Components and Characteristics of Soils

Soils are a mixture of weathered mineral rock particles, organic matter (i.e., both living, and dead and decaying), water, and air. Soils can be thought of as functional entities that are the resulting products of the interaction of physical, chemical, and biological processes. Soil mineral particles are usually described based upon their relationship to soil texture, and include sand (0.05-2.0 mm), silt (0.002-0.05 mm), and clay (<0.002 mm). The relative distribution of these particles is used to describe different soils, and affects soil properties such as bulk density, pore space, and particle density. Variation in these physical properties structure habitats for different soil microbes and fauna. Soils are also composed of organic materials, which include the partially fragmented and decomposed remains of plant material, animals, microbes, and feces. Organisms are also an important organic constituent of soils, and their activity can influence soil properties (e.g., porosity), the binding of inorganic materials into soil aggregates, and nutrient cycling in soils. Soil air is the gases that are present in soil pores that are not filled with water. Oxygen and carbon dioxide (CO2) are important constituents, and their concentration in the soil affects many processes (e.g., nitrification and denitrification). Soil water can contribute up to 30% of soil volume, and is essential for the activity and physiological functioning of organisms in the soil.

Soil structure is a description of the spatial arrangement of soil particles into aggregates. Several hierarchical schemes for classifying soil structure have been proposed that link soil aggregates across several orders of magnitude of scale, from the submicron level (microaggregates) to several square meters (the pedon). The aggregate structure of soils has important implications for the distribution of pores in soils, which affects the distribution

Controlling factors

Parent material

Climate Vegetation



Grazing Species Nutrient input Fire


Seeding and planting Site preparation Watershed management Fire Harvest

Cropland Cultivation Fallow Crop selection Residue management

Nutrient inputs Water management Fire

Harvest (removal)


Energy inputs and transformations Radiation

Primary production Decomposition Nutrient cycling Immobilization Mineralization Weathering Translocation Transport Erosion Gaseous Leaching

Development of ecosystem properties Vegetation Consumers Soil Base status Texture

Organic matter Phosphorus Sulfur Nitrogen Salinity

Figure 2 Factors that control soil formation. Soil formation is an ecological process that involves the interactions of climate, organisms, parent material, and relief. From Coleman DC, Crossley DA, Jr., and Hendrix PF (2004) Fundamentals of Soil Ecology, 2nd edn., Burlington, MA: Academic Press. Modified from Jenny H (1980) The Soil Resource: Origin and Behavior. Ecological Studies 37. New York: Springer.

and availability of water and gasses. While soil structure generally refers to this aggregate structure, soils also have a spatial structure with depth. Soils in general are composed of different horizons, if one views them in profile. On the surface is a layer of recently fallen plant litter, with smaller amounts of excreta and dead bodies. Below this litter layer is a horizon enriched in organic material from the breakdown of the litter layer, and further below are mineral horizons of various textures and thicknesses. This vertical structure derives from the varying influence of soil formation processes with depth. In the 1940s, Jenny described the formation of soils as an ecological process that results from the interactions of factors, including climate, organisms, parent material, and relief.More recent theoretical advances resulting from agroecological and urban ecosystem research have extended the

'organisms' category to explicitly include the actions of humans in the formation of soils and soil structure. These physical, chemical, and biotic formation factors lead to variations in the distribution to depth of various mineral, chemical, and organic components of soil, giving soils a depth profile (Figure 2).

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