Soil: Science and Alchemy


We’ve heard it a thousand times from enthusiastic science teachers, “the soil is teeming with life!” And it’s true. Get on your hands and knees and stare at the apparently motionless soil and you will see ants building cities, worms mining for nutrients, and all manner of creepy crawlies going about their business as oblivious to your presence as you usually are to theirs. Get on your hands and knees with a powerful microscope and you will really begin to see the meaning of “teeming with life.” The interactions of organisms in the soil is so complex and so microscopic that many people, dubbed soil scientists, devote their whole lives to its study. Perfecting the alchemy of the soil is so challenging that farmers with healthy soil are often quoted as saying “I wouldn’t trade my land for a thousand acres of yours” or “I wouldn’t trade my soil for [insert extremely valuable thing here].”

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Even the desert sand is a type of soil. Here in Saudi Arabia they add water and start farming.

The four basic components of soil—mineral solids, water, air, and organic matter—are combined in any number of ways to give us the huge diversity of landscapes we see on earth, or even on a single farm. At Bellair we plant in soils ranging from the famous Virginian red clay to sandy loams, often within a single field. Living components—bacteria, fungi, nematodes, algae, and protozoa—navigate these different soil textures recycling essential nutrients as they go. Some bacteria release enzymes into the soil to degrade dead organic matter, consuming all the nutrients they can and leaving the rest for plant uptake. Other bacteria are famous for their bartering relationships with legumes, fixing nitrogen from the air and then trading it with the plant for sugars. Fungi also form important bartering systems with many plants, exchanging water and nutrients for sugars. Nematodes, Algae, and Protozoa ingest other microbes which eventually become available to other organisms as they decompose. Larger insects, earthworms, and millipedes also pull their weight by tilling the soil and making air available to the many life forms beneath the surface as they burrow and tunnel.  In this case too many cooks make the broth just right.

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Although an ant’s ‘entire world’ is usually much more biodiverse, ant farms do give us a sense of how bugs give the soil a good tilling.

While many microbes live their whole lives beneath the surface of the soil, many of them owe their life, or death, to the human beings above the surface.  Farmers have historically been the great (though often hapless) alchemists of the soil, irrigating to add water, tilling to add air and mix things up, and adding minerals and organic matter in the form of fertilizers. In conventional agriculture the recipe is simple; ‘clean’ the field by killing all microbial lifeforms then add the nutrients a plant needs in the form of synthetic inputs. In a sustainable agricultural system, the farmer instead seeks to “build” the soil without synthetic inputs by tinkering with the balance of microbial life.

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An old-time alchemist works the soil, cultivating sweet potato vines on the world’s first tractor.

You might ask, as many agronomists and farmers do, why bother trying to control such a complicated, multi-species, genus, family, class, phylum and kingdom system?  The truth is that, for all of our talk, soil microbes are much better than humans at building soil, having practiced at it for millennia.  We can synthesize Nitrogen from the air (using large amount of fossil fuels) and mine for Potassium and Phosphorus until they run out but we cannot mimic the breakdown of complex molecules that happens in healthy soils; we cannot make humus without the help of soil microbes.  All farmers’ fertilizer recipes call for the limiting nutrients for plant growth, Nitrogen, Phosphorus, and Potassium but sustainable farmers are also concerned with making sure these nutrients ‘stick’ and don’t run off into the watershed or rise up into the atmosphere.

Unlike synthetic fertilizers which contain ‘ready-made’ nutrients, compost and many other organic fertilizers simply provide the types of organic matter that microbes were born to consume and then let them do their thing. Soil microbes slowly break down this organic matter, fixing Nitrogen, Potassium, and Phosphorus and creating new layers of Carbon-rich humus in the process. While synthetic fertilizers give plants the nutrients they need to grow, only a well-fed microbial community can produce the complex carbon molecules that make up the soil itself. Farms that ignore the world beneath the surface may end up with all the nutrients needed for plant growth but nothing to put them in!

Each season as our crops succumb to the fall frosts we wander the fields collecting soil samples to send in to a lab in Richmond. When the results are in, we find out whether our efforts to build the soil are paying off. The first column of the report, and perhaps the most important, is a measure of the organic matter in the soil. This years results are in and things are looking good for the Bellair’s microbial community. In the coming years we will continue doing what we’re doing to transition Bellair into sustainable agriculture; composting, using green manures, rotating crops, and spreading organic fertilizer on our fields. We also hope to integrate cattle, with their nutrient-rich brown manure, into our crop rotation.

That’s it from me for this week.  Check back next Thursday for details on the inner workings of a compost heap!

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