What makes your compost tick?

by Catherine Haug

After our discussion at the screening of our Root Cellars, Gardens & Greenhouses Slideshow, I’ve felt inspired to write a bit about composting theory.  This article includes the following main topics:Funk's compost bins

  • Five necessities of compost
  • Compost microorganisms
  • Stages of compost
  • Killing the pathogens
  • Other Issues:  Turn? Inoculate? Add lime or wood ashes? What shouldn’t be added to compost?
  • Toxic Chemicals; Pharmaceuticals
  • Troubleshooting
  • Sources

While my main source of information is  The Humanure Handbook by Joseph Jenkins, the basic theory applies whether your compost contains humanure or not.

Perhaps the scariest thing about humanure is the potential to pass on human pathogens through use of the compost in our gardens.  This issue has come up at several Olduvai discussions, and also at the ESP screening of our Root Cellars, Greenhouses & Gardens slideshow.

While it is painful to do so, we must admit that we are not immortal, that we will all succumb to something at some point.  And indeed, if we are truly concerned about human overpopulation, we must allow for the natural end to happen more frequently.

But I digress.  The point I want to make is that humans have unwittingly composted our wastes over thousands of years prior to the advent of the indoor toilet and sewer system.  Many cultures, such as the long-lived Hunzas, have actively composted their human wastes for hundreds of years.  And yes, sometimes we passed on scary pathogens, and people died.  This is nature, after all.

That said, even if we don’t consider composting humanure, we can still discuss the theory of composting in general, and come to a better understanding about how proper composting of kitchen scraps and yard waste can kill any pathogens and weed seeds. (And how the addition of manure, human or otherwise, improves the process).

Five Necessities of Compost

According to The Humanure Handbook (1), there are four essentials needed to create good, healthy compost. To these, I add a fifth essential.

  1. Moisture
  2. Oxygen
  3. Temperature
  4. Carbon / Nitrogen Balance
  5. Size.

Lets look at each of these in more depth.


It is more likely that you will need to add moisture to your compost, than curb the leakage of moisture from the pile.  This is because as the pile heats up from the action of microbes (more on this later), moisture is lost to evaporation.

Optimal moisture content is 50 – 60%; compost will stop working if moisture falls below 30% (2).  The amount of moisture needed to attain this optimum depends on the pile’s composition; but on the average, about 200 – 300 gallons of moisture is required per cubic yard of finished compost, per year. Most, if not all, of this requirement comes from rainfall, and moisture in kitchen scraps; and also urine, if you add humanure (1).

Collected graywater or rainwater are excellent sources of added moisture, if more is needed.


The desirable microbes that work on your compost are aerobic, which means they need oxygen to survive and generate the heat needed to fully compost the materials. Adding sticks and other coarse matter to your pile allows for oxygen to circulate through the pile.  Stirring a bit with a broom handle also helps (see ESP post Composting: Kitchen & Yard Scraps, by Sally Janover, for more on this.)

Anaerobic bacteria (these work in absence of oxygen) can also create compost, but it takes longer, and they produce a definite “stink.” If you add something to your pile that does not smell good, immediately cover it with a clean, organic, non-smelly material (see below). This is important with any compost that contains rotted food or manure, including humanure. Appropriate cover materials (1) for a:

Composting toilet:  sawdust, peat moss, leaves, rice hulls, and coco coir (coconut shell material).

Compost pile: include all of the above, plus straw, weeds, hay, and other bulky materials that can help trap oxygen in the pile.

If your pile devolops a definite stink, try covering with any of the materials listed above.


Compost piles will not work if frozen (1).  That is, they rest in a dormant stage until ambient temperatures warm sufficiently to thaw and warm the pile enough to re-activate the composting microbes.  It helps to cover your pile with natural insulating material such as straw, when temperatures dip below freezing.

My compost pile froze pretty solid this winter, even though it had a good straw cover.  However, I was able to insert a turning fork into it in March.  Much of it was still frozen, but in the very center, it was quite warm, about room temperature!

