Soil
organic matter (SOM) is the organic component of soil, consisting of three
primary parts including small (fresh) plant residues and small living soil
organisms, decomposing (active) organic matter, and stable organic matter
(humus).
Soil
organic matter serves as a reservoir of nutrients for crops, provides soil
aggregation, increases nutrient exchange, retains moisture, reduces compaction,
reduces surface crusting, and increases water infiltration into soil.
Components
vary in proportion and have many intermediate stages. Plant residues on the
soil surface such as leaves, manure, or crop residue are not considered SOM and
are usually removed from soil samples by sieving through a 2 mm wire mesh
before analysis.
Soil
organic matter content can be estimated in the field and tested in a lab to
provide estimates for Nitrogen, Phosphorus and Sulfur mineralized available for
crop production and adjust fertilizer recommendations.
Soil
organic matter impacts the rate of surface applied herbicides along with soil
pH necessary to effectively control weeds. Soil organic matter impacts the
potential for herbicide carryover for future crops, and amount of lime
necessary to raise pH.
By
the end of this article, you will be able to:
· Explain
what organic matter is
· Outline
the major sources of organic matter.
· Importance
of soil organic matter
What is Soil Organic Matter?
The
term “soil organic matter” (SOM) has been used in different ways to describe
the organic constituents of soil.
Baldock
and Skjemstad (1999) defined SOM as “all organic materials found in soils
irrespective of origin or state of decomposition”. SOM consists of C, H, O, N,
P and S. Included are living organic matter (plants, microbial biomass and
faunal biomass), dissolved organic matter, particulate organic matter, humus
and inert or highly carbonized organic matter.
Part
of soil organic matter consists of carbohydrates, lipids and proteins that are
abundant in fresh plant residues. These are rapidly metabolized, immobilized or
decomposed.
Organic
matter is categorized into aboveground and belowground organic. Aboveground
organic matter includes plants and animal residues while the belowground matter
comprises of living soil fauna and micro flora, partially decomposed plant and
animal.
7 Importance of Organic Matter to the Soil
1. Soil
organic matter serves as a reservoir of nutrients for crops
2. Provides
soil aggregation
3. Increases
nutrient exchange
4. Retains
moisture
5. Reduces
compaction
6. Reduces
surface crusting
7. Increases
water infiltration into soil
Healthy
soil is the foundation of the food system. It produces healthy crops that in
turn nourish people. Maintaining a healthy soil demands care and effort from
farmers because farming is not benign. By definition, farming disturbs the
natural soil processes including that of nutrient cycling - the release and uptake
of nutrients.
Plants
obtain nutrients from two natural sources: organic matter and minerals. Organic
matter includes any plant or animal material that returns to the soil and goes
through the decomposition process. In addition to providing nutrients and habitat
to organisms living in the soil, organic matter also binds soil particles into
aggregates and improves the water holding capacity of soil. Most soils contain
2-10 percent organic matter. However, even in small amounts, organic matter is
very important.
Soil
is a living, dynamic ecosystem. Healthy soil is teeming with microscopic and
larger organisms that perform many vital functions including converting dead
and decaying matter as well as minerals to plant nutrients. Different soil
organisms feed on different organic substrates. Their biological activity
depends on the organic matter supply.
Nutrient
exchanges between organic matter, water and soil are essential to soil
fertility and need to be maintained for sustainable production purposes. Where
the soil is exploited for crop production without restoring the organic matter
and nutrient contents and maintaining a good structure, the nutrient cycles are
broken, soil fertility declines and the balance in the agro-ecosystem is
destroyed.
Soil
organic matter - the product of on-site biological decomposition - affects the
chemical and physical properties of the soil and its overall health. Its
composition and breakdown rate affect: the soil structure and porosity; the
water infiltration rate and moisture holding capacity of soils; the diversity
and biological activity of soil organisms; and plant nutrient availability.
Many
common agricultural practices, especially ploughing, disc-tillage and
vegetation burning, accelerate the decomposition of soil organic matter and
leave the soil susceptible to wind and water erosion. However, there are
alternative management practices that enhance soil health and allow sustained
agricultural productivity.
Conservation
agriculture encompasses a range of such good practices through combining no
tillage or minimum tillage with a protective crop cover and crop rotations. It
maintains surface residues, roots and soil organic matter, helps control weeds,
and enhances soil aggregation and intact large pores, in turn allowing water
infiltration and reducing runoff and erosion.
In
addition to making plant nutrients available, the diverse soil organisms that
thrive in such conditions contribute to pest control and other vital ecological
processes. Through combining pasture and fodder species and manuring with food
and fibre crop production, mixed crop-livestock systems also enhance soil
organic matter and soil health. This document recognizes the central role of
organic matter in improving soil productivity and outlines promising technologies
for improved organic matter management for productive and sustainable crop
production in the tropics.
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 as well as bioturbation
(mixing by soil macrofauna), leaching by water and humus stabilization
(organomineral complexes and aggregates). Land use and management practices
also affect soil organic matter.
