Functions of Organic Matter in Soils

The SOM supplies nearly all the N, 50 to 60% of the P, perhaps as much as 80% of the S, and a large part of the B and Mo adsorbed by plants from unfertilized, temperate region soils. Indirectly, SOM affects the supply of essential elements from other sources. The amount of N2 fixation by the free-living bacterium Azotobacter, for example, is related to the amount of readily available energy sources in the soil, such as the carbohydrates in SOM. 

In humid soils, C, N, and S are found predominantly in SOM. With increasing aridity, the amount of organic matter decreases and the fractions of the inorganic forms of the elements (carbonate, sulfate, and nitrate) tend to increase. 

The mass ratio of C/N/S in the SOM of temperate region soils is roughly 100/10/1. Carbon supplies the energy for N and S reduction, as well as the matrix of compounds into which reduced N and S are incorporated and stabilized. Nitrogen 

Color The typical dark color of many soils is caused by organic matter May facilitate warming 

Water retention Organic matter can hold up to 20 times its weight in water Helps prevent drying and shrinking improves moisture retention in sandy soils 

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The rate of organic residue decomposition in soils and related environments is ultimately controlled by its biological stability, which is a function of the following four main factors, namely, (i) its biochemical recalcitrance, (ii) the biological capability and capacity of the environment, (iii) decomposition rate modifiers (e.g., temperature, moisture, exposure time) and (iv) physical protection mechanisms (Baldock et al., 2004). Recent studies have shown that the physical protection mechanisms, such as the spatial inaccessibility of organic matter in soil micropores, are the most important factors in controlling the stability of organic matter in soils (Mikutta et al., 2006 von Liitzow et al., 2006). 

The great importance of chelation in nature has been stressed by Schatz (1963), and also in several other articles by the same author. In one of these the opinion is expressed that since the chelating ability of organic matter may be its most important property and function, the relatively neglected metal chelates may turn out to be the most important constituents of soils. This statement may be a little optimistic but there is no doubt that one of the main benefits derived from the presence of organic matter in soils is... 

Carrizosa MJ, Hermosin MC, Koskinen WC, Cornejo J (2004) Interactions of two sulfonylurea herbicides with organoclays. Clays Clay Miner 52 643-649 Celis R, Hermosin MC, Cornejo J (2000) Heavy metal adsorption by functionalized clays. Environ Sci Technol 34 4593-4599 Chappell MA, Laird DA, Thompson ML, Li H, Teppen BJ, Johnston CT, Boyd SA (2005) Influence of smectite hydration and swelling on atrazine sorption behavior. Environ. Sci Technol 39 3150-3156 Chiou CT (1989) Theoretical considerations of the partition uptake of nonionic organic compounds by soil organic matter. In Sawhney BL, Brown K (eds) Reactions and movement of organic chemicals in soils. Soil Science Society of America, Madison, WI, pp 1-29... 

Herbert, B. E., and P. M. Bertsch. 1995. Characterization of dissolved and colloidal organic matter in soil solution A review. In Carbon Forms and Functions in Forest Soils (W. W. McFee and J. M. Kelly, Eds.), pp. 63-88, Soil Science Society of America, Madison, WI. 

In partnership with the clay fraction, organic matter has an extremely important influence on the chemical and physical properties of soils. Critical and beneficial functions of organic matter include ... 

FUNCTIONS AND EFFECTS OF ORGANIC MATTER IN MINERAL SOILS... 

FIGURE 9.11 Thermal conductivity of soil (a) thermal conductivity as a function of density at three moisture contents in % (sand), (b) thermal conductivity as a function of density at three moisture contents in % (clay, loam), (c) thermal conductivity of clay loam as a function of organic matter content in %. Data from Abu-Hamdeh and Reeder (2000). 

Figure 4.5. Cumulative sums of extractable elements in soil J3 (CaC03 33.3%) in consecutive steps of the selective sequential dissolution procedure as a function of the cumulative steps. 1 CARB (carbonate) 2 CARB (carbonate) + ERO (easily reducible oxide) 3 CARB + ERO + OM (organic matter) 4 CARB + ERO + OM + RO (reducible oxide) (after Han and Banin, 1995. Reprinted from Cornmun Soil Sci Plant Anal, 26, Han and Banin A., Selective sequential dissolution techniques for trace metals in arid-zone soils The carbonate dissolution step, p 573, Copyright (1995), with permission from Taylor Francis US)...

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