Dehydration, caused by heat generated in the pile, with insufficient moisture added back, will also cause the pile to stop working.  The addition of moisture will restart it again (1)

Carbon / Nitrogen balance

This is probably the trickiest part of composting because it is controlled by the mixture of materials added to the pile (1,2).

  • Carbon sources: plant cellulose materials such as:  cardboard, shredded paper and newspaper, rotted sawdust (best from a sawmill rather than lumberyard), corn cobs, wheat straw, rice hulls, and bark.
  • Nitrogen sources:  manure, grass clippings and hay.
  • Kitchen scraps are typically C:N balanced.

The optimal C:N ratio is about 30:1, to minimize nitrogen loss. If the ratio is less than 20:1 due to increased nitrogen content, the microbes will not be able to use all the nitrogen, and it is lost as gas. If you can smell ammonia from your pile, you’re adding too much nitrogen (1,2). Because it is very energy intensive to fix the nitrogen in ammonia, it is not desirable to lose it in this way.

Browns & Greens

A simplistic rule of thumb is that “browns” are carbon-contributors; “greens” contribute nitrogen.  A mix of 3 parts brown to 1 part green is the optimal recommendation.

This can be hard to control, however. Often, the compost pile is not very active due to insufficient nitrogen and moisture.  The addition of humanure resolves this problem (if urine is collected with the solid waste).(1)  Animal manures typically do not contain urine.


Guerrant Compost bins-hens_smlA pile that is too small (or not sufficiently insulated) will not generate enough heat to reach the thermophilic stage. One that is too large may hold too much water, and not allow air into the center, creating an anaerobic environment. Air naturally penetrates 18 to 24 inches into a pile from all directions (4).

Optimal size (4):

  • no smaller than 3 feet X 3 feet X 3 feet, and
  • no larger than 5 feet X 5 feet X 5 feet.

Compost Microorganisms

Microbe categories

Biologists have many ways of categorizing microbes in compost, such as bacteria, actinomycetes, and fungi. Another important categorizing criteria is the temperature at which the microbes thrive (1):

  • Psychrophiles’ optimum temperature is 59° F or lower;
  • Mesophiles live at medium temps, 68° – 113° F, and includes E. coli and other intestinal bacteria; human pathogens are also mesophiles.
  • Thermophiles thrive above 113° F.

It’s the thermophiles who are responsible for the heat in an active compost pile, and this heat is important for the destruction of pathogens and weed seeds.  If your compost doesn’t reach this stage, something is wrong.

Stages of compost

These are defined by the temperature of the pile and hence the microbes that are most active.  As the microbes digest the carbohydrates and proteins in your pile, heat is released. The stages are (1):

  • Mesophilic stage:  compost mass rises to 111° F as mesophilic bacteria work on the easily digested sugars and proteins;
  • Thermophilic stage: thermophilic bacteria begin to take over and temperatures optimize between 111° and 125° F.  This phase doesn’t last long; temperatures will drop when the readily-available nutrients have all been digested.
  • Cooling stage: the temperatures moderate and mesophilic fungi take over to digest the hard stuff.  This can take some time.
  • Curing stage: the longest (and often skipped) phase, during which remaining pathogens lose the competition battle for the remaining nutrients, and die off.  A long curing period, at least a year after the thermophilic stage, ensures that your compost produces no phytotoxins that are harmful to plants.

Many factors determine which microbes get the upper hand, and the progression through the stages:  competition for nutrients, inhibition and antagonism between microbes, consumption of other compost organisms, antimicrobials produced by compost organisms, and biological heat generated by compost organisms.

Killing the pathogens

A temperature of 122° F maintained for 24 hours should kill all known pathogens; a temperature of 115° F will take at least a week to accomplish this.  Many researchers have demonstrated that maintaining composting sewage sludge in a temperature range of  116° – 130° F for 3 days will kill all polio virus, salmonella, roundworm eggs, and Candida albicans (1).

