Soil, a conglomeration of inorganic materials, plant nutrients and roots, organic matter and innumerable organisms, provides indispensable services to humanity, not to mention all the rest of life on earth. Unfortunately, it has been terribly abused by much of human agriculture and other activities—and tremendously undervalued, externalized and simply assumed in most economic analyses. That most civilizations throughout history have declined as their soil base has deteriorated should give us some pause as mega-corporations push toward a "globalized" economic system.
Sustainability will require much more attention be paid to maintaining and renewing our soils. Unfortunately, while Aldo Leopold has provided an overarching perspective of soil, there are many other salient issues to consider. One of the most crucial components of soil health and productivity is the level of humus or organic matter they contain. Unfortunately, nearly all agricultural and forestry practices, and especially what has become "conventional" agriculture in North America and other areas which have emulated these techniques, result in decreasing levels of organic matter. Sustainable organic and related farming and forestry systems which include high levels of concern for "feeding the soil" offer many options for stabilizing and even reversing this disastrous trend in humus levels.
Organic matter in the soil is composed primarily of carbon. Indeed, the precipitous decrease in soil organic matter levels in North America over the past hundred or so years has contributed a significant amount of carbon released to the atmosphere as well as the hydrosphere. There is a large potential for helping to stabilize and even decrease atmospheric carbon through developing systems which feed more and more well decomposed and stabilized organic matter back to the soil.
Aerobic decomposition or composting, which requires oxygen, and anaerobic decomposition or digestion, which requires the absence of oxygen, are the two biological pathways by which organic matter is broken down for reuse, i.e., growth of other organisms. Much more will be said of these two bacterial pathways in "Agricultural Systems." Although the pathways are somewhat similar, they do result in different gases being given off, different compounds containing the many plant nutrients, and different forms of energy. Although a bit dated, my comparison of the effects of the differently decomposed materials on soils does deal with most of the relevant issues.
A Few Relevant Words of Wisdom
"In spite of the huge fossil fuel subsidies to conventional agriculture, and the general 1% efficiency of photosynthesis, United States' agriculture is still over 90% dependent upon solar energy." [Hall, et al., 1984]
"Since 1942 in the United States, insecticide use has increased by ten times while losses to insects have doubled." [Jackson, 1980]
"The United States has lost more than one third of its topsoil in its first 200 years." [Axell, 1982]
"Almost every past civilization we know of has perished because the effect of energy depletion of the soil was not appreciated by their citizens." [Stanford, 1981]