufficient oxygen for respiration, reactions provided by air or by frequently turning the pile, moisture maintained by adding water if needed, and heat which is maintained within the pile itself. These are not natural conditions easily found in nature. They are carefully manipulated by those monitoring the compost process.

The fungi and bacteria on the initial feedstock are part of the ecosystem and are generally not directly manipulated in the process. Fungi and bacteria have the enzyme systems necessary to break the bonds that link the glucose molecules and then utilize the energy in glucose for their own growth.

Composting does not help the environment

As I have discussed, composting liberates carbon dioxide increasing the amount of greenhouse gasses in the atmosphere. On a large scale composting adds many tons of CO2 to the atmosphere as well as oxides of nitrogen which are also potent greenhouse gases. Composting can also release mineral salts into underlying soil and runoff from large composting operations, especially manure composting, can pollute waterways. There is nothing about composting that is helping the environment per se.

Sheet mulching with cardboard cuts gas exchange below. Covering with fine textured compost as is often done will exacerbate gas exchange issues.

Compost is not full of life

Sometimes you hear that compost is “full of life”. Sort of true but not really. The biological processes that break down the compost happen in the pile. As compost matures microbes die, their growth is reduced and they form spores or other resting structures. Once compost is ready for use, it is not particularly biologically active because all the energy has been utilized to make heat and decompose the feedstock. When the energy (carbon, sugar, cellulose) is used up, the microbial activity declines.

What is it good for?

Since compost is a distillation of feedstock minerals it makes an excellent fertilizer. Since compost is mostly fine textured, it is suitable for use in soil as an amendment. The lignin molecules resident in compost help bind nutrients in organic matter and retain them for later uptake by plant roots. Compost can increase the fertility of a sandy soil which has low nutrient binding capacity. Compost is full of secondary metabolites left over from the microbial activity produced when the pile was hot. These compounds can confer disease protection when pathogens are present in soil. Since feedstocks are variable this can not be predicted. Finished composts with a carbon:nitrogen ratio (C:N) of less than 25:1 do not perturb the nitrogen dynamics of most soils and in many cases may be a source of nitrogen in the amended soil. Since compost is mostly broken down feedstock it does not deteriorate as fast when mixed in garden soil. It resides longer than other more labile amendments. Compost is also a great container medium if mixed with coarse materials to assure aeration. Because the compost feedstock is well decomposed, the material has a longer life as a growing medium.

Since composts get hot as the feedstock is broken down, they tend to sanitize the pile of pathogens. Composting kills food-borne pathogens and plant pathogens easily since most do not survive the high temperatures (>140F) found in an active compost pile for more than a day or so. For effective pathogen kill it is important to turn the pile frequently.
Some plants may survive high composting temperatures, e.g, tomatoes are a notorious compost weed. Yellow nutsedge and bermudagrass stolons can also resist the high temperatures found in compost piles.

Compost can be used in container media if enough aeration is provided by other media components

It’s not a good mulch

One of the amazing things about mulch is undergoes the same processes that make compost and it does have a place in your garden. The microbial processes that decay arborist wood chips on the soil surface happen slowly over months of time. The chips are mineralized but more of the carbon enters the soil rather than the atmosphere because soil fungi, especially mycorrhizal fungi, transform the energetic carbon molecules (labile carbon) into a soil stabile polysaccharide called glomalin. This in turn binds soil particles which increases soil structure. Note: When these processes happen in a compost pile they can not happen again in your garden. The energy is gone.
Texturally fine compost will make greater hydraulic conductivity with the underlying soil and allow for greater moisture loss through evaporation. In some cases compost layers may impede infiltration of water and prevent newly planted root balls from being watered. Compost layers may also impede gas exchange to underlying soils. Depending on the feedstocks, composts may also contain viable weed seeds or other propagules that contaminate landscape soils. Composts make bad mulches.


Daugovish, O.,Downer, J., Faber, B. and M. McGiffin. 2006. Weed survival in yardwaste mulch. Weed Technology 21: 59-65.

Downer, A.J.,D. Crohn, B. Faber, O. Daugovish, J.O. Becker, J.A. Menge, and M. J. Mochizuki. 2008. Survival of plant pathogens in static piles of ground green waste. Phytopathology 98: 574-554.

