Soils: The Residue of Weathering

On moderate and gentle slopes, plains, and lowlands, where erosion is less intense, a layer of loose, heterogeneous weathered material remains overlying the bedrock. It may include particles of weathered and unweathered parent rock, clay minerals, iron and other metal oxides, and other products of weathering. Geologists use the term soil to describe layers of material, initially created by fragmentation of rock during weathering, that experience additions of new materials, losses of original materials, and modification through physical mixing and chemical reactions. Organic matter, called humus, is an important component of most of Earth’s soils; it consists of the remains and waste products of the many organisms that live in soil. Leaf litter contributes significantly to the soil of forests. In addition, most soils have the ability to support rooted plants. Not all soils support life, however, and soils occur in places, such as Antarctica and Mars, where life is limited or possibly absent altogether.

Soils vary in color, from the brilliant reds and browns of iron-rich soils to the black of soils rich in organic matter. Soils also vary in texture. Some are full of pebbles and sand; others are composed entirely of clay. Soils are easily eroded, so they do not form on very steep slopes or where high altitude or frigid climate prevents the growth of plants that would hold them in place and contribute organic matter. Soil scientists, agronomists, and engineers, as well as geologists, study the composition and origin of soils, their suitability for agriculture and construction, and their value as a guide to climate conditions in the past.

Soils form at the interface between the climate and plate tectonic systems. They are crucial to life on Earth’s continents, and they are one of human society’s most valuable natural resources. Soils are the primary reservoir of nutrients for agriculture and the ecological systems that produce renewable natural resources. They filter our water and recycle our wastes, and they provide the necessary substratum for our buildings and infrastructure. In addition, they help regulate the global climate by storing and releasing carbon dioxide. Soils contain twice as much carbon as the atmosphere and three times more than all of the world’s vegetation.

Soils as Geosystems

As we have seen, the concept of Earth as a set of interacting geosystems is of great value in understanding geologic processes. Soils, like many other components of the Earth system, can be described as a geosystem with inputs, processes, and outputs (Figure 16.11).

Figure 16.11: Soils are geosystems that develop through inputs of new materials, losses of original materials, and modification through physical mixing and chemical reactions. Soil modification processes can be divided into two basic types: translocations and transformations. The distinct soil horizons that make up the soil profile are also visible in this diagram.

Inputs: Weathered Rock, Organisms, and Dust

Soils develop from weathered rock, with additional inputs of organic matter from the biosphere and dust from the atmosphere. As discussed earlier, physical weathering breaks down rock into smaller pieces, and chemical weathering transforms minerals in that rock (such as feldspar) into other minerals (such as clays). Plants and other organisms may colonize the soil, and when they die, their tissues decompose to form humus. The atmosphere also contributes matter to the soil, but this material is predominantly inorganic dust.

Processes: Transformations and Translocations

As soil ages and matures, the materials added to or removed from it cause it to undergo a set of transformations. The addition of humus, for instance, provides a source of nutrients that encourage further plant growth and add more humus—a positive feedback process within the soil geosystem. Many soil transformations involve chemical weathering of feldspar and other minerals to form clay minerals.

Translocations are lateral and vertical movements of materials within the developing soil. Water is the main agent of translocation, usually transporting dissolved salts. Water selectively removes some materials as it percolates down through the soil after rainfall in a process called leaching. However, it may also rise from below the soil surface when temperatures increase and evaporation draws more water to the surface. Organisms also play an important role in translocation by moving components of the soil as they burrow through it.

Soils are dynamic and respond to changes in climate, interactions with organisms, and perturbations by humans. Five factors are important in their formation and development:

• 1. Parent material: the solubility of minerals, the sizes of grains, and the patterns of fragmentation, such as joints and cleavage, in the bedrock
• 2. Climate: temperatures, precipitation levels, and their seasonal patterns of variation
• 3. Topography: slope steepness and the direction the slope faces: gentler slopes that face toward the Sun promote better soil development
• 4. Organisms: the diversity and abundance of organisms living in the soil
• 5. Time: the amount of time that a soil has to form

Outputs: Soil Profiles

Most soils form distinct layers as they develop. The composition and appearance of a soil is known as a soil profile. Soil profiles consist of up to six horizons: distinct layers of varying color and texture, usually parallel to the land surface, that are visible in vertical sections of exposed soils (see Figure 16.11).

The soil’s topmost layer, called the O-horizon, is usually thin and consists of loose leaves and organic detritus. Beneath this topmost layer is the A-horizon, typically not much more than a meter or two thick and usually the darkest layer because it contains the highest concentration of humus. Next down is the E-horizon, which consists mostly of clay and insoluble minerals such as quartz, as soluble minerals will have been leached from this layer. Beneath the E-horizon is the B-horizon, in which organic matter is sparse. Soluble minerals and iron oxides accumulate in this layer. The climate influences the specific types of minerals that accumulate in the B-horizon; carbonate minerals and gypsum, for example, are found there in arid climates. The next layer, the C-horizon, is slightly altered bedrock, broken and decayed, mixed with clay produced by chemical weathering. Unaltered bedrock forms the lowest level, or R-horizon.

The five soil development factors listed above interact to create 12 different soil types, each with a distinct profile, that are recognized by scientists who study soils (Table 16.3).

Paleosols: Working Backward from Soil to Climate

Recently, there has been much interest in ancient soils that have been preserved as rock in the geologic record. These paleosols, as they are called, are being studied as guides to ancient climates and even to the amounts of carbon dioxide and oxygen in the atmosphere in former times. The mineralogy of paleosols billions of years old, for example, provides evidence that there was no oxidation of soils at that early stage of Earth’s history, and therefore that oxygen had not yet become a major component of Earth’s atmosphere.

Soil formation is just one step in the evolution of a landscape. Weathering and rock fragmentation often destabilize topographic features and lead to the more dramatic changes caused by mass wasting. This process is an important part of the general erosion of the land, especially in hilly and mountainous regions.