Putting The “Carbon Footprint” of Farming in Perspective

no-till corn


When thinking about “carbon footprints” it helps to have real numbers to put things in perspective. The EPA estimates that for the US, agriculture represents about 8% of total human-related greenhouse gas emissions. The following is a list with a little of the detail of what makes up the footprint of an acre of a rain-fed Midwestern corn crop with a few other things thrown in for comparison. Since we grow 80-90 million acres of corn its something that matters. The values are all expressed as pounds of CO2 equivalents. If you want Carbon equivalents multiply by 12/44

Some “Carbon Footprints”

  • embedded carbon in the seed treatment chemicals that help get the corn growing strong – 0.56
  • Embedded carbon in a fungicide sprayΒ  – 2.7
  • The gas to drive 2 miles each way to the grocery store for a car that gets 20 mpg – 3.9
  • Running a 2-stroke lawn mower for an hour – 6.1
  • Fuel to drive a sprayer over an acre to make a pesticide application – 7.3
  • Fuel to drive the no-till planter in the spring – 31
  • All the embedded carbon for a pest control program on the acre that would really optimize corn yield – 52
  • Fuel to run the combine over the acre at harvest – 59
  • Fuel for all the farming operations on that acre if it is no-till – 169
  • Fuel for all the farming operations if it is conventional till – 231
  • Embedded carbon in a synthetic fertilizer program (120 lbs N, 40 lbs P, 35 lbs K) – 502
  • Nitrous oxide emissions if 1.5% of the applied nitrogen gets converted to that gas (fairly typical range) – 823
  • Fuel to harvest a processing tomato crop (for a vegetable crop comparison) – 1,348
  • The total carbon footprint of an acre of no-till corn not counting carbon sequestration – 1,829
  • Methane emissions from stored manure for 120 lbs of N (1.5% conversion) for an acre of Organic corn – 1,936
  • All the fuel to grow an acre of the processing tomato crop – 3,627
  • The EPA estimated average annual household carbon footprint – 22,880

In the acre of no-till corn was also “cover-cropped” on the order of 1,100 lbs of carbon dioxide could have been removed from the atmosphere. This sort of corn might produce 225 bushels/acre so the carbon footprint if farmed with conventional tillage comes out to 0.15 lbs CO2/lb corn. In the no-till with cover crop scenario that is dropped to 0.059 lbs CO2/lb corn. If this isn’t something you’ve seen before I would be interested to know what did or didn’t fit your expectations about the relative “footprint” of different farming elements.

Cornfield Photo by 1 Riverrat

  1. russ

    Well done Steve – it is pleasant to see someone writing who has an idea of what they are writing about. Too many just write green stuff with little or no thought involved.

  2. Steve Savage

    Thanks. There is a tendency for well-intentioned people to propagate “green” assumptions that they have heard that might actually not be quite true if you look into it

  3. Michael Bomford

    Very interesting. This post led me to your composting post (https://sustainablog.org/2009/07/27/an-inconvenient-truth-about-composting/) and to your calculations on the embedded carbon in fertilizers (http://www.scribd.com/doc/17356325/Carbon-Footprint-of-Organic-Fertilizer). I truly appreciate you showing your calculations and sources. I have worked through your calculations and they look solid. The paper by Hao et al. (2004) is useful and surprising. I’ll keep it on file.

    I do have some problems with your underlying assumptions. You make the case in your ‘carbon footprint of fertilizer’ calculations that greenhouse gas produced by storing or composting animal manure is the responsibility of organic (or conventional) farms that use composts and manures as fertilizers, not the livestock operations that produce the manure. I disagree.

    Your claim that manure is only stored or composted to satisfy demand for organic fertilizer does not reflect reality. The confined animal feedlot operations that produce most of our country’s animal products have a waste disposal problem. They store manure because they can’t spread it whenever they like (e.g. the soil is too wet, or frozen) and because their land base can’t support the quantities of manure they produce. They compost because composting reduces bulk, odors, leaching, runoff, and pathogen spread. Although compost is a saleable product, for-profit composting operations make most of their money on tipping fees, not on sales (http://www.p2pays.org/ref/12/11502.pdf). Compost sales often account for as little as 10% of facility operating costs. It is not unusual for compost to be given away for free. Most livestock producers would be thrilled if somebody offered to haul away their manure for free. The economics hardly support the notion that manure storage and composting is done solely to provide fertilizer.

