THE TEXTBOOKS WILL TELL YOU that humus is “deeply altered, black organic matter” lying in the top layer of the soil. But I only grasped what humus is about when my friend Pamela Morton showed me a picture of it.
Pamela spends parts of her summers in boreal Canada and has long been fascinated with the forest floor, where bits of blackening twigs, needles, bark, and the carcasses of small creatures decay. Humus is this organic part of the soil, the final residue of those matted leaves and cold bodies, intercalated, lapped, melding, losing their distinction, dry on top but inside turning the shades of brown and black that we associate with rain-wet wood or a man’s study. The deeper you go, the blacker it gets, and the fewer of the bits survive intact. A few inches down, it is pure black acrid matter having a texture like a cross between cotton candy and damp sawdust. This is the stuff from which all life on the land is born.
Pamela wanted to make a picture that would show its power. She began to experiment, making homemade paper out of leaf litter and deriving vegetable dyes from the material of the forest floor. She got not only deep blacks, but a range of browns, oranges, yellows, and reds, with a range of blue from robin’s egg to midnight.
All this was hidden in the substances themselves. She made of it a small collage, no bigger than the cover of this book, showing matrices of shifting color patterns, with a deep black enclosing them all. I have never seen an image that more dramatically showed the huge energy contained in humus. It was a dynamo of pure hues shining out of the blackness.
It set me to wondering about humus.
Humus, human. The dictionaries say there is a connection between the words, but they don’t elaborate. What does the root hum- mean?
It must have to do with humble, or with humilis, humiliate. Those words come from roots meaning “of the ground, lowly.” But humus does not refer to the ground itself It refers to the end product of decaying litter and dead creatures. It also has to do with being humorous, that is, in the original meaning, “wet.” Both people and humus are wet inside.
Wetness is opportunity. It represents the openness of nature to what falls from heaven. As Meister Eckhart put it, the humble man is “he who is watered with grace.”
The processes of growth, decay, feeding, digestion, excretion, attack, and repulsion all need wetness and generate heat. To understand them, you have to study the interconnections, not the essences. You have to put physiology first, not molecular chemistry.
For more than a century, chemists have been trying to answer the question, What is humus? And to this date, no one knows. Probably, no one will ever know. Even Hans Jenny, who knew more about the soil than anyone, remarked with a sigh, “Humus is imperfectly understood.” Every time you attempt to break it down into its basic components you get acids of a slightly different nature. All of them have similar properties, for example, a carbon-to-nitrogen ratio of 10:1, but none of them are chemically alike. In fact, as soil scientist Dr. James Rice puts it, “It is very possible that no two humus molecules are or have ever been alike.” Like snowflakes or people.
This makes it virtually impossible to apply to humus the quantitative experimental methods upon which modern agricultural chemistry is based. In fact, to establish that chemistry, the great nineteenth-century chemist Justus von Liebig had to try to descredit the then-prevailing theory that humus was the chief source of plant nutrients.
Liebig established that to grow a crop successfully, you need only supply the requisite mineral elements plus nitrogen in a sufficient quantity for the plant in question. Whether these substances are drawn from humus or not is immaterial, he argued. And he ridiculed the humus theory for its insistence that all plant carbon is derived from humus. In this respect, he was right, since most plant carbon is ultimately derived from the carbon dioxide of the air.
Today we still do not know what humus is, but we know a little about what it does. Some of it releases nitrogen and trace elements for the reuse of plants; all of it nourishes the microbes, which decompose it and whose bodies add to its substance. The microbes, furthermore, secrete sticky substances that help bind humus and clay together into stable aggregates. We know, too, that, unique among biological substances, humus resists the processes of microbial decay, so that it can remain in the soil, sometimes for ten thousand years or more. And we know that this is a good thing, because it can hold mineral nutrients for a plant’s use twice as well as the best clays. We are also aware that stable humus helps a porous soil to hold more water, and a heavy soil to hold less water. (A pound of sand absorbs one fourth pound of water; a pound of humus absorbs two pounds.) And finally, we see that in this ensemble of properties, it is the habitat for a diverse microflora and microfauna that tend to suppress or eliminate disease organisms, in order to continue their own robust lives.
Radical disorder is the key to the functions of humus. At the molecular level, it may indeed be the most disordered material on Earth. No two molecules of humus may be alike. Though no one has difficulty recognizing a humus molecule, it is quite likely unique, because it works upon fractal principles. Simple geometries define any given part of it, but the modes for the combining of these shapes produce a vast array of different manifestations at different scales. For humus, similarity is rampant, but identity nonexistent.
Neither humus nor humans are humble at all. We are audacious, like nature herself. We are wet, fecund, protean, dangerous. When we start to comprehend this in widening circles of the world, we know something worth knowing. We know that we must become responsible.