Lesson 2 Introduction

As introduced in Lesson 1, Critical Zone processes are represented by coupled physical, biological, and chemical processes that involve study by experts in geology, soil science, biology, ecology, geochemistry, geomorphology, and hydrology, to name a few of the relevant sciences. While these various disciplines are equally important for understanding the Critical Zone, they are linked by the presence of soil, considered by many to be the central component of the Critical Zone. This concept is perhaps best illustrated by the SoilCritZone logo shown here: a four-leaf clover-like emblem of yellow (at top, representing atmosphere), green (right, biosphere), blue (left, hydrosphere), and brown (bottom, lithosphere) arcs surrounding the acronym SoilCritZone; the design of the logo symbolizes how soil and the Critical Zone exist within the overlapping region between the four arcs or "spheres." The spheres closely overlap with the state factors of soil formation you will be introduced to later in this lesson: parent material, climate, topography, biota, and age.

Many of you may consider soil to be the mud you played in as a child, the material into which you plant your garden or the dirt you scrape off the bottom of your shoes or wipe from your dog's muddy feet. To most of humanity, soil may even seem to be a nuisance—the word itself has various meanings depending on the usage: as a verb, soil means to make dirty; to disgrace or tarnish; to corrupt or defile; or to dirty with excrement (Free Online Dictionary).

To begin to focus your attention on the positive aspects of soil, please consider the following four definitions. Soil is:

1. the upper layer of Earth that may be dug or plowed and in which plants grow; and, the superficial unconsolidated and usually weathered part of the mantle of a planet and especially of the Earth (Merriam-Webster).
2. that Earth material which has been so modified and acted upon by physical, chemical, and biological agents that it will support upright rooted plants. The term as used by engineers includes, in addition to the above, all regolith (American Geological Institute Dictionary of Geologic terms).
3. (i) the unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants. (ii) The unconsolidated mineral or organic matter on the surface of the Earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time. A product-soil differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics (Soil Science Society of America Glossary).
4. the naturally occurring, unconsolidated or loose covering of broken rock particles and decaying organic matter (humus) on the surface of the Earth, capable of supporting life. In simple terms, soil has three components: solid, liquid, and gas. The solid phase is a mixture of mineral and organic matter. Soil particles pack loosely, forming a soil structure filled with voids. The solid phase occupies about half of the soil volume. The remaining void space contains water (liquid) and air (gas). Soil is also known as earth: it is the substance from which our planet takes its name (Wikipedia).

Though the four definitions share common attributes, they do differ. For example, agronomists and most soil scientists focus their studies of soil on the rooted zone, or rhizosphere (approximately to depths of one meter), while the geologist's perspective is much broader and deeper, encompassing the full thickness of material down to the original parent material (shallow to depths up to one hundred meters or more). Nonetheless, most scientists agree that soil is a complex biomaterial that promotes the growth of terrestrial organisms, that it is crucial to life on Earth, and that it is the product of material derived from weathering of parent material, decomposing plant matter, and atmospheric deposition. Furthermore, as soil resources are finite, humanity should view them as non-sustainable and learn to care for and sustain these important resources. For these reasons, we will now turn our attention in Lesson 2 to the study of soil, the "heart" of the Critical Zone.