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Soil, Sunlight, and Symbiosis What can be so commonplace and natural as the soil beneath our feet? Separated as we are from contact with it by cement and shoe leather, we take it for granted and hardly ever notice it. And yet, it is one of the greatest gifts of nature to us, worthy of meditation and gratitude. The formation of topsoil is a dynamic process in which loss through erosion is balanced by production of new soil, at least for the period before the last few centuries. In more recent times, of course, the pressure of human population and the practice of industrialized agriculture have resulted in much loss of topsoil, to the detriment of humans and other animals which depend on plants for food. The slow but steady process which produces soil begins with the weathering of rock. Lichens and microscopic organisms open up minuscule cracks on the surface of the rock, allowing the entrance of water. In winter, the water expands as it freezes, opening up the cracks a bit more. (Water is unique among common substances in expanding on freezing. How lucky for us!) Slowly over time, the rocks crumble into smaller pieces, then into sand, and finally into the fine particles which constitute the inorganic portion of soil. Soil can range from the relatively large particles found in sand to the smallest ones in clay. Sandy soil drains quickly, but lacks the extensive surface area to hold the inorganic nutrients—principally nitrates and phosphates, but including numerous other lesser and trace elements—so necessary for plants. At the other extreme, clay has plenty of surface area, but it holds water and nutrients so strongly that the water does not easily flow to bring the nutrients to the roots of plants. The best soil, therefore, is a mixture of sand and clay, in which the fine particles of clay settle in between the larger particles of sand, providing the surface area to hold nutrients while retaining enough porosity to allow good drainage. Topsoil, however, includes much more than its inorganic components; there are living creatures, earthworms turning the soil by passing it through their bodies, and the web of insects and burrowing mammals serving as food for one another. Not the least of these are the microscopic decay organisms, which turn organic detritus, consisting of dead plants and animals as well as animal waste products, into the inorganic form needed by plants. Thus we have this remarkable cycle. Plants grow and become food for animals from microbes to rabbits to human beings. For plants, this self-sacrifice, so common in nature, is essential for animals. Even carnivores, of course, depend ultimately on herbivores, which depend on plants. For their part, animals grow on the food provided by plants and produce organic waste and ultimately dead tissue when they die. Here we have another form of self-sacrifice, giving of self to provide the inorganic nutrients for plants. The process is sublime. What engineer could ever have designed anything like it? And yet there is more. What has been described so far would not work; it would be forbidden by the laws of thermodynamics. Since the return cycle would involve converting low-energy inorganic waste into high-energy organic food; there would need to be some additional input of energy. Fortunately, there is a handy source of energy constantly at hand, sunlight. In its own act of self-sacrifice, our sun constantly consumes its store of hydrogen in intense fusion reactions, producing enormous amounts of energy, sending it out to the universe (including our own little planet) in the form of sunlight, which warms the planet to a range of temperatures suitable for the various forms of life found on earth. (How fortunate that the earth is positioned just far enough from the sun to avoid extremes of temperature! How lucky we have an additional safeguard of carbon dioxide to provide a bit of extra warmth by means of the greenhouse effect!) There is a problem with sunlight, however. Although solar radiation leaves the sun at an extremely high temperature, and is therefore a high-quality form of energy in terms of the second law of thermodynamics, by the time it reaches the surface of the earth it is so diffuse that it normally is unable to do more than warm the surface. For this reason, it is unable by itself to provide the energy needed to turn inorganic nutrients into plant tissue. Along comes another ingenious solution. The green pigment which we so much prize when we admire pastures or hillsides turns out to have other amazing properties. Chlorophyll is able to capture sunlight, take in carbon dioxide, and act as a catalyst through a most complex succession of chemical reactions to produce organic compounds. As a bonus, the process produces oxygen as a waste product, replenishing the supply of oxygen in the atmosphere so necessary for animals. The symbiosis, then, is more far-reaching than we thought at first. It is not only organic and inorganic compounds which are cycled, but carbon dioxide and oxygen as well. The waste product from animal respiration, carbon dioxide, is necessary for plant photosynthesis; the waste product from photosynthesis is necessary for animal respiration. (Who could ever have thought up such a remarkable and efficient arrangement?) Of course, everything becomes more complicated when human beings come onto the scene. With the advent of self-reflection, human beings can pay attention as they wish, can evaluate the world around them as it strikes them. For some, the natural world will not be worth their notice; striving, worrying, and escape will take all their energy. Others will stand in awe, feeling the allure of the natural world. For these, poets, mystics, and ordinary human beings, the world will be seen as filled with spirit. Beginning with a bit of dirt, we can see a drama involving all of life, extending our reflections to such interrelated phenomena as insects transporting pollen, the social structure of animals, the migration of birds, the development of human culture. In fact, we find everything related to everything else in an incredibly complex web. Incredibly, we ourselves are not only part of the web but also co-creators of it. We are on intimate terms with the universe. Dom Roberti |