Download Soil Organic Matter

Soil Organic Matter
• Organic matter
encompasses all organic
components of a soil:
– Fresh residues
– Decomposing
organic matter
– Stable organic matter
– Living organisms
Fresh Residues
• Up to 15% of organic
matter is fresh residue
• Comprised mainly of
litter fall
• Much can be recognized
as plant residue
Decomposing Organic Matter
• Plant material is
transformed from one
organic compound to
another mainly by
organisms in the soil
• Organisms create byproducts, wastes, and cell
tissue
• Compounds released as
waste by one organisms
can often be used as food
by another
Soil Organic Matter =SOM
• SOM is labile -it can decline rapidly if the
soil environment changes and renewable -it
can be replenished by inputs of organic
material to the soil.
• Adequate levels of SOM can be maintained
with proper fertilization, crop rotations,
and tillage practices if crop residues are
returned to the soil.
Soil Organisms
• A cubic meter of healthy soil =
• home to billions of bacteria and fungi,
millions of nematodes, tens of thousands of
spring tails and mites and several hundred
earthworms
• These "critters" are nature's recyclers,
converting plant residue and animal
manures into usable nutrients and soil
organic matter.
Soil Organisms
• The soil microflora (bacteria, fungi
and algae) are responsible for 90% of
the decomposition of organic
material.
• Plant residue is the main fuel for
sustaining the broad spectrum of
organisms residing in the soil.
Soil Organisms
• The microbial population can increase
rapidly to take advantage of a favorable
change in the soil environment, doubling in
a few hours.
• The size of the microbial population is
soil moisture,
aeration, temperature and their
distribution in the soil.
usually controlled by
Active Fraction
• 10 to 30% of the soil organic matter
(active fraction) is responsible for
maintaining soil microorganisms.
• The active fraction of organic
matter is most susceptible to soil
management practices. (Inactive =
humus)
Adding Fresh OM
• In a soil which at first has no readily
decomposable materials, adding fresh
tissue under favorable conditions:
• 1) immediately starts rapid multiplication
of bacteria, fungi, and actinomycetes,
• 2) which are soon actively decomposing the
fresh tissue.
Fresh SOM
• as most readily available energy
sources are used up, microorganisms
again become relatively inactive,
leaving behind a dark mixture usually
referred to as humus – a stable
organic compound
Stable Organic Matter
• Soil organic compounds
become stabilized and
resistant to further
changes by
microorganisms
• Bound inside soil
aggregates
• Stabilized organic
matter acts like a
sponge and can absorb
six times its weight in
water
HUMUS
• Newly-formed humus - a) combination of
resistant materials from the original plant
tissue, b) compounds synthesized as part
of the microorganisms' tissue which remain
as the organisms die. (Fluvic and Humic
Acid)
• humus is resistant to further microbial
attack- N and P are protected from ready
solubility.
Function of Humus
• holds water and nutrients;
• it sticks together & helps establish and
maintain a strong crumb structure & thus
reduce soil erosion
• it provides some nutrients (N & P) as it is
slowly decayed by microbial activity,
• Buffers effects of pesticides
• humus decomposes at the rate of 2.5% per
year
SOM Maintains soil “Tilth”
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aiding infiltration of air and
water
promoting water retention
reducing erosion
SOM = SOIL HEALTH
• Measuring SOM is one step in
assessing overall soil quality or
soil health • measuring various key attributes
of soil organic matter quantity
and quality will give an indication
of the health of the soil.
Soil Food Web
• The community of organisms living all
or part of their lives in•the
soil
Use sun’s energy to
fix carbon dioxide
• Fueled by primary producers
from atmosphere
– plants, lichens, moss, photosynthetic
• Get energy/carbon
bacteria, and algae
by consuming
organic compounds
found in plants, other
organisms, and waste
by-products
Soil Food Web Diagram
Microbial Biomass
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Around roots
In litter
On humus
On the surface
of soil aggregates
• In spaces between
soil aggregates