Soils and Decomposition


Soils are dynamic, complex systems of inorganic, organic and biotic components that have the capacity to support plant life. Soils are classified on the basis of their chemical and physical properties. These properties include texture, structure, colour and the nature and properties of soil horizons. Soils are an important component of natural capital, providing for food and fibre production and physical support of cultural infrastructure, but soils are degrading. Soils are the major site for plant nutrient regeneration through the process of decomposition. Decomposition processes also generate longā€term soil organic matter and play an important role in the global carbon cycle. Decomposition of organic matter in soils is accomplished largely by microorganisms, often in association with animals. The rate and course of decomposition is influenced by climate, organic matter composition and nutrient availability from the environment.

Key Concepts:

  • Soils are dynamic systems composed of organic, inorganic and living components.

  • Soils are classified according to measurable and observable properties, but there are many different classification schemes around the world.

  • Key physical characteristics of soils are texture, bulk density and porosity all of which influence nutrient and water dynamics in the soil.

  • Key chemical properties of soils include acidity (pH), cation exchange capacity (CEC), base saturation, organic matter content and nutrient availability.

  • Soils are a slowly renewable natural resource that is being degraded worldwide due to erosion, desertification, salinisation and overuse.

  • Decomposition of organic matter is regulated by climate and the nature and chemical composition of the organic matter.

  • Soil organisms, particularly microorganisms (fungi and bacteria) and animals, play a major role in the decomposition process.

  • Decomposition is important in regulating soil nutrient availability to plants.

  • Soils are an important reservoir for carbon in the global carbon cycle and sequester significant amounts of atmospheric carbon dioxide.

Keywords: soil taxonomy; paedogenesis; soil formation; soil degradation; organic matter; biogeochemistry; organic matter decomposition; humus; carbon sequestration; global carbon cycle

Figure 1.

Idealised soil profile. Many soils lack one or more of the master horizons or layers (O, A, E, B, C and R). O‐horizons are dominated by organic material. A‐horizons are formed at the surface or beneath an O‐horizon and are characterised by either humified organic material or surface disturbances such as pasturing or cultivation. E‐horizons are characterised by a loss of clay or minerals, leaving a higher concentration of sand and silt. B‐horizons are commonly called subsoil and are characterised by an influx and accumulation of clays. C‐horizons show little soil development and often resemble the parent material. R‐layers are bedrock. Subscripts are used to indicate particular characteristics. For example Oi, Oe and Oa denote organic horizons in which the litter is slightly decomposed (i), intermediately decomposed (e) or highly decomposed (a).

Figure 2.

Textural triangle. Soils textural classes can be determined on the basis of the percentage of sand, silt and clay. Knowing any two of these percentages allows the determination of textural class. From US Department of Agriculture (1996), Keys to Soil Taxonomy, 7th edn.

Figure 3.

Relationship between soil water availability and soil texture. As the textural class includes more silt and clay the total, available and unavailable portions of soil water all increase and gravitational water decreases. Data taken from Klocke and Hergert .

Figure 4.

Munsell Color chart (Munsell Color, ). Page from a field book indicating soil colour 5YR5/3 (reddish brown). The hue is the page (5YR), the row is the value (5/) and the column is the chroma (/3). Colours in this figure may not reproduce well and should not be used to describe soils.

Figure 5.

Schematic diagram of the structure of aluminosilicate clays. The basic units are silicon tetrahedra and aluminium octahedra. These units join together by sharing oxygen atoms to create two‐dimensional sheets. The sheets then align with one another in alternating layers to create a variety of crystalline lattices.

Figure 6.

Comparison of three models of decomposition. The single exponential model is the classical view and implies that all the material decays at a constant rate. The double exponential model assumes that organic matter consists of two parts, a rapidly decomposing component and a slowly decomposing component, and that each decays simultaneously with its own exponential rate of decay. The asymptotic model assumes that a certain portion of the organic matter will be metastable or resistant to decay and will persist for a long time. For these models the decay rate is 0.2 for the single exponential model. For the asymptotic model, the initial rate, which initially was 0.4, decreases continuously to reach the value of 0 at the asymptote, in this case 20% remaining, leaving 20% as metastable organic matter. The rates are 0.3 and 0.05 for the fast and slow components, respectively, of the double exponential model.



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Further Reading

Berg B and McClaugherty C (2008) Plant Litter: Decomposition, Humus Formation, Carbon Sequestration, 2nd edn. Berlin: Springer Verlag.

Brady NC and Weil RR (2010) Elements of the Nature and Properties of Soils, 3rd edn. Upper Saddle River, NJ: Prentice Hall.

Braimoh AK and Vlek PLG (2008) Land Use and Soil Resources. Dordrecht, The Netherlands: Springer Science & Business Media B.V.

Coleman DC and Crossley DA (1996) Fundamentals of Soil Ecology. San Diego, CA: Academic Press.

Parker SS (2010) Buried treasure: soil biodiversity and conservation. Biodiversity and Conservation 19(13): 3743–3756.

Roose EJ, Lal R, Fellar C, Barthès B and Stewart BA (2006) Soil Erosion and Carbon Dynamics. Boca Raton, FL: CRC Press.

Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture (2011) Official Soil Series Descriptions. Available online at (accessed 30 March 2011).

Swift MJ, Heal OW and Anderson JM (1979) Decomposition in Terrestrial Ecosystems. Berkeley, CA: University of California Press.

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McClaugherty, Charles, and Berg, Björn(Sep 2011) Soils and Decomposition. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0003187.pub2]