Chalker-Scott, L. and A. J. Downer. 2022. Garden Myth-Busting for Extension Educators:The Science Behind the Use of Arborist Wood Chips as Landscape Mulches. Journal of the NACAA 15(2).

Published by

Jim Downer

Dr. Downer has 34 years of experience as a horticulture and plant pathology Advisor with the University of California Cooperative Extension in Ventura County. Dr. Downer’s academic training is from California Polytechnic Univ., Pomona, (BSc. horticulture & botany, 1981; MSc. Biology, 1983;. In 1998 he earned a Ph.D. in plant pathology, from University of California, Riverside. Dr. Downer’s research is focused on mulch, soil microbiology and disease suppression in mulched soils, diseases of shade trees and cultural practices to maintain landscape plants. Dr. Downer is a member of the American Society of Horticultural Science, the American Phytopathological Society, The International Soc. of Arboriculture, and the Western Chapter of the ISA, and the International Society for Horticultural Science. Dr. Downer is an Adjunct professor at California Polytechnic University in Pomona. Dr. Downer serves on the Board of the John Britton Fund for tree Research as the chair of the research advisory committee, and currently chairs the regional conference committee for WCISA. Dr. Downer has a love of shade trees, Shinrin roku (forest bathing/walking) tree work, wood working, horses, gardening, horticulture and the study of plants and their biology.

24 thoughts on “The Yin Yang of Compost”

  1. We never add weeds to the compost bin. We have a separate yard waste bin. We have a problem with the right % of paper (whatever). What are your suggestions?

    1. Generally I don’t put much paper or cardboard in my compost. I put it out to recycling. There are microplastics in cardboard that I object to in my garden. If you are doing a lot of paper you will need to add nitrogen. Drink more coffee! The grounds are high in N.

      1. starbucks and other high volume coffee shops will happily give you lots of used coffee grounds. Become a friendly and regular customer and strike up a relationship with the branch manager, and they soon give you loads of the stuff for free!!

  2. I can see why Prof. Downer thinks he is “constantly slaying horticultural dragons.” Much of this article is in direct opposition to at least 50 years of a great volume of published work on the benefits of composting. I have composted kitchen and garden “waste” for at least 40 years on a fairly large scale for a home gardener. I use the compost on site in my own vegetable and flower gardens. My own anecdotal experience is that composting has greatly benefitted my local environment. All the CO2 that my compost has released into the atmosphere had been in that same atmosphere just a year or two earlier before the photosynthesis going on around me had temporarily sequestered it in my vegetables and flowers, so that is a zero sum change. The dragon lives on. I think part of the issue in trying to say that compost does not benefit the environment is the good professor’s limited, perhaps misguided, operating opinion or definition of what benefitting the environment includes.

    1. What do you consider to be “published work?” Is it in scientific journals? There are a lot of authors with a lot of opinions, but our blog focuses on peer-revivewed information. Past that, I’ll let Dr. Downer (who has a PhD in plant pathology and has published articles on composts) discuss his “limited or misguided” opinions on the topic.

      1. Everyone is entitled their own educated opinion. I happen to agree with most of what Dr. Downer says. His work makes good common sense. He didn’t say composting has no benefits environmentally, but perhaps not the benefits many people believe. We need to understand the process to make a good judgement on composting’s benefits and Dr. Downer lends a great deal to that understanding.

  3. We have been experimenting with different forms of composting. We have small pile that we add vegetable and other plant materials (primarily kitchen scraps and coffee grounds) that seems to break down relatively effectively over time. We also have a worm bin that seems to work really well in breaking down materials and providing us with a usable byproduct (compost?). We have heard that by composting our food scraps, that we are diverting food waste from going to landfills that would otherwise transform into methane gas if left sealed up in trash bags or buried under trash (can you please confirm or dispel this notion). We also have started a windrow pile that is much larger where we are putting virtually all of our other landscape plant debris in an attempt to get it to break down as we do not have access to a green waste pick-up service nor do we want to use fuel to haul the green waste to a local landfill. This article leaves me with the impression that composting may not be the best solution for the garden nor the environment. What are your recommendations for handling food scraps and other large scale plant debris that would better serve the environment and the garden?

    1. Shred and apply mulch. The shredding will require fuel so some carbon used there. Food wastes should always be composted due to the hazardous nature of food waste. The main point is don’t compost your tree trimmings spread them out as mulch after chipping

  4. One has to question an author who start a topic with the idea that most content on the subject is in some form misinformation. This implies they are an expert on the subject and what they are about to present isn’t possibly misinformation.