However, as mentioned above, it isn’t just the heat that gets the pathogens, but several other factors.  Don’t fall to the temptation of trying to achieve the highest possible temperatures, as this may backfire and kill the desirable composting microbes.

Other Issues

To turn or not to turn?

Early modern advocates of composting assert that turning the compost is essential for proper mixing, heating, and aeration of the components. Rodale recommends turning three times in the first few months, and three times a year thereafter, for a year.

But Joseph Jenkins believes otherwise; especially for home composters, who add to their pile daily (as opposed to commercial composters who compost an entire batch at a time).  Aeration can be also accomplished by (1):

  • optimizing the size of the pile (see above);
  • poking holes in the pile by driving pipes (or broom handles) into it; or
  • use of twigs and other coarse material (straw, hay, weeds) in the pile.

It is this later method which he espouses: proper construction of the pile by layering coarse material between softer materials, and by using a bin that is well ventilated (using mesh or leaving gaps between boards).  Such a pile does not need turning.

He asserts (1):

[Daily additions to the pile] “keep the thermophilic stage near the top; the thermophilically ‘spent’ part sinks lower and lower, to be worked on by fungi, actinomycetes, earthworms and lots of other things. Turning [this type of] compost dilutes the thermophilic layer with the spent layer, and can quite abruptly stop all thermophilic activity… and lead to loss of agricultural nutrients.”

It’s important to remember, here, that it is the thermophilic activity that is responsible for killing pathogens.

Refer also to the Garden Web Forum (5) for an interesting discussion of this issue.


There’s probably no harm in this practice, but it’s a cost you don’t need to spend. The microbes that work on your pile are present everywhere:  on the foods you harvest and the leftovers you toss on the pile; in the soil upon (or in) which you build your pile; and even in the air you breathe. And, incredibly, in your gut.  In fact, your life and health depend on those microbes in your gut.

Researchers reported in the Journal of Composting and Recycling, October 1998, page 52 [per The Humanure Handbook (1)]:

“No data in the literature indicate that the addition of inoculants, microbes or enzymes accelerate the compost process.”

Add lime or wood ashes?

Lime (or wood ash) is added to soil to increase its pH; that is, to decrease the acidity. It is not necessary to add it to compost. Properly aged compost is not acidic, even with the use of sawdust. In fact, its pH should be just on the alkaline side of neutral.(1)

Thermophilic composting occurs in a pH range of 7.5 to 8.5, which is slightly alkaline.  Your compost may start out slightly acidic, but the microbes will quickly change that, if you compost properly. (1)

Lime has the potential to kill microbes in your pile, which would not be good.  In fact, it is added to sewage sludge to stabilize the sludge by killing microbes (7). Another reason not to add lime is that in the presence of organic acids, lime tends to drive of nitrogen, reducing the nitrogen value of your compost (6).

If your soil is too acidic, add the lime or wood ash directly to the soil, not the compost. That way, you have more control over how much lime is added, and where. (1)

What shouldn’t be added to compost?

Many compost ‘experts’ say that you should not add meat or animal products to your compost.  Well, if you have a problem with bears or other invaders, then this is perhaps good advice.  But not because of the effect of animal products on the compost.

Similarly, oily products are often listed as no-nos, including oils and fats, peanut butter, mayonnaise, salad dressing, sour cream, and other dairy products.  This is because these foods require thermophilic conditions in order to break down.  So if your compost does not reach the thermophilic phase, you could avoid adding these items. (1)

However, it would be better to explore WHY your compost is not thermophilic, because otherwise your compost will take much longer to complete. In fact, adding these fatty foods can actually help thermophilic conditions to prevail. (1)

Bones do not compost very well, but they do no harm in the compost, especially if they are broken up.  However, I prefer to make bone broths with my chicken or beef bones before composting them.

Manures are often on the ‘banned list,’ for the same reasons that fatty foods are listed. But most people recognize the value of composting manure with other waste materials, because manure and other nitrogen-rich wastes help your pile reach the thermophilic phase. (1)

There is one category that definitely should be excluded from your compost:  toxic substances.  For example, sawdust from pressure treated lumber, which contains toxic heavy metals; and vegetation that has been sprayed with pesticides or herbicides.

