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Monday, May 28, 2012

Soil organic matter SOM;

How does vegetation affect soil organic matter?

What are soil forming factors?

What is the organic matter in soils?

Soil organic matter SOM is a complex mixture of material with heterogeneous biological, physical, and chemical properties. Soils represent a major pool in the cycling of carbon from the atmosphere to the biosphere and are the habitat for terrestrial photosynthetic, about half of which usually finds its way into soils.

What is the organic matter in soils

Organic matter in soils is the plant debris or litter in various stages of decomposition of decay vegetations to humus and includes the living organisms in the soil. While above ground plants are excluded from consideration of soil organic matter, but living roots are considered of soil organic matter, and see the impact overgrazing on soil organic matter.

Soil studying

It is likely to consider the following definitions:
1.    Soil organic matter: is a complex mixture of material with heterogeneous biological, physical, and chemical properties.
2.    Natural C-containing: organic living or dead but charcoal is excluded.
3.    Phytomass: is the above ground portion of plant material origin usually living and may include standing dead trees.
4.    Microbial biomass: is the living population of soil microorganisms.
5.    Litter: comprises the dead plant and animal debris and decay on the surface.
6.    Macro organic matter: these are organic fragments from any source and generally less decomposed than humus.
7.    Organic carbon: carbon content is commonly used to characterize the amount of organic matter on soils.
8.    Humus: material remaining in soils after removal of macro organic matter.
9.    Humic acids: dark colored amorphous materials that can be extracted from the soil by variety of reagents like strong bases neutral salts and that are insoluble in dilute acid. Humic acids are considered to be polymerization products of fulvic acids and other decay products.
10. Fulvic acids: the organic materials that are extracted with humic acid but remain in solution upon acidification with dilute acid. Fulvic acids are considered to be decay products of higher plants and microbial residue.
11. Humin: the strong base insoluble fraction.
12. Carbon cycle: the carbon cycle describes how carbon is circulated through the atmosphere, biosphere, lithosphere and hydrosphere. The dead organic matter of the soil is useful for microorganisms, which derive energy for growth from the oxidative decomposition of complex organic molecules.
13. Decomposition: is the biochemical break down of mineral and organic materials. During decomposition, organic elements are converted from organic compounds, this process is called mineralization.
14. Humification: is the formation of humus from raw organic materials such as fulvic acids, humic acids, or humin. 

Soil forming factors

Climate, precipitation, and temperature influence the amount and type of vegetation, as well as the rate of decomposition. The organic matter content of soil increases with increasing decomposition up to the limit set by temperature. Cold and arid climate tends to slow the microbial process within soil particularly in decomposition and mineralization. The soil moisture content also has a remarkable effect of soil organic matter decomposition and accumulation.

Vegetation and Soil organisms

Vegetation affects soil organic matter by the amount and placement of organic residues. The composition of organic matter in soil can be related to the nature of the soil floral and faunal community. 


Topography affects the amount of surface runoff, erosion and deposition. When erosion removes soil from the shoulder or back slope areas of the hill thinner and light colored soils remain where the organic matter content is low. Soils found on foot slop or toe slope areas always show a higher organic matter content, as soil moisture often differs across a hill slope, microbial activity is affected as well. 

Parent Material

Parent material, it is found less organic matter on sandy soils than on silty or clay soils. Sandy soils are well aerated and tend to have low soil moisture content, whereas clayey soils are less aerated with high amount of fine micro pores and tend to have higher soil moisture content than medium that means to have higher organic matter content. 

Turnover time

Turnover times for organic C in soils can be derived by dividing the organic matter content of the soil by the annual biomass input and expressing the answer in years. The turnover time for global C is 30 to 40 years, but varies by orders of magnitude for different ecosystems. This turnover time differs from place to place as well as according to the different climate conditions.
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