    To be very clear, composting is a natural system, one that has been cultivated by humans to function in specific ways for specific purposes. This does not remove it from nature and sometimes we need to be reminded we as humans are a part of the natural systems we live. In fact we come out of nature.

    The more we use the systems nature provided us, such as composting, the better off all living organism will be – composting is a nature-based solution to many of the problems we face.

    Composting is an art just as fermenting food is. It provides for our well-being. Composting is a method of recycling the good nutrients via microbes back into the system hence we and all other organic matter comes. The more we use the systems nature provided us the better off we’ll all be and there is no time to lose, as most of us now realize.

    Question those who claim to be experts and where they come from. Do they really know the topics they are going on about or do they just see themselves as knowledge gods because they have some letters behind their names.

    Composting is a natural system and it works very well if managed with some basic understanding of the functions at hand.

    1. As I mentioned to an earlier skeptic, Dr. Downer has a PhD in plant pathology and has published extensively on many topics, including compost.

      Composting is NOT a natural process. It is a human-driven activity that does not occur in nature. To quote another university site, “composting is a method of speeding natural decomposition under controlled conditions.” Just because we are all “part of nature” does not automatically make everything we do natural. That should be quite obvious by the current state of our natural environment.

    2. Dr. Downer did all of what you are saying. And his credentials are top notch. I would call him an expert. I fail to see your point. Also, you mention composting to be the answer to many of our problems but you fail to mention any.

    3. What part do you feel is incorrect? When someone corrects your own mistake don’t call it an error, say thank you instead. If you’re certain of an error provide conflicting data to support the position. Tantrums and thinly veiled ad hominem attacks weakens your credibility.

    4. The decomposition of dead organic matter is a natural process; we call it “rotting”. The creation of compost is a skill or craft, or even an art, as you say: An activity requiring human agency, that makes use of natural processes to create something that is similar to, but different from, what we find in nature. In the same way that a bottle of Morgon is similar to, but different from, a bunch of moldy grapes left on the vine.

  5. Veronica Alice; exactly where in nature is there a combination of green plant material mixed in with ‘browns’? There certainly is slow decompostion of leaf litter; but where ‘naturally’ are there piles of green grass clippings, left over vegitable matter, pruning trimmings, etc… those are human created; and they change the decomposition of natural dead plant material into a very hot pile that decomposes much more rapidly.

      1. I delete argumentative comments that provide no value. If you wish to have a rational discussion on the topic, then provide objective information to discuss. Not opinions, anecdotes, or attacks on the author.

  6. I found Dr. Downer’s explanation of the respiration process that occurs in a compost pile, and the subsequent release of carbon into the atmosphere, most enlightening. But this blog gives rise to a few questions:
    1. Looking at it from an environmental protection perspective, is there an argument to be made that slow or cold composting will actually contribute less to global climate change than fast or hot composting?
    2. Also, Dr. Downer maintains that “There is nothing about composting that is helping the environment per se.” But when applying a total system analysis, is it not better for the environment for property owners to maintain a home compost pile for their food and yard waste? Home composting eliminates the carbon footprint associated with transporting that organic material to a landfill (or even worse an incinerator)? And then once at a landfill, anerobic decomposition results in a significant release of methane.
    The only other alternative would be to allow yard waste to decompose “naturally” but even then, there would still be carbon released to the atmosphere, albeit at a lower rate than what would be released from a hot compost pile.

    1. 1.) Yes the argument is that some of the carbon will make into the soil and will slowly be converted by fungi into soil carbon. That does not happen in compost piles most of the carbon is lost as carbon dioxide
      2.) Yes we need to reduce the amount of waste to landfills. This is a good idea and a reason for home composting of food wastes. What I object to is the industrial composting of food wastes which will release more carbon dioxide faster and possible other gasses if the piles are large.

      1. It was not clear from this article that your objection is to industrial composting compared to home composting. It would be great if you could write another article clarifying this. I would love to be able to share this with my gardening club and start getting people to follow the science!

        1. Industrial composting uses fuel to run machines; home gardeners do that with their labor.
          Home composting takes away the feedstock from the industrial process.
          Also the trucks required to take waste from homes adds to the the carbon expended into air.

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