An Inconvenient Truth About Composting Revisited

  • Published on November 21st, 2010

Although composting has a very strong “green” reputation, it isn’t without its environmental issues.  Compost is an important source of fertilizer for Organic crops, and is widely promoted as a “green” alternative to synthetic nitrogen.  The inconvenient truth is that, as a nitrogen fertilizer, compost has a carbon footprint more than 10 times as large as that for the synthetic nitrogen used in conventional farming.  It will never happen, but if a significant percentage of crops were ever to be fertilized with compost, it would be a very bad thing in terms of climate change.

In July of 2009 I posted an obscure document about this topic titled, “The Carbon Footprint of Organic Fertilizers” on Scribd.  It just passed 5000 reads, and that inspired me to write another post about the conversations I’ve been having on this topic for the last two years.

Why Such A Big Footprint?

The reason that compost-based nitrogen has such a big carbon footprint is because, during the composting process, micro-sites in the pile run out of oxygen because there is so much being consumed by the microbes.  Under those circumstances, other organisms make methane or nitrous oxide (21 and 295 times as potent as CO2 as a greenhouse gas).  Two to three percent of the carbon is emitted as methane even in a very well run, commercial-scale composting operation.  Because it takes many tons of compost to provide the fertilizer for an acre of a crop – the greenhouse gas contribution per acre is very large.  The conclusion from this is not that composting is a bad thing, but rather that it is definitely not an acceptable fertilizer alternative for the bulk of agriculture.

The Discussion

Since January of 2009 I’ve discussed this topic with dozens of qualified  academic scientists, with scientists that work for the Rodale Institute and the Soil Association in the UK, and with representatives of several Environmental Groups.  The basic conclusion has held up – “there really is a large carbon footprint associated with fertilizers that come from composting.”

Still, many interesting issues have been raised and need to be considered:

Whose Footprint Is This?

Manure is a major waste product of animal agriculture.  It has many environmental downsides, but it also provides the fertilizer for about 5% of US crops. Greenhouse gas emissions that come from “manure management” are certainly related to animal product production, but there are areas that pass the boundary. If manure needs to be composted to fit the USDA Organic rules and/or to be safe to apply to a food crop, then the emissions that occur during composting can be assigned to the farm that uses it.  This is no different than assigning farms the footprint for the energy-intensive manufacturing of synthetic nitrogen fertilizers.

What Would Have Happened To This Waste If It Wasn’t Composted?

This is an important question. If you compost your vegetable scraps instead of sending them to the landfill, you are way ahead even if small scale composting has some greenhouse gas emissions (I’ve never seen a study on this).  This is because far more methane would have been generated in the landfill.   If manure is not composted but simply stored for a while and spread on a non-Organic, non-food crop like field corn (this is the normal scenario), there are still some methane emissions during storage.  Thus it is fair to deduct that storage-associated level of emissions from the footprint of  composted manure (it still comes out larger than for synthetic nitrogen).

What Would Be The Best Use Of The Waste Stream?

“Waste is a terrible thing to waste.”  Whether it is manure or some other organic waste stream (yard waste, food scraps…), there is energy potential in every ton, and it often has more economic and environmental value as a renewable energy than as fertilizer .  There are technologies like anaerobic digestion or fast pyrolysis that could convert this waste to energy and offset fossil fuels.


Composting definitely has its legitimate place in our need to deal with wastes.  Compost can also be very good for building soil quality.  It just isn’t a good way to provide nitrogen for crops.

You are welcome to comment on this site or to email me at

Commercial compost image from Tie Guy II

About the Author

Born in Denver, now living near San Diego. Agricultural scientist for 30+ years with a Ph.D. in Plant Pathology. Have worked for Colorado State University, DuPont and Mycogen and for the last 13 years consulting for all sorts or companies, universities and grower groups. Experience in biological control, natural products, synthetic chemicals, genetics, GMOs and agronomic practices. Have given multiple invited talks on the interaction between agriculture and climate change (both ways)
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  • Similar to the lament of the cartoon character Augie Doggy to his father, Doggy Daddy, I must reply to your posting that, “You’re intentions are sincere but misguided”.

    Your major premise is based upon using compost as a singular nitrogen fertilizer source. Considering the unrealistically narrow argument, “that as a nitrogen fertilizer, compost has a carbon footprint more than 10 times as large as that for the synthetic nitrogen”, I concede, you are probably correct. However, if all that we consider in plant nutrition continues to focus exclusively or even primarily on nitrogen, we will all certainly lose more than a simple argument in the end. The similarity between inorganic nitrogen and compost is that manufactured nitrogen stimulates plant growth, as does compost. But the similarity ends there. The consequence is that compost builds topsoil, whereas repeated and overuse of manufactured nitrogen without the addition of organic matter destroys soil structure and its plant productivity by concentrating mineral salts where applied. As a result, additional mineral fertilizer is required year after year to get sustained production from crops. Furthermore, the highly volatile, water soluble, inorganic nitrogen provided by the most commonly used manufactured nitrogen sources (anhydrous ammonia, urea, ammonium nitrate, ammonium sulfate and water-based liquid nitrogen fertilizers) are only partially utilized by the plants to which they are applied. According to the USDA, estimated average annual per-acre nitrogen loss summed over all loss pathways for managed croplands ranges between 35 and 98 pounds of nitrogen loss per acre. ( Losses are from volatilization into the atmosphere, vertical leaching into ground water and lateral runoff into surface waters such as lakes, streams, rivers and bays.
    The nitrogen in compost is water insoluble, organic nitrogen, requiring soil microbes to break it down into a plant usable form. These microbes become active when soil temperatures rise above 54 degrees Fahrenheit – the same temperature that plants start actively growing. The microbes go dormant and nitrogen is preserved in the compost when temperatures drop below 54 degrees.
    The mechanisms by which plant growth is enhanced using compost as an amendment and critical nutrient source include the following. Compost:
    a) Improves soil structure and porosity – creating a better plant root environment;
    b) Increases moisture infiltration and permeability, and reduces bulk density of heavy soils – improving moisture infiltration rates and reducing erosion and runoff;
    c) Improves the moisture holding capacity of light soils – reducing water loss and nutrient leaching, and improving moisture retention;
    d) Improves the cation exchange capacity (CEC) of soils;
    e) Supplies organic matter;
    f) Aids the proliferation of soil microorganisms;
    g) Supplies beneficial microorganisms to soils and growing media;
    h) Encourages vigorous root growth;
    i) Allows plants to more effectively utilize nutrients, while reducing nutrient loss by leaching;
    j) Enables soils to retain nutrients longer;
    k) Contains humus – assisting in soil aggregation and making nutrients more available for plant uptake;
    l) Buffers soil pH.