    Most of the manure and compost applied as fertilizer does not end up on organic farms. Although organic farms frequently use manure and compost when it is available, they also rely heavily on other nutrient sources, such as nitrogen fixation by legumes. To hold organic farms that use manures or composts accountable for the greenhouse gas emissions of the livestock industry does not offer a realistic distribution for lifecycle assessment purposes.

    Organic livestock operations are less likely to stockpile waste manure because organic standards are designed to promote land-based animal production instead of confinement and concentration. The organic ideal is integrated crop and animal production systems that support cycling of resources such as carbon and nitrogen.

    I agree with your suggestion that anaerobic digestion could offer a more sustainable way of dealing with stored manure than composting. Not only is there potential to capture methane and generate heat or electricity, but byproducts of anaerobic digestion are rich in nitrogen and phosphorus, and could be useful fertilizers.

    Synthetic fixation of nitrogen accounts for about a third of agricultural energy use in the USA. The amount of nitrogen fixed synthetically each year is roughly equal to the amount that is fixed biologically, representing a tremendous anthropogenic impact on global nitrogen cycles. We see it in the form of air and water pollution. We need to find ways to promote nitrogen cycling, not more nitrogen fixation. Blaming organic farmers for turning a waste into a resource won’t help make farms more sustainable.

  4. Steve Savage

    Christopher asked where I get the numbers. There is a vast scientific literature on this subject. Use Google Scholar and put in “nitrous oxide” or “Carbon sequestration no-till” and you will see them by the score. I’ve digested about 600 of them. A good starting place is West and Marland 2002. It is in Agriculture, Ecosystems and Environment 91:217-232.

    1. Steve Savage

      Farmers are experiencing climate change. Pests are moving to regions where they could not over-winter before. Rain is shifting to spring from winter snows and leading to disease for California apples. Who knows what other changes are in store.

  5. Fancy Dress

    Farmers are very important to each and everyone of us. They are the ones who are responsible of the rice we eat and all farm products we eat so its very important to treat them nicely.

  6. Andrew S

    Very interesting Steve.
    I grow veg in my garden more or less organically and as I dig, try to do some math about my contribution or otherwise to the world!
    I had in mind a much greater carbon figure for transportation and mechanisation; its interesting to see how relatively little it seems to contribute. But for manure and foodwaste, surely you have to subtract the figure that equates to the decomposition of these things that would take place anyway? What do you think those offsets would be? Plus, one might need to consider the contribution of organic methods to soil flora and fauna and to local wildlife diversity – I can personally attest to those benefits.

    In addition, I find that my use of manure on previously manured (with charcoal and fish waste) drops greatly once the soil has reached a certain fertility level.

    But if it could be shown that manufactured NPK does not seriously damage soil structure or flora then I really would consider switching to it.

    1. Steve Savage

      The source of the NPK does not have much effect on soil organisms. The biggest issue is tillage. When soil is farmed “no-till” on a commercial scale the populations build just fine with synthetic NPK. If you think about it, there is no where in nature where something like adding tons/acre of manure occurs.

      1. Andrew S

        Hmm – but everything I put on my heap (manure from local cowshed, food waste, tree and shrub clippings, paper and grass) is something that would decompose in the cowshed, landfill or just on the soil surface. Not sure about the seaweed but it too would eventually rot. That would even include cover crops. Putting that decomposition to work to grow more food surely cannot be making a net contribution to greenhouse gases. We could certainly do with having less cows around to produce less manure in the first place though. And if we were instead to take all of that waste and digest it anaerobically, burning the resulting gases usefully that would certainly be better. Then we could use the effluent mixed with the charcoal.

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