HANS JENNY USED TO SAY that soil is a Body in Nature. I wondered what he meant.
To try to understand the soil by taking a few trowelsful and submitting them to chemical tests is like trying to understand the human body by cutting off the finger, grinding it to paste, and performing the same tests. You may learn a lot about the chemistry of pastes, but about the intricate anatomical linkage of systems—and about the body’s functions as a whole—you will learn nothing at all.
Like our bodies, the soil participates in the recirculation and transformation of the four major elements: earth, air, fire, and water. Like our bodies, too, it is full of channels and pathways, directing the elements into fertile combinations and transformations at distinct, organized levels of the whole structure. And like our bodies, it has a definite genetic form.
Jenny once described a dog digging dirt out of a hole. “That,” he said, pointing to the mound of earth, “is not a soil.” To be more precise, he added, “Or it has only begun to be a soil.” In essence, it is no different from the ash just spewed out of Mount St. Helens. Its particles are more or less isomorphically arranged, randomly ordered. Its pebbles are still weathering to spheres, the shape of least resistance, not reorganizing and building energy at the microscopic level of the clays and iron oxides, as a soil does. Its tissues have not become differentiated. But the moment it is exposed on the Earth’s surface it begins to acquire a body, to incarnate, as it were, to become a soil.
The body of a soil is a sky where seeds and worms and ions fly. Just as the sky links outer space to Earth’s surface by means of increasingly dense atmospheric layers, so the soil links the surface to planetary bedrock by means of increasingly dense layers called, appropriately, horizons. Where the bottom layer of the sky rubs up against the top horizon of the soil, all terrestrial life is found.
In a fresh roadcut or beside a beach, you often can see the soil’s body exposed. It can be very beautiful. One February morning, I slammed on the brakes a few miles from the Tallahassee Airport. Cars zipped by. Drivers looked at me in anger or puzzlement. But I had found a very sexy soil-cut maybe twenty feet deep, created by the highway department as a source for fill to line the roadside ditches that eroded every spring.
Atop it was a forest of small second-growth pine, but the dramatic thing was the soil. Its upper horizon, maybe six inches deep, was dark with the decayed remains of pine needles, bark, and the root and stems of grasses. Beneath that was a sandy layer taller than me, the color of bone china or beach sand, looking like a Sahara in the making. And beneath that, stretching another ten feet down to the base of the cut, and who knows how far into the underground, was an orange-red horizon, variegated like a sunset on the ocean.
How is this picture different from, say, the lovely strata of Grand Canyon sandstones? Geological strata are the product of differing climates and different influences, separated by large periods of time. They express a calendar or an almanac. The horizons of a soil, on the other hand, express a single life, rising out of a single environment.
The soil therefore has an age, an expression of the life it has lived in its own peculiar place. In Tallahassee, where temperatures and rainfall are both high, and where the parent material was marine sand left high and dry after the end of the surge in sea level when last the glaciers melted, a soil on level ground very quickly becomes deep.
Rainwater, made more acid and chemically active by picking up CO2 in the air and by the residue of the pine trees, sluices through the sand at a rate of twenty inches per hour, or even higher. It might as well be going down a bathtub drain, except that here the water chemically extracts and moves aluminum and especially iron, carrying it down into the subsoil, which it makes red, leaving the unmoved silica above white. Within a few thousand years, the soil is fifty feet deep, and the native slash pines are sending their taproots down to find water.
The cut was a place of surprises. Not only were some tree roots exposed, showing a taproot more than three times as deep as the sapling was tall, the red subsoil was also the scene of unexpected developments. Nothing at all was growing in the exposed whitish horizon; for all the life evident in it, we might have been flying in the clouds. But beneath it, in the red layer, streaks of green and purplish lichen were colonizing rough runnels, and here and there, the tiny green bottle brushes of the club mosses had begun to emerge. The exposed subsoil, like the dirt the dog dug up, was already beginning to become a new soil in itself, with its own biota. In the structure of the old soil were contained a new heaven and new earth.
