The Critical Zone: Soil
Soil is the complex mixture of minerals, organic matter (humus), water, air, and living organisms that covers the land surface. It is a vital natural resource that supports plant life, and thus, nearly all terrestrial ecosystems. The study of soil is called pedology.
The Five Factors of Soil Formation (CLORPT)
The type of soil that forms in a location is a result of a combination of five factors:
1.Climate: Temperature and precipitation are the most important factors. They determine the rate of chemical weathering and the amount of water moving through the soil.
2.Organisms: The types of plants, animals, and microbes living in and on the soil. Plants provide organic matter, and decomposers (like bacteria and fungi) create humus, the dark, nutrient-rich component of soil.
3.Relief (Topography): The shape of the land. Steep slopes have higher rates of erosion and thus thinner soils than flat, low-lying areas where sediment accumulates.
4.Parent Material: The underlying geologic material (usually bedrock or sediment) from which the soil is formed. It determines the basic mineralogy of the soil.
5.Time: Soil formation is a very slow process. A mature, well-developed soil can take thousands of years to form.
The Soil Profile
A vertical cross-section of the soil from the surface down to the parent material reveals distinct layers called soil horizons.
O Horizon: The top layer, consisting of organic matter like decomposing leaves and twigs (litter).
A Horizon (Topsoil): A mixture of mineral particles and dark, rich humus. This is the most biologically active layer and is crucial for plant growth.
B Horizon (Subsoil): This layer has less organic matter and is a zone of accumulation. Minerals and clay particles leached from the A horizon are deposited here. Leaching is the process where water carries dissolved substances downward through the soil.
C Horizon: This layer consists of partially weathered parent material. It is a transition zone between the soil above and the bedrock below.
R Horizon: The unweathered bedrock.
Soil Texture
Soil texture refers to the proportion of sand (largest particles), silt, and clay (smallest particles) in the soil. The texture is critical as it influences the soil's ability to hold water and nutrients. A soil with a balanced mixture of all three, called a loam, is generally the most fertile and best for agriculture.