Sunday, November 21, 2010

An Inconvenient Truth About Composting Revisited

(here is the link to all my posts)
Although composting has a very strong “green” reputation, it isn’t without it’s 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:

Who’s 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 land fill.   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


  1. If the composting is done aerobically does it still emit greenhouse gasses?

  2. The more aerobic the compost, the less GHG emissions. If it is small scale, if it is actively aerated etc, it will have lower emissions. The challenge is that when a compost is really running "hot" (which is something you want to kill weed seeds, human pathogens etc) that means that it has a very high oxygen demand. It is very hard to supply that much oxygen when you consider that even a micro-site less than a millimeter across can be a site where methanogens can take over.

    I'm not saying composting is a bad idea. It is a far better fate for many waste streams than landfills etc. I'm just making the argument that this can't be the really large scale solution that we need


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