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Soil Organic Matter
Beneficial impacts of SOM on soil properties:
- Physical - stabilizes soil structure, improves water holding characteristics,
lowers bulk density, dark color
may alter thermal properties
- Chemical - higher CEC, acts
as a pH buffer, ties up metals, interacts with xenobiotics
- Biological - supplies energy and body-building constituents
for soil organisms, increases microbial populations
and
their activities, source and sink for nutrients,
ecosystem resilience, affects soil enzymes
Each year, about 1 to 4% of nutrients
in the soil organic matter are released through microbial
transformations
to become available to plants. Release is highest under
warm, moist conditions and slowest in cool dry climates.
Microorganisms are the driving force for nutrient release
to plants.
The soil food web
(figures on this page from USDA NRCS 2001 Soil Planning
Guide)
Soil Microorganisms
Microorganisms constitute < 0.5%
(w/w) of the soil mass yet they have a major impact
on soil properties
and processes. 60-80 % of the total soil metabolism is
due to the microflora. In numbers, soil microorganisms
beat out all other organisms. One gram of topsoil may
contain:
- as many as one billion bacteria
- up to 100 million actinomycetes
- one million fungi
- 100 nematodes
Importance of Soil Organisms
- Responsible for cycling
of C, N and other nutrients
- Enhance soil structure
- Relocate and decompose organic materials
- Maintain soil quality and health
- Increase soil aeration and penetrability
- Involved in disease transmission and control
Plant Roots: the Rhizosphere
The narrow region of soil
directly around roots, teeming with bacteria that feed
on sloughed-off plant cells and the proteins and sugars
released by roots. Protozoa and nematodes graze
on bacteria also concentrated near roots.
Next page: The
soil fauna (or zoo)
Updated
July 15, 2004
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