Topsoil Formation

Topsoil is the upper layer of the soil profile. It is composed of minerals and rock particles, humus, dead and decaying organic matter, water, and an array of living creatures. The kinds of animals are specific to the location, but in general they include rodents, earthworms, insects, fungi, bacteria, protozoans, and viruses. It is these life forms that digest and decompose the organic matter by feeding on their dead and dying tissues, creating humus. These organisms leave their waste behind as well as their dead bodies, thereby contributing again to the soil by becoming part of its organic matter. Proteins are changed to usable nitrogen compounds, and minerals such as phos phorus, potassium, and calcium are changed to soluble compounds.

As decomposition takes place, sugars, starches, and organic acids are destroyed quickly; fats, cellulose, proteins, waxes, and lignins take longer. Because lignins are resistant to breakdown, they become more abundant as the process of humification takes place. Carbohydrates are attacked by a variety of bacteria and fungi when the humus is well aerated. Cellulose, a fibrous structure in many plants, is decomposed more slowly than sugars and starches, and by a limited number of types of bacteria and fungi. These organisms require nitrogen to accomplish their cellulose decomposition, depriving living plants of nitrogen while the process is going on if an excess of cellulose material is present. Proteins are decomposed by numerous kinds of fungi and bacteria into amino acids and then ammonia, and finally into nitrate.

Rodents and earthworms move through the topsoil, mixing it and allowing air and water to easily permeate the material. The chemistry of the water in the soil varies in composition and concentration, and thus in the chemical changes effected. These conditions, in turn, determine the variety and abundance of living organisms that in turn influence the development and properties of the soil.

Topsoil is distinguishable from the other horizons in the soil profile because of its position and because humus usually is much darker, because organic matter coats the mineral particles. The thickness of topsoil depends on the rate of erosion and the rate of accumulation of organic matter, as well as the level of nutrient demand. It may be an inch thick, or several feet, but, in general, it is thicker on flat surfaces and thinner on slopes. The uppermost part of top-soil consists of fresh or partially decomposed litter and other plant parts that have recently fallen onto the surface. It is most conspicuous in forests but not so in grasslands, where it seldom occurs. When it does, cultivation destroys it. On grasslands the organic material dies annually, adding to the soil humus, including the root systems of perennial grasses that are replaced every few years.

The decomposition of fallen leaves and needles in forests produces organic acids, which increase the leaching power of percolating water and hasten the removal of soluble materials. As a result, grassland soils are neutral to basic, while most forest soils are acidic.

—Sidney Horenstein

See also: Deposition; Erosion; Geology, Geomor-phology, and Geography; Soil


Brady, Nyle, C. 2001. The Nature and Properties of Soils. Upper Saddle River, NJ: Prentice Hall; Montgomery, Carla. 1996. Fundamentals of Geology, 3rd ed. New York: McGraw Hill Professional Publishing; Tate, Robert 1992. Soil Organic Matter: Biological and Ecological Effects. Malabar, FL: Krieger Publishing.

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