Horizons are what make some people become soil scientists. They are that lovely. To describe the variety of soils and their profiles, the thinkers have come up with a whole set of strange names that they nevertheless pronounce lovingly, like a best friend’s nickname. Joe Shuster, of the Soil Conservation Service in Tallahassee, helped me wonder about the name of the soil I’d seen. “It sounds a little like a haploquod called Leon,” he reflected. “That’s close to a Myakka soil, the state soil, of course.” (It was news to me that there was a state soil, but Shuster didn’t miss a beat.) I described the odd little club mosses growing in the subsoil. “Sandy, was it?” asked Shuster. Yes, I said, it was. “And kind of dry?” Yes. “And what was the relief?” he asked, warming to the game. I told him it was flattish.
“No, no, I take back that Leon,” he concluded. “I’ll bet it was a Mandarin soil, that’s a haplahumod, a little bit drier than a haploquod.” I told him thanks, that’s exactly what I needed to know, but would he mind spelling the terms?
While he was doing that, he evidently had time to consider. “You know” he added, with a tone of awe in his voice. “You almost saw a Re-sota.” I checked myself before asking what kind of hapla that was, for fear it might be something else even more difficult to spell, like an argialboll, an ustochrept, or a quartzipsamment. Dr. Shuster, however, needed no urging.
“R-E-S-O-T-A,” he spelled. “It has a much thicker spodic horizon, and there are long tongues of the Albic horizon that extend down into the yellow Bw horizon. It’s very impressive.” He paused to let that salvo take effect, then added, “And you should see the spodic horizon. It’s intermittent!”
I couldn’t flip the pages in my nomenclature book fast enough to figure out just what he was talking about, but his tone made me feel like hitting the road in search of the great Resota. I kicked myself for having missed it, even as Dr. Shuster reassured me that my Mandarin was almost as spectacular.
It’s easy to laugh at a nomenclature so arcane that it includes ten soil orders, fourteen thousand soils with proper names, and twenty-one letter designations to distinguish the different characteristics of soil horizons. Yet from another point of view, this naming by kinship admits life to this matter. In being related to each other, they are at another level related to us.
The assembly of horizons, the profile, is the signature of any soil. And the profile is created by the dynamic interactions of the four elements. Earth is present in the parent material, the rocks whose weathering gives the soil its bulk. Usually, earth is passive in the reactions that make soil, being transformed by what reaches it. Water is the most obviously active matter, the agent by which silica, irons, aluminums, clays, and humus are driven down from the surface into the subsoil, creating horizons like my Mandarins white and red ones.
Air and fire are also active. In a soil that has lime in the parent material, like the fertile soils of the Midwestern prairie, carbon dioxide is a prime mover in the creation of the soil. It engages in a dance of solid and liquid phases, whose result is the storage of lime in a white horizon in the subsoil. The roots breathe CO2 out into the soil, as do the millions of soil microbes and fauna as they digest organic matter. That gas changes ordinary calcite, the comparatively immobile calcium carbonate, into calcium bicarbonate, a form that dissolves readily in rainwater. The dissolved matter then sinks through the profile, until it reaches a depth at which there is no longer sufficient carbon dioxide to keep the bicarbonate reaction going. The matter reverts to calcium carbonate and falls from solution, creating a white horizon of calcium, like a layer of cirrus clouds.
A hotter soil goes deeper faster. The rate of chemical reactions doubles for every ten-degree Celsius rise in temperature, transforming primary minerals—silica and aluminum into clay minerals, and iron into oxidized iron compounds—and freeing salts, acids, and bases to further work on the soil profile. Another dance of matter takes place. Dispersed and stable clays, in the presence of electrically energetic compounds, curdle into heavy lumps. Gravity drags these lumps deeper into the profile, until the decrease of salts allows the clay particles to disperse once more, forming a clay layer in the subsoil.
By the same token, when chemical reactions release hydrogen ions into the soil, it becomes more acid. The more acid in the soil solution, the more iron compounds become soluble. In fact when the pH (a measure of hydrogen ion concentration) goes from 8.5 to 6—a logarithmic increase of 250 percent—iron becomes 100,000 times more liable to dissolve. The rainwater washes it deeper into the profile, where it forms a rust-red horizon at the place where the hydrogen concentration drops.
A soil is not a pile of dirt. It is a transformer, a body that organizes raw materials into tissues. These are the tissues that become mother to all organic life.