SOIL ORGANIC MATTER NOTES
Soil Organic Matter (OM)
Includes ALL the organic components of the soil: 1) living biomass (plant and animal tissue and microbes), 2) detritus (e.g., dead roots and other recognizable plant residues), and 3) humus (i.e., humic substances and non-humic substances) is a largely amorphous and colloidal mixture of complex organic substances no longer identifiable as plant tissues.
OM may be roughly estimated as 1.72 times the value of the organic carbon (~58% of
OM is equal to the organic carbon).
The upper 6" generally represents the surface soil which is most readily influenced by land use and soil management.
Soil OM greatly affects the biology of the soil because it provides the "main" food source for the community of heterotrophic soil organisms.
Soil OM is the sum of different POOLS of soil OM, i.e., ACTIVE (e.g, living biomass, some detritus, and non-humic-sub; comprises about 10-20% of the total soil OM), SLOW
(intermediate in properties between the active and passive fractions), and PASSIVE fractions (includes most of the humus physically protected in clay-humus complexes, most of the humin, and much of the humic acids; the passive fraction accounts for 60-
90% of the OM in most soils).
The susceptibility of the ACTIVE fraction to rapid changes explains why even relatively small changes in total soil OM can produce dramatic changes in important soil properties, such as aggregate stability and N mineralization, which are associated with this OM fraction.
Humic substances comprise about 60-80% of the soil OM; includes fulvic acids, humic acids, and humin which are the most complex organic carbon, and are the most resistant to microbial attack. However, despite its relative resistance to decay, humic substances are subject to continual microbial attack (consequently, without the ANNUAL addition of sufficient plant residues, microbial oxidation results in a reduction in soil OM levels).
Non-humic substances: about 20-30% of the humus in soils consist of non-humicsubstances (e.g., polysaccharides, which are "important" in enhancing soil aggregate stability); these substances are less complex and less resistant to microbial attack than those of humic-substances. They includes specific biomolecules (e.g., organic acids) with definite physical and chemical properties. They are microbially modified plant compounds, while others are compounds synthesized by the soil microbes as BY-
PRODUCTS of decomposition. Granulation and aggregate stability are encouraged by the non-humic-substances.
Plant Residues
The amount of plant residues returned to the soil is generally higher in fine-textured soils because of the greater nutrient and water-holding capacities of these soils promote greater plant production.
The management that most influences soil OM contents are cropping and tillage.
The composition of representative green plant materials includes: 1) cellulose (45%), 2) hemicellulose (20%), 3) lignin (20%), 4) proteins (8%), 5) sugars and starches (5%), and
6) fats and waxes (2%).

Green plant material is approximately 75% water and 25% dry matter.
The organic carbon content of most plant residues is about 40%.
The PRIMARY source of soil OM is plant residues.
Micro-Organisms
About 1/3 of the organic carbon metabolized by microbes is incorporated into their cells
(the remainder is respired and lost as CO
2
).
The release of organic carbon as CO
2
by the oxidation of soil OM (mostly by microbial respiration) is balanced by the input of organic carbon into the soil as plant residues.
OM supplies "ENERGY" and "body-building constituents" for most of the microbes.
Autochthonous organisms are the principal microbes (small populations) that are slowly and steadily metabolizing the very resistant, soil OM (humus).
On the average soil microbes must incorporate into their cells about 8 parts of organic carbon for every one part of Nitrogen (N) (i.e., the microbes have an average C:N ratio of
8:1).
As soon as the plant residues contact the soil, the microbial community responds to the new FOOD SUPPLY (i.e., the heterotrophic zymogenous microbes become active, multiply rapidly, and yield CO
2
in large quantities). Because of the microbes demand for
N, little or no mineral N is available to higher plants during this period.
Microbial Respiration
In a well-aerated soil, ALL of the organic compounds found in plant residues are subject to oxidation. * Approximately 2.5% of the total soil humus is mineralized per year
(consequently, under equilibrium conditions, about 2.5% humus is formed each year
(humification) from plant residues).
Microbial decomposition proceeds most rapidly in the presence of plentiful supplies of
O
2
, which aerobic microbes use to accept the electrons produced as they oxidize OM. The peak of microbial activity often stimulates the breakdown of even the original, resistant,
OM (i.e., humus); a phenomenon known as the priming effect.
If the C:N ratio of the OM (e.g., plant residues and manure) added to the soil exceeds about 25:1, the microbes will "scavenge" the soil solution to obtain enough N.
N, phosphorus, sulfur, and micro nutrients are stored as constituents of soil OM until released by mineralization.
Carbon/Nitrogen (C/N) Ratio
Because the C:N ratio is relative constant in soils, the maintenance of organic carbon and hence soil OM, is constrained by the soil N level. The C:N ratio is therefore an important consideration in developing sound soil management schemes.
The C:N ratio of arable (cultivated) surface (Ap) horizon commonly ranges from 10:1 to
12:1 (this ratio is generally lower for subsoils).
Generally, one can expect organic N to begin to be mineralized when the C:N ratio of the remaining OM (plant residues or manure) drops below about 20:1.
Organic Nitrogen can be estimated from the organic carbon values by dividing by 12 for most soils.
Humus Formation

While most of the organic carbon (from plant residues or manure) has returned to the atmosphere as CO
2
, about 20-33% remains in the soil as newly formed humus.
Because the N content of soil OM generally remains constant at about 5%, the amount of
OM that can be maintained in any soil is largely dependent on the amount of N present; the soil’s OM content cannot be increased without increasing its organic N content and vice versa.
Humus interaction (i.e., complexes) with clay provides another means of stabilizing soil organic N and organic carbon. In many soils more than ½ the OM is associated with clay and other inorganic constituents. Soils high in clay and silt are generally higher in OM.
Humus plays a "very important" role in aggregate formation and stability.