One of the first distinctions made by a soil scientist is that "soil" and "dirt" are not the same. Dirt is what collects on the car or in the corner of the bedroom when it has been months since the last time it was vacuumed. Soil, on the other hand, is a highly structured matrix of inorganic and organic particles that form the substrate for terrestrial ecosystems . The substrate is the foundation where plants, ranging in size from minute ferns to tall trees, are rooted. The inorganic particles are formed from minerals in rock through weathering; a process that produces them by physical means (for example, erosion, freezing and thawing, and wind abrasion) or chemical means (for example, oxidation , dissolving crystals, or the action of acids). The organic particles originate from plant and animal tissues through fragmentation, decomposition, and chemical transformation.

The climate, rainfall, and temperature determine the pattern of soil weathering in a particular area. The weathering process often produces horizontal layers of soil of varying thickness called soil horizons because each layer is roughly parallel to Earth's horizon. The uppermost horizon often contains the most organic matter and others have differing nutrient contents and physical properties.

Soil provides physical support for plants, and the pores between particles provide spaces that contain water used by the plants and animals living within the soil. Oxygen from air diffuses into the pores when the water drains through the soil. This allows plant roots, aerobic microorganisms, and invertebrates to survive. Root systems may be located just below the surface, or may penetrate many meters deep. Too much water prevents air from reaching roots. Because of this, too much continuous water can kill many species of plants just as effectively as the absence of water during an extended drought. Only certain specially adapted plants are successful in water-saturated soils.

The particles that make up the soil may occasionally be all of the same size, as in the case of river sand deposits, or a silt layer that settled out on the bottom of ancient lakes. Sand particles are fairly large, only slightly smaller than gravel used in a fish tank, while silt particles are smaller than sand grains and clay particles are even smaller, approaching the fineness of talcum powder or baker's flour. Soils that are composed predominately of one of these particle sizes are known respectively as sands, silts, and clays. However, very often there is a mixture of particle sizes and the soil is referred to as a loam (a sandy loam has a mixture of particle sizes, but is mostly composed of sand). Loams are generally the best soils for plants to grow in. The larger sand particles facilitate drainage and oxygen penetration, while the small clay or organic humus particles provide a large amount of surface area where nutrient ions can become attached. Examples of these nutrients include nitrate, potassium, calcium, phosphate, and iron. They can be provided by commercial fertilizers, but are present naturally in nutrient-rich soils. The ions are attracted to electrically charged sites on clay or fine humus particles and gradually released into the water as they are exchanged with other ions. This nutrient-rich soil solution provides nutrition to plants through the roots.

Finally, the soil is a habitat for millions of small organisms per cubic meter such as bacteria, algae, nematodes , insects, and mites. These organisms make nutrients available through metabolic activity or the production of feces. They also die and add to organic matter and in general contribute to good soil quality. Larger organisms also inhabit the soil. Earthworms are particularly important because they mix the soil and process organic matter, which passes through their intestinal tracts and is released as feces. This helps produce loose textured soils with a high organic content and nutrient-holding capacity. In addition, their burrowing increases oxygen penetration. Larger animals such as moles, rabbits, foxes, and groundhogs create burrows that provide them with amenities such as shelter and food storage areas. This allows them to survive and thrive within the subterranean part of the ecosystem.

SEE ALSO Biogeochemical Cycles ; Mycorrhizae ; Nematode ; Nitrogen Fixation ; Plant Nutrition ; Roots

Dean Cocking


Killham, Ken. Soil Ecology. New York: Cambridge University Press, 1994.

Miller, Raymond W., and Duane T. Gardiner. Soils in Our Environment, 9th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

Paton, T. R., G. S. Humphreys, and P. B. Mitchell. Soils: A New Global View. New Haven, CT: Yale University Press, 1995.

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