The pie chart represents organic matter in soil before cultivation.
After land has been cultivated for one or two decades, much of the
active fraction is lost and stabilized organic matter makes up more
than half of the soil organic matter.
SOM's effects on soil functioning includes improvements
related to soil structure, aggregation, water retention,
soil biodiversity, absorption and retention of pollutants,
buffering capacity, and the cycling and storage of plant
nutrients. SOM increases soil fertility by providing cation
exchange sites and acting as reserve of essential
nutrients, especially nitrogen (N), phosphorus (P), and
sulfur (S), along with micronutrients, which are slowly
released upon SOM mineralization.
As such, there is a significant correlation between SOM
content and soil fertility.
The mass of SOM in soils as a percent generally ranges from 1 to
6% of the total topsoil mass for most upland soils.
Soils whose upper horizons consist of less than 1% organic matter
are mostly limited to desert areas, while the SOM content of soils
in low-lying, wet areas can be as high as 90%.
Soils containing 12-18% SOC are generally classified as organic
A good soil should have an organic matter content of more than
3.5 per cent. But in Bangladesh, most soils have less than 1.7%,
and some soils have even less than 1 % organic matter.
Organic matter status of Bangladesh Soils
AEZ No. Main Locations Total
1, 7, 8, 10,
11, 16, 25,
26, 27, 28,
Dinajpur, Sherpur, Jamalpur,
Rajshahi, Pabna, Kushtia, Bogra,
Naogaon, Rangpur, Khagrachari,
Adjoining areas of Tista, Dharlus
Noakhali, Bhola, Barisal
Patuakhali, Narsingdi, Dhaka
Kishoreganj, Sherpur, Jamalpur,
Sylhet, Moulvibazar, Feni, Cox's
Panchagaher, Natore, Naogaon,
Khulna, Satkhira, Madaripur,
Gopalganj, Munshiganj, Habiganj
Source: Land Degradation Situation in Bangladesh, Soils Division,BARC, 1999
The Changing Forms of Soil Organic Matter
Additions. When roots and leaves die, they become part of the soil
Transformations. Soil organisms continually change organic
compounds from one form to another. They consume plant residue
and other organic matter, and then create by-products, wastes, and
cell tissue. Microbes feed plants. Some of the wastes released by soil
organisms are nutrients that can be used by plants. Organisms release
other compounds that affect plant growth.
Stabilization of organic matter. Eventually, soil organic compounds
become stabilized and resistant to further changes.
Role of Soil in carbon cycling
Soil plays a major role in the global carbon cycle, with the
global soil carbon pool is estimated at 2500 gigatons. This
is 3.3 times the size of the atmospheric pool (750 gigatons)
and 4.5 times the biotic pool (560 gigatons).
The pool of organic carbon, which occurs primarily in the
form of SOM, accounts roughly 1550 gigatons of the total
global C pool, with the remainder accounted for by soil
inorganic carbon (SIC).
The pool of organic C exists in dynamic equilibrium
between gains and losses; soil may therefore serve as either
a sink or source of C, through sequestration or greenhouse
gas emissions, respectively, depending on exogenous
factors (Lal, 2004).
What Does Organic Matter Do In Soil?
Of all the components of soil, organic matter is
probably the most important and most
Organic matter serves as a reservoir of nutrients and
water in the soil, aids in reducing compaction and
surface crusting, and increases water infiltration into
Yet it's often ignored and neglected.
What Are the Benefits of Organic Matter?
Nutrient Supply: Organic matter is a reservoir of
nutrients that can be released to the soil. Each percent
of organic matter in the soil releases 20 to 30 pounds
of nitrogen, 4.5 to 6.6 pounds of P2O5, and 2 to 3
pounds of sulfur per year.
The nutrient release occurs predominantly in the
spring and summer, so summer crops benefit more
from organic-matter mineralization than winter crops.
Water-Holding Capacity: Organic matter behaves
somewhat like a sponge, with the ability to absorb and
hold up to 90 percent of its weight in water. A great
advantage of the water-holding capacity of organic
matter is that the matter will release most of the water
that it absorbs to plants. In contrast, clay holds great
quantities of water, but much of it is unavailable to
Soil Structure Aggregation: Organic matter causes soil
to clump and form soil aggregates, which improves soil
structure. With better soil structure, permeability
(infiltration of water through the soil) improves, in
turn improving the soil's ability to take up and hold
Erosion Prevention: This property of organic matter is
not widely known. Data used in the universal soil loss
equation indicate that increasing soil organic matter
from 1 to 3 percent can reduce erosion 20 to 33 percent
because of increased water infiltration and stable soil
aggregate formation caused by organic matter.
How Much Organic Matter Is in the Soil?
An acre of soil measured to a depth of 6 inches weighs
approximately 2,000,000 pounds, which means that 1
percent organic matter in the soil would weigh about
20,000 pounds per acre.
