In the Dietary Guidelines for Soil, Pass the Carbon, S'il vous plaît

January 20, 2016 | 2:55 pm
Marcia DeLonge
Former Contributor

Between New Year’s resolutions and the recent release of the US Dietary Guidelines, a lot of us have healthy foods on our minds. In that spirit, I’d like to give a shout out to the real hero behind healthy food: soil. Although eating dirt may not be trendy, there’s more and more evidence that healthy soils actually do produce healthy humans.


Building healthier soils affects a lot of life, because in just 2.5 cups of soil (only as much volume as your recommended daily vegetable intake) there can be over 500 billion organisms (compare to the relatively measly US population of less than 325 million). Science indicates that there is a lot to gain from feeding this population well. So do the French, for that matter. When it comes to diets, I’ll always listen to their two cents (ahem, centimes).

So, what’s on the menu?

1. More low C, less high C

Carbon (the big C on the Periodic Table of Elements) may more likely conjure up images of diamonds and pencils (thanks, Chemistry 101) than healthy soils. Even for me, this simple symbol has often triggered more visions of carbon copies or radiocarbon dating than anything else. Yet earthworms, farmers, and I are all rejoicing in the fact that the carbon in soils looks quite a bit different, and so should you.

The carbon pool party is in the soil. The more carbon below our feet, the better. Soil carbon is essentially code for soil organic matter, because carbon makes up the majority (about 58%) of this critical part of “healthy soil” (where it helps to grow both your nutritious dinner and the jeans you resolved to fit in). There is actually already a lot more carbon in soils than anywhere else (as we scientists say, it’s “the biggest pool of carbon on the planet”).

Scientists estimate that the top 1 meter of soil contains approximately 2300 gigatons of the world’s carbon. Compare this to the 824 gigatons in the atmosphere or the 620 in “life on earth” (the “biotic pool”) and you’ll start to realize why soils are so important. (What’s a gigaton? Over 100 million elephants or 6 million blue whales…so, it is a lot of mass).

Given how much carbon soils already have, it may seem greedy (or perhaps unrealistic?) to ask for more. In reality, there is lots of room for more—especially in the millions of acres that we have degraded by causing carbon loss over many years. Estimates indicate that 24% of the world’s soils—including about half of all agricultural soils—are degraded, many of them depleted in soil carbon (this, by the way, can be an expensive problem).

Don’t forget your straw. Moving more carbon into soils means storing less in the atmosphere, mitigating climate change, and better health for all—talk about a balanced diet! But if you’re wondering how carbon gets from one pool to the other, remember that plants suck in CO2 and use it to create roots, stems, leaves, and produce. Some of this carbon, naturally, ends up in our soils, until the plants, you, animals, microbes, and I work on turning it back into CO2. This give-and-take is continuous—24 hours a day and 365 days a year. When things are running smoothly, the give balances the take. Today, with so much C in the atmosphere and too little in the soil, it is time to nudge the system. But how?

2. Soil humus, better hummus, repeat

Farmers and scientists know that it is possible to manage soils to increase soil organic carbon (just one form of which is known as humus), by means of many practices I have mentioned before in this blog (including cover cropping, well-managed grazing, and other agroecological practices). One way to build soil organic matter is to rotate crops, incorporating leguminous crops like chickpeas and lentils onto farms. Legumes, in addition to producing loose seeds in pods, tend to form a mutually beneficial relationship with soil bacteria that can draw nitrogen from the atmosphere, saving on the need for external fertilizer inputs. In turn, these crops and practices can build soils, leading to more resilient and healthy soils for future seasons. Since it is officially the start of the International Year of the Pulses (which are edible legume seeds, like lentils, chickpeas, pinto beans, black beans, and more), there seems like no better time to “spill the beans” about these great tricks of the healthy soil trade.

3. Carry more water

Bonus: soils stashing more carbon hold their drinks better, too, which means happier plants (and farmers) for more of the season, rain or shine. This favorable outcome is due to the known relationship between increased carbon or increased organic matter and water available for plants in the soil.

A high-performance diet to fend off climate change?

Remember the old adage about how an apple a day keeps the doctor away? A recently recommended diet for soils has a similar spirit, though it’s not quite as catchy. 4 per thousand, referring to an annual percentage growth rate of soil carbon stock to strive for, has been proposed as a prescription to prevent continued risky increases in atmospheric CO2. Okay, so it doesn’t roll off your tongue and a lot remains to be worked out, but the core idea is on track—a small and consistent change, scaled up over space and time, can make a big difference to soils and farmers and diners worldwide.

Just look at it this way. If you had a whale in your bathtub, you might want to find a way to move it on over into the great blue sea—even if you heard rumors that the sea was a little busy. Doing so might just be a small change for the sea, but a really big deal for your bathtub.

No magic diet pill, but plenty of recipes for success

Yes, folks, healthier, carbon-rich soils are a key to a better future. It may not be the end-all be-all, but making soils healthier is an important piece of the puzzle. If you don’t want to take it from me, try this beautiful video from the Center for Food Safety, narrated by Michael Pollan.