Farming
systems have tended to mine the soil for nutrients and to reduce soil organic
matter levels through repetitive harvesting of crops and inadequate efforts to
replenish nutrients and restore soil quality. This decline continues until management
practices are improved or until a fallow period allows a gradual recovery
through natural ecological processes. Only carefully selected diversified
cropping systems or well-managed mixed crop-livestock systems are able to
maintain a balance in nutrient and organic matter supply and removal.
Farmers
can take many actions to maintain, improve and rebuild their soils, especially
soils that have been under cultivation for a long time. A key to soil
restoration is to maximize the retention and recycling of organic matter and
plant nutrients, and to minimize the losses of these soil components caused by
leaching, runoff and erosion.
However,
rebuilding soil quality and health through appropriate farming practices may
take several years, especially in dryland areas where limited moisture reduces
biomass production and soil biological activity. Thus, the challenge is to
identify soil management practices that promote soil organic matter formation
and moisture retention and ensure productivity and profitability for farmers in
the short term.
Sources of Organic Matter
Soil
organic matter consists of diverse, heterogeneous components. It was reported
that living C rarely exceeds 4% of total soil organic C and is present as
roots, microorganisms and soil fauna. Non-living C represents the major portion
of organic C, consisting of surface litter, root litter, microbial metabolites
and humic substances. The living and non-living C component constantly
interacts, as do the saprophytic organisms that acquire metabolites from
non-living C in the soil and then die.
The
term “soil organic matter” (SOM) has been used in different ways to describe
the organic constituents of soil. Baldock and Skjemstad (1999) defined SOM as
“all organic materials found in soils irrespective of origin or state of
decomposition”. SOM consists of C, H, O, N, P and S. Included are living
organic matter (plants, microbial biomass and faunal biomass), dissolved
organic matter, particulate organic matter, humus and inert or highly
carbonized organic matter.
Part
of soil organic matter consists of carbohydrates, lipids and proteins that are
abundant in fresh plant residues. These are rapidly metabolized, immobilized or
decomposed. Organic matter is categorized into aboveground and belowground
organic.
Aboveground
organic matter includes plants and animal residues while the belowground matter
comprises of living soil fauna and micro flora, partially decomposed plant and
animal.
1. Crop Residues
Crop
residue is divided into two types- field and process residue.
i.
Field residue is materials which are left in an agricultural field or orchard
after the crop has been harvested.
This
includes stalks, stems or leaves and seed pod. The residue can be ploughed
directly into the ground or burned first.
ii.
Process residue: They are materials which are left after the crop is processed
into useable resource. It includes husks, seed and root which can be used as
soil amendment, fertilizer and in manufacturing. They have high CN ratio with low
N content but fairly high Potassium and silica content which help to improve
the resistance of crops to disease and lodging.
Fibrous
materials provide an energy source for soil microorganisms which improve soil
physical properties.
2. Green Manure and Cover Crops
Green
manure can be defined as a practice of ploughing or turning into the soil under
composed green plant tissues for improving physical structure and soil
fertility. The green manure crops supplies organic matter as well as nitrogen,
particularly if it is a legume crop.
3. Animal Waste
Amending
soil with animal waste has been old practice. Animal waste can supply
nutrients, OM and enriched soil with beneficial organisms. Dung’s comes mostly
as undigested material and the urine from the digested material. More than 50%
of the organic matter that is present in dung is the form of complex product
consists of lignin and protein which are resistant to further decomposition and
therefore the nutrients present in dung are released very slowly
4. Compost
Compost
is any organic material that undergoes decomposition under controlled
conditions. Any organic material can be converted to compost, but there are
rules regarding what material can and cannot be used. Compared to some
uncomposted animals waste, it may have low nutrient levels.
Nutrients
from compost are often less available to the crop; thus, compost may be more
useful for building SOM. Compost causes less water pollution. Nyamangara et
al., (2003) reported that management of soil organic matter by using composted
organic waste is the key for sustainable agriculture.
Read: 6 Types of Farm Animals and Its Characteristic
Conclusion on what is Soil Organic Matter? – Importance & Sources
Soil organic matter may be defined as “all organic materials found in soils irrespective of origin or state of decomposition”. SOM consists of C, H, O, N, P and S. Soil organic matter (SOM) is the organic component of soil, consisting of three primary parts including small (fresh) plant residues and small living soil organisms, decomposing (active) organic matter, and stable organic matter (humus).
You
have leant about soil organic matter as well as their sources.
In
this article, you leant the following;
That
soil organic matter (SOM) is the organic component of soil, consisting of three
primary parts including small (fresh) plant residues and small living soil organisms,
decomposing (active) organic matter, and stable organic matter (humus).
Importance
of organic matter to the soil includes; organic matter serves as a reservoir of
nutrients for crops, provides soil aggregation, increases nutrient exchange,
retains moisture, reduces compaction, reduces surface crusting, and increases
water infiltration into soil.
Sources
of Organic matter include; crop residue, green manure, animal waste and
compost.
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