Readers of the Washington Post may recall a book reviewed by Adrian Higgins a few weeks before Christmas called ‘Life in the Soil’ by James Nardi. After having Jane buy it for me as a Christmas gift and reading it, I would consider this a ‘must read’ for anyone interested in natural history, or, more to the point, life on earth. Here’s a clear and concise presentation that enables the reader to make the connection between the sciences of geology and biology. You are virtually taken by the hand and shown how all the life forms on earth are able to flourish here, using only the minerals obtainable from the earth’s rocks, water and air, and fueled by the energy from the sun.
What Mr. Nardi presents is the basic tenet that soils are a “marriage” of both the mineral world and the organic world, but it takes the living world component to complete the formation.
The contribution to soils from the inorganic rock minerals is first presented. We all know about moisture and carbon dioxide in the air forming a weak carbonic acid that accounts for the breakdown of calcium carbonates of limestone. This is what creates the marvelous limestone caves of Shenandoah Valley with their stalagmites and stalactites. But, in easily read narrative and using simple chemical equations, he illustrates how this same acidic solution chemically breaks down feldspars of granites and schists to create sands, silts, and clays. Through this inorganic process, mineral soils are created. In fact, every element that is necessary for plant life can be gotten from these inorganic mineral soils but for one: nitrogen. And here is where we begin our rock and plant association.
In order for life to emerge on earth, there was a need to obtain nitrogen from the air and convert it from a gas to a form usable by plant life. The first organisms on earth that could ‘fix’ nitrogen were a few specialized bacteria, blue-green algae (photosynthetic cyanobacteria), and certain actinomycetes. While we’re familiar with the nitrogen-fixing bacteria that live in the root nodules of leguminous plants, such as soybeans and locust trees, it was interesting to learn that these legume roots actually secrete substances that attract the nitrogen-fixing bacteria. The advantage of the actinomycetes is that they are not limited to legumes. They have been found on most plant roots. The interesting thing about these actinomycetes, which number in the billions in each square meter of soil, is that their waste, or byproduct, is responsible for the earthy smell we notice from freshly turned soils.
More rock-plant connections reveal that, not only do inorganic carbonic acids make sands, silts and clays from rocks, but also plant roots can do the same. As the roots break down sugars through respiration, the waste, or byproduct, carbon dioxide is released in the soil. In the presence of water; carbonic acid is created, and we already know this chemically breaks down rocks to create sands, silts, and clays. Not only plants, but lichens and some fungi can organically break down rocks as well.
What Mr. Nardi is presenting is the story of energy transformation throughout the soil world. As we read this book, we are shown how all of the many life forms affect, or are obligates of, other life forms. Bacteria, fungi, algae, protozoans, invertebrates and vertebrates, all complement (and control) the others in a wonderful natural balance. A balance of fuel; a balance of energy; a balance of organisms that can most efficiently utilize the energy presented in the particular niche. When organisms die, many, organisms compete for the available stored energy. Waves of organisms consume the organics, using up what energy they can, depositing their waste products, then declining in numbers when the targeted organics are consumed. This feeding frenzy of the remaining organic material continues by various other organisms, getting what energy they can derive from the remaining original organism, the newly deceased first line decomposers, and their byproducts, and then leaving (or dying). These waves of decomposers continue mining what little energy remains, until, ultimately; all of the organic molecules are consumed, leaving only inorganic minerals, water and air. This ‘mineralization’ process completes the cycle of nutrients within the soil.
The affects of exotic, or introduced, organisms on this balance are discussed. For example, exotic worms of the upper mid-west have created such numbers that they have removed virtually all organics from the soil, severely depleting the populations of the natural communities. Garlic mustard is identified as one of the very few plants that, not only doesn’t establish mycorrhizal associations with fungi, but also actually releases chemicals into the soil that disrupts the mycorrhizal associations of other fungi and trees.
The numbers of organisms that inhabit the soils are overwhelming. Statistics become meaningless as you read about the thousands of billions of bacteria, ten billion protozoa, several million of invertebrates, and 100,000 – 400,000 mites in the top 6 inches of a square meter of soil! But you leave understanding the importance of these organisms to the health and structure of the soil. You appreciate the balance of life in the soils and how humans have affected this ‘biomantle’.
Humans have disturbed between one third and one half of the earth’s surface from agriculture, grazing, timbering and urbanization. These activities increased soil erosion from 1 foot of soil per 10,000 years to 1 foot of soil per 1,000 years. With this loss goes the topsoil; the humus and the nutrients that are found in it, and the organisms that keep our soils healthy.
Mr. Nardi puts it best when he says, “In a healthy soil, with its mix of decomposers, diggers, and predators, a harmony pervades both the mineral world and the organic world. The creatures of the soil community depend on each other to maintain the balance between processes of growth and processes of decay.”