    Compost cures a problem that inorganic nitrogen cannot. According to Cecil Adams on the website, Straight Dope (, Lester Brown of the Worldwatch Institute estimated in 1992 that 24 billion tons of cropland topsoil are lost annually worldwide, or 66 million tons per day. However, environmentalist and author Bruce Sundquist, reviewing the literature on the earth’s “carrying capacity” (, comes up with a net topsoil loss rate of 78 billion tons per year, or 215 million tons per day.
    I strongly suggest that anyone interested in the topic watch the documentary, “Dirt, The Movie” at

    The idea that compost is significantly different from inorganic fertilizers is not new; however, until recently, the unique properties of compost were overlooked and instead compost has been generally classified as a nutrient-poor “soil amendment”. While there is nutrient content in compost, its true value is not in the singular nutrient, nitrogen it supplies but in the multiple other benefits it provides – including making existing and supplemental nitrogen (as well as the other essential nutrients for plant growth) more available. The fact is that, in many states it is illegal to call compost a fertilizer or to make any nutrient claims.

    I would also point out that your conclusion is also flawed and short sighted, as it assumes an “either/or” scenario between anaerobic digestion (AD)/fast pyrolysis to produce energy versus composting to provide nitrogen to crops. In fact, neither AD nor pyrolysis is an ultimate solution for manure management as both processes result in a residual material that must be further handled and managed. Furthermore, in both of your selected processes, it is the carbon and hydrogen in various compounds, which are cleaved from the organic matter. The nitrogen is almost entirely untouched in the process and remains available in the resulting residual materials. The reality and most beneficial reuse of manures is to use the AD and then composting processes in sequential manner.

    In the case of anaerobic digestion, there is minimal pathogen reduction and no vector attraction (flies, vermin, birds, etc.) reduction, to say nothing of odor mitigation. The residue from AD needs further processing before it is used safely in the required quantities. Composting achieves both of these objectives and the compost can then be applied to croplands.

    So which is the best source of nitrogen – mineral fertilizer or compost? The answer is that the organic matter in compost enhance the uptake of nutrients – those existing in the soil and those applied by the farmer. When compost is applied and nutrient requirements are supplemented with mineral fertilizers, a synergistic effect is achieved and crops produce better than with either alone. Moreover, the planet benefits, as well.

    (Mr. Hill is the Manager of Legislative and Environmental Affairs for the US Composting Council)

    • Mr Hill,
      Many good points, but compost at the very best compost is a good solution for intensive crops like vegetables that cannot be no-tilled and cover-cropped (the other way to build all the soil benefits you describe without having to import the carbon from some other field. You can see from some of my other posts that I am not a supporter of business as usual for the vast areas of row crop farming.

      The reason I ever raised the GHG issue about compost is because of publications by the Soil Association and the Rodale Institute claiming that it was a major part of the solution to climate change. It is great for many things, but not for that.

      Thanks for the feedback

      • Wow what a miss-informed sentinment. Have you actually seen any of the data regarding this. Start with the largest report I know on the GHG impacts and benefits of compost:

        Then check out the protocols that are used to give Carbon offsets for composting (primarily at the Climate Action Reserve, but also at CCX and CDM), and the research done by Dr. Sally Brown at the University of Washington.

        Also take a look at and at the work done by California’s state agencies (the California Air Resources Board and CalRecycle).

        Based on what I’ve seen, composting is one of the most important and cost-effective greenhouse gas reduction measures.

        • Nick,
          All the papers you are citing are about the GHG advantages of Composting vs land-filling. That is not the issue I was addressing as I stated clearly. I’m talking about people who say that compost as a fertilizer (generally based on manures) is a better nitrogen source for climate change because it does not involve fossil energy. That is what is false. I have read dozens of papers about both topics. Compost is a good waste management method for things like municipal organic waste and green waste. It is not a good approach for manures that are to be used for fertilizer compared to anaerobic digestion or just using the manure to fertilize a non-food crop

  • Edward,
    In some settings you are right about the food scraps. The earthworm thing might not be so good because there are anaerobic bacteria in their guts that will generate nitrous oxide. I still think that fuel is the best fate for manures, either directly or after conversion to methane in a digester. What comes out of the digester is still a good soil conditioner

  • Instead of composting food waste, it should be fed to suitable animal(s) which we can then use as a food source. The manure can be collected for earthworms.