reading material :UNIT 7 Soil Organic Matter &Soil Erosion


Soil organic matter
Cation exchange capacity
Dry combustion


Soil organic matter is not a single, readily definable entity, but includes a heterogeneous set of entities. Broadly interpreted, it includes all carbon-based components found in soil. These would include a biotic, or living, component comprised of plant roots and a myriad of micro- and mesoflora and fauna. The non-living segment represents material of plant, animal, or human origin in various stages of decomposition. Carbon compounds in the soil ecosystem face three fates; 1)oxidation and return of the carbon to the atmosphere as carbon dioxide, 2)assimilation into biomass, or 3)incorporation into humus.

Although organic matter is a minor component of soils (soils typically contain 1-5% organic matter), it performs and influences many essential functions in the ecosystem far out of proportion to the small quantities present (Brady and Weil, 1996). Practically all physical, chemical, and biological processes in soil show some dependence on organic matter. Because of this importance, organic matter has earned respect as the "life blood of soils" and warrants our investigation.

The origin of most soil organic matter is plant tissue. Soil organisms decompose plant tissue and synthesize from it a dark, amorphous colloid called humus. Humus is the active component of soil organic matter and enhances water retention, nutrient adsorption, aggregate stability, and pesticide adsorption, to identify a few key contributions. A simplified picture of humus formation from plant residue shows the involvement of enzymatic oxidation by microbes.

Plant residue + O2 Enzymatic oxidation CO2 + H2O + Energy + Humus
(reduced carbon )                                                              (microbial)

Humus is a relatively stable form of soil carbon, but it, too, can eventually be lost as microbes slowly oxidize the energy stored in its structure. A soil having 4% organic matter contains up to 180 million kilocalories of potential energy per acre furrow slice, equivalent to the heat value of 25 tons of coal. This energy is released slowly as microbes oxidize soil organic matter to support their growth. Microbial decomposition of humus operates continuously in nature, but slowly enough that scientists believe its half-
life to be decades, if not centuries. In this exercise, oxidation will be accelerated through use of a high-temperature laboratory combustion.

Organic Matter Content

An important criterium of soil quality is organic matter content. Color provides an estimate of organic matter content because the humus coating on mineral particles darkens the soil. Dark soils typically have chemical, physical, and biological conditions superior to those of light soils. Using color to judge organic matter content can be misleading, however, unless the comparison is made between soils of similar texture. Because of their low specific surface area, coarse-textured soils require less organic matter to look dark than do fine-textured, high specific surface area soils.

Accounting for soil organic matter content is an important management consideration. Soil testing labs can provide an accurate determination of soil organic matter content. This value has implications in decisions involving water relations, nutrient application, pH buffering, and especially, pesticide management.

Contribution of Organic Matter to Aggregate Stability

An important role of soil organic matter is through its impact on binding sand, silt, and clay particles into aggregates. Organic matter stabilizes these aggregates by forming bonds between the particles. The polysaccharide component of soil organic matter is frequently credited with initiating aggregate stability while other organic compounds lend long-term integrity to the aggregates.

Aggregation is necessary to create a favorable pore environment in the soil. Without sufficient porosity, or pores of the appropriate size, plant growth suffers and many soil processes are adversely affected. Aeration and drainage improve with the presence of macropores, while water distribution is more highly correlated to the presence of mesopores. Micropores are principally used for water storage.

When subjected to agricultural use for tilled crops, organic matter loss from soils through erosion and oxidation is frequently more that the crop's residue replaces. This net loss of soil organic matter is quickly manifested by the symptoms of poor aggregation. Residue conservation, green manuring with grass or legume cover crops, and the addition of composted materials all promote organic matter conservation.

Determination of Organic Matter Content by Dry Combustion

Dry combustion analysis of organic matter in soils is accomplished by measuring the weight loss in a soil sample following high temperature treatment. Heat oxidizes organic matter to CO2 and H2O which escape from the sample.

Note: high temperature can also cause weight loss from dehydration of minerals or decomposition of carbonates. Thus, this method only approximates soil organic matter content and should not be used on soils containing free carbonates.

C6H12O6 + 6 O2 + heat -->6CO2 + 6H20
(Organic Matter)

1. Place approximately 5g each of a low, medium, and high organic matter content soil into separate porcelain crucibles and weigh to the nearest 0.01 gram. These soils should have been previously oven-dried and stored in a dessicator.

2. Heat the crucibles to a red color over a Bunsen burner. Stir occasionally to aid complete oxidation of the organic matter. Oxidation is complete when the soil becomes light tan, usually in 1-2 hours. Alternatively, this step can be accomplished using a muffle furnace at 550 0C for 24 hr


3. Cool the samples and reweigh. Determine the loss in weight and calculate percent organic matter.

4. Save the soil for use in Part II.


Determination of Organic Matter Content by Dry Combustion.

                                                                                                  Soil Organic Matter Content

  Low Medium High
1. weight of soil and crucible before combustion      
2. weight of crucible      
3. weight of soil      
4. weight of soil and crucible after combustion      
5. weight lost (organic matter)      
6. organic matter, %      


1.Why did the soils change color after extensive heating?

2.What correlation can you draw between combustion losses of organic matter from soils in the lab and organic matter losses caused by soil management practices?