What is Soil?
The word “soil” has been defined differently by different scientific disciplines. In agriculture and horticulture, the soil generally refers to the medium for plant growth, typically material within the upper meter or two (Figure 1). We will use this definition in this chapter. Soil consists predominantly of mineral matter but contains organic matter (humus) and living organisms. The pore spaces between mineral grains are filled with varying proportions of water and air.
In common usage, the term soil is sometimes restricted to only the dark topsoil we plant our seeds or vegetables. In a more broad definition, civil engineers use the term soil for any unconsolidated (soft when wet) material that is not considered bedrock. Under this definition, soil can be several hundred feet thick! Ancient soils, sometimes buried and preserved in the subsurface, are called paleosols (Figure 2) and reflect past climatic and environmental conditions.
Importance of Soil
Soil is important to our society primarily because it provides the foundation of agriculture and forestry. Of course, soil is also critical for terrestrial ecosystems and thus important to animals, plants, fungi, and microorganisms.
Soil plays a role in nearly all biogeochemical cycles on the Earth’s surface. Global cycling of key elements such as carbon (C), nitrogen (N), sulfur (S), and phosphorous (P) all pass through the soil. In the hydrologic cycle, soil helps mediate precipitation flow from the surface into the groundwater. Microorganisms living in soil can also be important components of biogeochemical cycles through decomposition and other processes such as nitrogen fixation.
Soil Forming Factors
The fundamental factors that affect soil genesis can be categorized into five elements: climate, organisms, relief, parent material, and time. One could say that the relief, climate, and organisms dictate the local soil environment and act together to cause weathering and mixing of the soil parent material over time. As soil is formed, it often has distinct layers, formally described as “horizons.” Upper horizons (labeled as the A and O horizons) are richer in organic material and so are important in plant growth, while deeper layers (such as the B and C horizons) retain more of the original features of the bedrock below (Figure 3).
Climate
The role of climate in soil development includes aspects of temperature and precipitation. Soils in very cold areas with permafrost conditions tend to be shallow and weakly developed due to the short growing season. Organic rich surface horizons are common in low-lying areas due to limited decomposition. In warm, tropical soils, soils tend to be thicker, with extensive leaching and mineral alteration. In such climates, organic matter decomposition and chemical weathering are accelerated.
Organisms
Animals, plants, and microorganisms are all important in soil development processes, supplying organic matter and/or nutrient cycling. Worms, nematodes,