by Donella Meadows

— October 26, 2000 —

Awhile ago I wrote about Dr. Jonathan Foley, a climate researcher at the University of Wisconsin, who is so worried about global warming that he has reorganized his life so he and his family don’t contribute to it.  No net carbon dioxide emissions.  That means no burning gas, oil, or coal.  Or, if there is any burning, there has to be a tree planted or a patch of prairie restored to re-absorb the carbon dioxide.

Now I discover that Jonathan Foley has been carrying on a correspondence with his brother David, a “green” architect in Maine, about whether David can rack up carbon-absorbing credit for his organic garden.  When David started gardening about 10 years ago, his soil tested just one percent organic matter.  Now it tests 7.7 percent.  That difference is made up of carbon, taken by plants from the atmosphere, now locked into the soil.

Jonathan, the numbers man, went to work estimating David’s carbon credits.  Here, for you gardeners who want to quantify your own contribution to the climate, is how he went about it.  The quotes are from Jon’s emails to David.  I’ve translated the units from his proper scientific metrics back to the crazy American system we all understand.

“The biggest uncertainty relates to how deep the organic matter is going into the soil.  I assume that the change in soil organic matter (SOM) is confined to the top 8 inches.  But I suspect that you’re leaching humus into the deeper soil, which would affect the result a lot.  So this is a conservative estimate.”

Area: 0.4 acres * (1 ha / 2.47 acres) = 0.162 hectares

0.162 hectares * (10,000 m2 / 1 ha) = 1620 m2 (about 40 meters on a side)

Soil Volume: density of a silt-loam soil is about 1.35 g/cm3, or 1350 kg/m3

if you assume that your soil organic test is representative of the top 20 cm of soil, then you have 1350 kg/m3 * 0.20 m = 270 kg of soil (per m2) effective soil mass that is being affected

if your soil started out at 1% (by weight) SOM, then this is 2.7 kg-SOM/m2 (in the top 20 cm)

if your soil is now at 7.7% (by weight) SOM, then you now have 20.7 kg-SOM/m2 (in the top 20 cm)

soil organic matter is roughly 58% carbon (and about 3-6% nitrogen) so

1.0% SOM =  2.7 kg-SOM/m2 = 1.56 kg-C/m2 (in the top 20 cm)

7.7% SOM = 20.7 kg-SOM/m2 = 12.0 kg-C/m2 (in the top 20 cm)

the difference is +10.44 kg-C/m2

10.44 kg-C/m2 * 1620 m2 = 16,912.8 kg-C = +16.9 tons of carbon

That is the mass of sequestered carbon (C); to get the equivalent mass of carbon dioxide (CO2) just multiply by 44/12 (the ratio of atomic masses of CO2 to C).

16.9 tons carbon sequestered = 61.97 tons of CO2 sequestered (remember these are metric tons!)

“You have easily sequestered 16.9 tons of carbon into your garden over the last 10 years.  If you think that the soil test is more representative of a deeper soil profile (let’s say 16 inches instead of 8), then scale that number up.”

“This is impressive!  The average American releases 5.5 to 6.0 tons of carbon into the atmosphere each year.  So you have offset about 3 years of an average American’s emissions.”

David is no average American, given his self-designed energy-efficient house and frugal consumption habits.  I’m willing to give him credit for offsetting all his carbon emissions over the past ten years.  But, I pointed out, he’s not likely to get his garden soil much richer in humus.  What’s he going to do to offset the next ten years?

He replied, “It’s true that we’ve ‘shot our wad’ and only can earn a one-time credit for tree-planting or soil organic matter.  But if everyone would do that, it would give us a great breathing space to make the transition to sustainable energy sources.”

“What I like about building up soil organic matter as a carbon-sequestering strategy is its sneakiness.  Imagine farmers being able to supplement their incomes by farming the way they should.  Imagine a transfer of income from CO2-emitting corporations back to farmers.  Wow!”

“And, of course, the soil organic matter we’ve built up helps the garden grow wonderfully and holds onto water to help us get through droughts.”

(If you’d like to reply to David or Jonathan, their email addresses are, respectively, Hollandfoley@acadia.net and jfoley@facstaff.wisc.edu.)

Copyright Sustainability Institute 2000