Environmental Science 15th Edition by Scott Spoolman,Tyler Miller

Exercise 5

SOIL IS THE FOUNDATION OF LIFE ON LAND
S oil is a complex mixture of eroded rock, mineral nutrients, decaying organic matter, water, air, and billions of living organisms, most of them microscopic decomposers. Soil formation begins when bedrock is slowly broken down into fragments and particles by physical, chemical, and biological processes, called weathering. Figure 10.A shows profiles of different-aged soils.
Soil, on which all terrestrial life depends, is one of the most important components of the earth's natural capital. It supplies most of the nutrients needed for plant growth and purifies and stores water. Organisms living in the soil remove carbon dioxide from the atmosphere and store it as organic carbon compounds, thereby helping to control the earth's climate as part of the carbon cycle (see Figure 3.14, p. 53). You might think of topsoil as something found only in farm fields, but it underlies all forests and grasslands, as well as croplands.
Most soils that have developed over long periods of time, called mature soils, contain horizontal layers, or horizons (Figure 10.A), each with a distinct texture and composition that vary with different types of soils. Most mature soils have at least three of the four possible horizons.
The roots of most plants and the majority of a soil's organic matter are concentrated in the soil's two upper layers, the O horizon of leaf litter and the A horizon of topsoil. In healthy soils, these two layers teem with bacteria, fungi, earthworms, and small insects, all interacting in complex food webs. Bacteria and other decomposer microorganisms, found by the billions in every handful of topsoil, break down some of the soil's complex organic compounds into a mixture of the partially decomposed bodies of dead plants and animals, called humus, and inorganic materials such as clay, silt, and sand. Soil moisture carrying these dissolved nutrients is drawn up by the roots of plants and transported through their stems and into their leaves as a key component of chemical cycling, the basis for one of the principles of sustainability.
The B horizon (subsoil) and the C horizon (parent material) contain most of a soil's inorganic matter, mostly broken-down rock consisting of varying mixtures of sand, silt, clay, and gravel. Much of it is transported by water from the A horizon. The C horizon lies on a base of parent material, which is often bedrock.
The spaces, or pores, between the solid organic and inorganic particles in the upper and lower soil layers contain varying amounts of air (mostly nitrogen and oxygen gas) and water. Plant roots use the oxygen for cellular respiration. As long as the O and A horizons are anchored by vegetation, the soil layers as a whole act as a sponge, storing water and nutrients, and releasing them in a nourishing trickle.
Although topsoil is a renewable resource, it is renewed very slowly, which means it can be depleted. Just 1 centimeter (0.4 inch) of topsoil can take hundreds of years to form, but it can be washed or blown away in a matter of weeks or months when we plow grassland or clear a forest and leave its topsoil unprotected.
Critical Thinking
How does soil contribute to each of the four components of biodiversity described in Figure 4.2 (p. 65)?

ANIMATED FIGURE 10.A Generalized soil profile and formation of soil. Questions: What role do you think the tree in this figure plays in soil formation? How might the soil formation process change if the tree were removed?
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