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, termites, ants, gophers, moles, etc., all cause considerable mixing of soil and help to blend soil, aerate and lighten the soil by creating pores (which help store water and air).

Plant life provides organic matter to the soil and helps to recycle nutrients with uptake by roots in the subsurface. The type of plant life in a given area, such as types of trees or grasses, depends on the climate, parent material, and soil type. With the annual dropping of leaves and needles, trees add organic matter to soil surfaces, helping to create a thin, organic-rich A or O horizon over time. Grasses have considerable root and surface masses that add to the soil each fall for annuals and short-lived perennials. For this reason, grassland soils have much thicker A horizons with higher organic matter contents and are more agriculturally productive than forest soils.

Relief (Topography and Drainage)

The local landscape can surprisingly strongly affect the soil on site. The local topography (relief) can have important microclimatic effects and affect soil erosion rates. Compared to flat regions, areas with steep slopes overall have more soil erosion, more runoff of rainwater, and less water infiltration, all of which lead to more limited soil development in very hilly or mountainous areas. In the northern hemisphere, south-facing slopes are exposed to more direct sunlight angles, thus warmer and drier than north-facing slopes. The cooler, moister north-facing slopes have a more dynamic plant community due to less evapotranspiration and, consequently, experience less erosion because of plant rooting of soil and have thicker soil development.

Soil drainage affects organic matter accumulation and preservation and local vegetation types. Well-drained soils, generally on hills or side slopes, are more brownish or reddish due to the conversion of ferrous iron (Fe²⁺) to minerals with ferric (Fe³⁺) iron. More poorly drained soils, in lowland, alluvial plains or upland depressions, tend more be more greyish, greenish-grey (gleyed), or dark colored due to iron reduction (to Fe²⁺) and accumulation and preservation of organic matter in areas tending towards anoxic. Areas with poor drainage also tend to be lowlands into which soil material may wash and accumulate from surrounding uplands, often resulting in over-thickened A or O horizons. In contrast, steeply sloping areas in highlands may experience erosion and have thinner surface horizons.

Parent Material

The parent material of soil is the material from which the soil has developed, whether it be river sands, shoreline deposits, glacial deposits, or various types of bedrock. In youthful soils, the parent material has a clear connection to the soil type and has a significant influence. Over time, as weathering processes deepen, mix, and alter the soil, the parent material becomes less recognizable as chemical, physical, and biological processes take their effect. The type of parent material may also affect the rapidity of soil development. Highly weatherable parent materials (such as volcanic ash) will transform more quickly into highly developed soils, whereas quartz-rich parent materials, for example, will take longer to develop. Parent materials also provide nutrients to plants and can affect soil internal drainage (e.g., clay is more impermeable than sand and impedes drainage).

Time

Generally, soil profiles tend to become thicker (deeper), more developed, and more altered over time. However, the rate of change is greater for soils in the youthful stages of development. The degree of soil alteration and deepening slows with time and, after tens or hundreds of thousands of years, may approach an equilibrium condition where erosion and deepening (removals and additions) become balanced. Young soils (< 10,000 years old) are strongly influenced by parent material and typically develop horizons and character rapidly. Moderate-age soils (roughly 10,000 to 500,000 years old) are slowing in profile development and deepening and may begin to approach equilibrium conditions. Old soils (>500,000 years old) have generally reached their limit beyond soil horizons and physical structure but may continue to alter chemically or mineralogically.

Soil development is not always continual. Geologic events can rapidly bury soils (landslides, glacier advance, lake transgression), can cause removal or truncation of soils (rivers, shorelines), or can cause soil renewal with additions of slowly deposited sediment that add to the soil (wind or floodplain deposits). Biological mixing can sometimes cause soil regression, a reversal or bump in the road for the normal path of increasing development over time.