A meditation on the transforming power of soil

In Jewish tradition, a butcher knife was purified by burying it in the ground for several weeks. This seemingly paradoxical use of dirt to clean is evidence that humans long ago recognized a mysterious transforming power in soil, "that element where," as a poet said, "life and death are exquisitely balanced." When we study the chemistry and biology of soil, we can see examples of the soil's transforming power.

The soil taken as a whole can be looked on as a catalytic system. This system has many interrelated subsystems, some biological, some abiotic. The abiotic catalysts are surfaces. Soil is a region where surfaces, or interfaces between phases, are dominant. The ocean is all wet. Rocks are merely solid. The atmosphere is fairly homogeneous. But in soil, the three phases are intimately commingled. The solid surfaces grow and dissolve, bind species from the liquid phase, and release them again, exchanging them for other ones. In these processes, electrons sometimes jump from one species to another. For example, certain organic molecules trade electrons fairly readily with iron and manganese ions. The organics can lose electrons to solid iron(III) and manganese(IV) oxides, which become reduced to soluble Fe2+ and Mn2+. Thus, the oxides are gradually dissolved. If oxygen is present, it may be able to reoxidize the metal cations (even though it was unable to directly oxidize the original organic molecule). In this case, the oxides are not dissolved, but act strictly as catalysts for the oxidation of the organic compounds.

Life has elaborated on the theme of surface reactions. Cells are full of membranes, which are a kind of surface, although a liquid liquid one. And in an enzyme, the protein molecule forms a surface of just the right shape and charge distribution to catalyze a particular reaction. Many soil transformations depend on these biological catalysts. Nitrogen and oxygen are shy strangers going their separate ways in the well lit atmosphere, but once they are pulled into the dark nightclub of the soil, they lose their inhibitions and are lured into an intricate dance of combination and recombination, fueled by the energy in reduced carbon and lubricated by the enzymes of soil bacteria such as Nitrosomonas and Nitrobacter. Nitrogen ends up losing its innocence and five electrons, but then is redeemed when oxygen becomes depleted and the denitrifiers come in and give the electrons back. N2 limps back to its home in the sky, but is no wiser for the experience, for the next weekend the whole thing happens again...and again...and again, in an endless cycle.

The surfaces in the soil are stages on which chemical and biochemical dramas are played out.


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