Toxic Chemicals

Compost can degrade some toxic chemicals, so if you’re concerned about adding humanure if you take pharmaceuticals, you may be able to rest easy.  The compost microbes, especially the fungi, break down the chemicals into simpler, benign, organic molecules.  This includes the following toxins (1):

  • gasoline & diesel fuel
  • automotive oil
  • grease
  • wood preservatives
  • PCBs
  • insecticides and herbicides
  • some pharmaceuticals

Compost also seems to bind (chelate) metals to prevent their uptake by plants and animals.  For example, one researcher fed lead-laced soil to rats, some with compost added and some without. The soil with compost produced no ill effects, but the soil without compost did produce some toxic effects (1).


Pharmaceutical drugs are a huge class of potential toxins in soil and compost, especially if you use manure from animals on feed that contains antibiotics, or if you plan to compost your humanure.

Recent research has shown that compost will indeed break down antibiotics. (8,9)  For example, “a study using Oxytetracychne (OTC) – a broad-spectrum antibiotic used in livestock production. The OTC did not affect the composting process and within the first six days of composting, showed a 95% reduction.” (8)

And here’s more, from the San Francisco Chronicle, by Deborah K. Rich (10):

“A recent study of composting examined its ability to break down 10 pharmaceutical and personal-care product residues in biosolids collected from a wastewater treatment plant in San Diego. Fatih Büyüksönmez, of San Diego State University’s department of civil and environmental engineering, found that composting for 45 days reduced residues of 9 out of 10 products by at least 85 percent.

Hormone residue in sewage and animal waste appears to be similarly reduced by the composting process; they are natural compounds and highly susceptible to microbial degradation.”

Time is a big factor in how much of the drugs are broken down.  This could be an issue for commercial composters, who need to clear their lots for more compostable materials in short order.  But for home composters who allow time for proper curing of the pile, this should not be a problem.

Certainly, you have a better chance of breaking down pharmaceuticals through compost, than through sewer sludge treatment!  That is, flushing your drugs or drug-infected human waste down the toilet will merely introduce them to the water supply.  Adding them to your compost pile will at least partially break them down.

Troubleshooting Compost Problems

Refer to Table 4-1 of the Cornell University site (2), for solutions to the following problems:

  • Pile is wet and smells like a mixture of vinegar, rancid butter, and rotten eggs;
  • Pile does not heat up;
  • Pile is attracting animals.


  1. The Humanure Handbook by Joseph Jenkins (3rd edition)
  2. Cornell University:  www.css.cornell.edu/compost/CIC.html, especially Chapter 3: cwmi.css.cornell.edu/chapter3.pdf & Chapter 4:  cwmi.css.cornell.edu/chapter4.pdf
  3. AgroEcology.org:  www.agroecology.org/Case%20Studies/humanmanure.html
  4. University of Illinois Extension: web.extension.uiuc.edu/homecompost/building.html
  5. Garden Web Forum: How often do you turn your compost? forums2.gardenweb.com/forums/load/organic/msg1214252411790.html
  6. North Carolina State University Cooperative Extension’s Master Gardener Decision Support Guide: /www.ces.ncsu.edu/cabarrus/staff/dgoforth/limefaq.html
  7. PennLive.com: Lime in the Compost, by George Weigel  blog.pennlive.com/gardening/2008/03/lime_in_the_compost.html
  8. Joseph Jenkins Publishing message board: www.jenkinspublishing.com/messages/messages/1366/2167.html?1237408114
  9. Journal of Environmental Quality, May 1, 2008; 37:1245-1253; Dollivera, Guptan and Noll:  jeq.scijournals.org/cgi/content/abstract/37/3/1245
  10. San Francisco Chronicle, by Deborah K. Rich, May 5, 2007:  Questioning the Compost Supply Chain; www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/05/05/HOGD3PJOUT1.DTL

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