2. SOM
Soil organic matter is any material
•produced originally by living organisms
(plant or animal)
•that is returned to the soil and
•goes through the decomposition process
At any given time, it consists of a range of
materials from the intact original tissues of
plants and animals to the substantially
decomposed mixture of materials known
as humus
3. SOM
Soil organic matter content is a function of
organic matter inputs (residues and roots) and
litter decomposition.
It is related to
•Moisture, temperature and aeration,
•Physical and chemical properties of the soils
• Bioturbation (mixing by soil macrofauna),
leaching by water
• Humus stabilization (organomineral complexes
and aggregates).
Land use and management practices also affect
soil organic matter.
4. SOM
Soil is a complex array of mineral material,
air/water and organic matter.
Organic matter itself is composed of three basic
types:
• living organisms (microorganisms, earthworms,
insects etc)
•recently dead (micro, plant, and animal),
•very dead components (humic substances).
There can be a wide range of organic content
distribution among various soil types:
5. SOM
Soil organic matter is composed of
<10% fresh organic residue
(leaf litter or mulch in an agricultural setting)
• <5% living organisms
•33-50% humus
(plant material that has transformed from one organic
compound to another and is considered stabilized
organic matter)
•33-50% decomposing organic
matter (active fraction of SOM) (Brady and Weil, 2002).
6. Organic matter may be divided into
aboveground and belowground fractions.
Aboveground organic matter comprises
plant residues and animal residues;
Belowground organic matter consists of
living soil fauna and microflora, partially
decomposed plant and animal residues,
and humic substances.
7. The C:N ratio is also used to
indicate the type of material and
ease of decomposition
hard woody materials have a high
C:N ratio and more resilient
soft leafy materials have a low
C:N ratio.
9. Although soil organic matter can be partitioned conveniently
into different fractions, these do not represent static end
products.
Instead, the amounts present reflect a dynamic equilibrium.
The total amount and partitioning of organic matter in the soil is
influenced by soil properties and by the quantity of annual
inputs of plant and animal residues to the ecosystem.
For example, in a given soil ecosystem, the rate of
decomposition and accumulation of soil organic matter is
determined by such soil properties as texture, pH, temperature,
moisture, aeration, clay mineralogy and soil biological activities.
A complication is that soil organic matter in turn influences or
modifies many of these same soil properties.
10. Organic matter constitutes from 1 to 6% of the
topsoil weight of most upland soils.
Topsoils with < 1% organic matter are mostly
limited to desert areas.
At the other extreme, the organic matter
content of soils in low wet areas may be as high
as 90% of more.
Soils with more than 12-18% organic carbon
(approximately 20-30% organic matter) are
called “organic soils.”
11. Under stabilized environmental conditions, whether in virgin
soil or cultivated soil, there is eventually established a fairly
constant level of organic matter.
This state of equilibrium is reached when carbon loss, chiefly
as carbon dioxide, balances the gain through the additions of
vegetable matter. Such equilibrium may be reached under
some soil conditions when the soil contains less than 1% of
organic matter
Under other conditions there may be 10% or more.
Under very extreme conditions, where drainage and soil
aeration may be unsatisfactory, the soil may contain more
organic matter than mineral matter, in which case it is
classified as a muck or peat.
12. • In general, more organic matter is present near the
surface than deeper in the soil
• This is one of the main reasons that top soils are
more productive than sub soils exposed by erosion
or mechanical removal of surface soil layers.
• Some of the plant residues that eventually become
part of the soil organic matter are from the above
ground portion of plants.
13. • In most cases, plant roots are believed to
contribute more to a soil’s organic matter
than the crop’s shoots and leaves.
• But when the plant dies or sheds leaves or
branches, depositing residues on the surface,
earthworms and insects help incorporate the
residues on the surface deeper into the soil.
• The highest concentrations of organic matter,
however, remain within 1 foot of the surface.
14.
15. • Litter layers that commonly develop on the surface
of forest soils may have very high organic matter
contents (figure 3.5a).
• Plowing forest soils after removal of the trees
incorporates the litter layers into the mineral soil.
The incorporated litter decomposes rapidly.
• An agricultural soil derived from a sandy forest soil
or a silt loam would likely have a distribution of
organic matter similar to that indicated in figure
3.5b.
• Soils of the tall-grass prairies have a deeper
distribution of organic matter (see figure 3.5c).
• After cultivation of these soils (prairies) for 50
years, far less organic matter remains (figure 3.5d).
16. Most soils Horizon A contains between 1 and 6% humic acid,
this represents between 20,000 and 120,000 lbs of organic matter in
an average acre furrow slice (about 2 million lbs of soil).
The organic matter in the full solum (horizons A and B) is considered
to be 2 to 3 times that of the furrow slice (Franzmeier, Lemme, and
Miles 1985).
Fresh residues typically represent less than one tenth of the weight
of soil organic matter – the rest being Humus. Some consider the
difference too small to make any distinction between humus and soil
organic matter,
others choose to classify the organic materials according to mode of
origin:
Living Macroorganisms
Identifiable remains of dead macroorganisms
Living Microbes
Finely divided nonliving organic materials (essentially all humus)
17. The estimated average contents of Organic Materials per acre in a soil profile formed under grass in
a subhumid temperate region can be found in the following table:
18. Organic matter occurs outside of aggregates as living roots or larger organisms
or pieces of residue from a past harvest.
Some organic matter is even more intimately associated with soil. Humic
materials may be adsorbed onto clay and small silt particles, and small to
medium-size aggregates usually contain particles of organic matter.
The organic matter inside very small aggregates is physically protected from
decomposition because microorganisms and their enzymes can’t reach inside.
This organic matter also attaches to mineral particles and thereby makes the
small particles stick together better.
The larger soil aggregates, composed of many smaller ones, are held together
primarily by the hyphae of fungi with their sticky secretions, by sticky
substances produced by other microorganisms, and by roots and their
secretions.
Microorganisms are also found in very small pores within larger aggregates.
This can sometimes protect them from their larger predators—paramecium,
amoeba, and nematodes.
19. There is an interrelationship between the amount of fines
(silt and clay) in a soil and the amount of organic matter
needed to produce stable aggregates.
The higher the clay and silt content, the more OM is needed
to produce stable aggregates, because more is needed to
occupy the surface sites on the minerals during the process
of organic matter accumulation.
In order to have more than half of the soil composed of
water-stable aggregates, a soil with 50% clay may need
twice as much organic matter as a soil with 10% clay.
20.
21. Functions of SOM
Decaying organic matter acts as a food material for bacteria, fungi and
other organisms.
Presence of organic matter dissolves many insoluble soil minerals and
make them available to plants
It plays an important role in the nutrient supplying power of soil as it has
got high cation exchange capacity (CEC)
It increases the water holding capacity of the soil, particularly in sandy
soils.
It improves aeration and infiltration in heavy soils.
It reduces loss of soil by water and wind erosion
It regulate soil temperature
It serves as an important source of certain plant of food element (N, P, S
etc).
The buffering nature of the organic matter is considered to be
advantageous in the residue management of pesticides, herbicide and
other heavy metals.