Remember that it takes at least 10 pounds of organic
material to decompose to 1 pound of organic matter, so
it takes at least 200,000 pounds (100 tons) of organic
material applied or returned to the soil to add 1
percent stable organic matter under favorable
Measuring soil organic matter
The most common methods for measuring soil organic matter in
current use actually measure the amount of carbon in the soil.
This is done by oxidising the carbon and measuring either the amount
of oxidant used (wet oxidation, usually using dichromate) or the CO₂
given off in the process (combustion method with specific detection).
Laboratories these days generally report results as soil organic carbon.
Those that report as soil organic matter have usually measured carbon
and converted to organic matter by multiplying by 1.72.
However, this conversion factor is not the same for all soils, and it is
more precise to report soil carbon rather than organic matter.
How Can We Maintain or Improve Soil Organic Matter Levels?
Building soil organic matter is a long-term process but
can be beneficial.
Here are a few ways to do it.
Reduce or Eliminate Tillage: Tillage improves the
aeration of the soil and causes a flush of microbial
action that speeds up the decomposition of organic
matter. Tillage also often increases erosion. No-till
practices can help build organic matter.
Reduce Erosion: Most soil organic matter is in the
topsoil. When soil erodes, organic matter goes with it.
Saving soil and soil organic matter go hand in hand.
Soil-Test and Fertilize Properly: You may not have
considered this one. Proper fertilization encourages
growth of plants, which increases root growth.
Increased root growth can help build or maintain soil
organic matter, even if you are removing much of the
Cover Crops: Growing cover crops can help build or
maintain soil organic matter. However, best results are
achieved if growing cover crops is combined with
tillage reduction and erosion control measures.
What Determines Soil Organic Matter Levels?
The amount of organic matter in soil is the result of
two processes: the addition of organic matter (roots,
surface residue, manure, etc.), and the loss of organic
matter through decomposition.
Five factors affect both additions and losses.
Practices that increase plant growth on a field (cover crops,
irrigation, etc.) will increase the amount of roots and residue
added to the soil each year.
Intensive tillage increases the loss of organic matter by speeding
decomposition. While tillage primarily burns younger organic
matter, older, protected organic compounds can be exposed to
decomposition if small aggregates are broken apart. In addition
to changing the amount of soil organic matter, tillage practices
affect the depth of soil organic matter.
2. Soil texture:
Fine-textured soils can hold much more organic matter than
sandy soils for two reasons. First, clay particles form
electrochemical bonds that hold organic compounds. Second,
decomposition occurs faster in well-aerated sandy soils. A sandy
loam rarely holds more than 2% organic matter.
High temperatures speed up the degradation of organic matter. In
areas of high precipitation (or irrigation) there is more plant growth
and therefore more roots and residues entering the soil.
4. Landscape position:
Low, poorly-drained areas have higher organic matter levels, because
less oxygen is available in the soil for decomposition. Low spots also
accumulate organic matter that erodes off hill tops and steep slopes.
In prairies, much of the organic matter that dies and is added to the
soil each year comes from grass roots that extend deep into the soil. In
forests, the organic matter comes from leaves that are dropped on the
surface of the soil. Thus, farmland that was once prairie will have
higher amounts of organic matter deep in the soil than lands that were
Differences in organic matter appear in the color
changes across a bare field. Less organic matter is
produced on the drier hilltops, and some is lost to
soil erosion and deposited in low spots.
How do organic matter levels change?
Most organic matter losses in soil occurred in the first
decade or two after land was cultivated.
Native levels of organic matter may not be possible
under agriculture, but many farmers can increase the
amount of active organic matter by reducing tillage
and increasing organic inputs.
An illustration of soil organic matter losses and
gains in response to tillage.
Does more carbon mean that crops grow better?
This is a hard question to answer, because there are so
many things that affect how crops perform.
We have looked at this in two ways: how crop yields
and soil properties vary in different parts of the same
paddocks, and secondly, how crop yields and soil
properties vary from paddock to paddock.
Results comparing different poppy paddocks show no
clear relationship between soil carbon and yield. This
is perhaps not surprising because other important
factors like water, fertilizer, pest and disease
management also varied from paddock to paddock.
Organic matter is an important soil property. It is
associated with a variety of other important soil physical,
chemical and biological characteristics.
Cool climate favours the retention of organic matter to a
greater extent than is the case in warmer regions.
It is not easy to show a clear link between soil organic
matter concentrations and crop yield and quality. Many
other factors affect yield and quality to a greater extent.
Nevertheless, there is sufficient evidence linking organic
matter and yield to suggest that farmers should strive to
maintain or increase organic matter in their soils.
Maximising inputs of organic matter by incorporating crop
residues and including green manures and pastures in the
rotation, where practical, should be a goal for all farmers.