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What is Soil Organic Matter?
Follow the appropriateness of the season,
consider well the
nature and conditions of the soil, then and only then
least labor will bring best success. Rely on one's own
idea and not on the orders of nature, then every effort
will be futile.
Jia Si Xie, 6th century, China
Soil consists of four important parts: mineral solids,
water, air, and organic matter. Mineral solids are sand, silt, and
clay. Sand has the largest particle size; clay has the smallest.
The minerals mainly consist of silicon, oxygen, aluminum, potassium,
calcium, and magnesium. The soil water, also called the soil solution,
contains dissolved nutrients and is the main source of water for
plants. Essential nutrients are made available to the roots of plants
through the soil solution. The air in the soil, which is in contact
with the air above ground, provides roots with oxygen and helps
remove excess carbon dioxide from respiring root cells. The clumping
together of mineral and organic particles to form aggregates of
various sizes is a very important property of soils. Compared to
poorly aggregated soils, those with good aggregation usually have
better tilth and contain more spaces, or pores, for storing water
and allowing gas exchange.
Organic matter has an overwhelming effect on almost
all soil properties, although it is generally present in relatively
small amounts. A typical agricultural soil has 1 to 6 percent organic
matter. It consists of three distinctly different parts living organisms,
fresh residues, and well-decomposed residues. These three parts
of soil organic matter have been described as the living, the dead,
and the very dead. This three-way classification may seem simple
and unscientific, but it is very useful.
The living part of soil organic matter includes a
wide variety of microorganisms, such as bacteria, viruses, fungi,
protozoa, and algae. It even includes plant roots and the insects,
earthworms, and larger animals, such as moles, woodchucks, and rabbits,
that spend some of their time in the soil. The living portion represents
about 15 percent of the total soil organic matter. Microorganisms,
earthworms, and insects help break down crop residues and manures
and, as they use the energy of these materials, mix them with the
minerals in the soil. In the process, they recycle plant nutrients.
Sticky substances on the skin of earthworms and those produced by
fungi help bind particles together. This helps to stabilize the
soil aggregates, clumps of particles that make up good soil
structure. Organisms such as earthworms and some fungi also help
to stabilize the soil's structure (for example, by producing channels
that allow water to infiltrate) and, thereby, improve soil water
status and aeration. A good soil structure increases water filtering
into the soil and decreases erosion. Plant roots also interact in
significant ways with the various microorganisms and animals living
in the soil. Another important aspect of soil organisms is that
they are in a constant struggle with each other (figure 2.1). Further
discussion of the interactions between soil organisms and roots,
and among the various soil organisms, is provided in chapter
3.
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| Figure 2.1 A nematode
feeds on a fungus, part of a living system of checks and balances.
Photo by Harold Jensen. |
The fresh residues, or "dead" organic matter,
consist of recently deceased microorganisms, insects, earthworms,
old plant roots, crop residues, and recently added manures. In some
cases, just looking at them is enough to identify the origin of
the fresh residues (figure 2.2). This part of soil organic matter
is the active, or easily decomposed, fraction. This active fraction
of soil organic matter is the main supply of food for various organisms
living in the soil microorganisms, insects, and earthworms. As organic
materials decompose, they release many of the nutrients needed by
plants. Organic chemical compounds produced during the decomposition
of fresh residues also help to bind soil particles together and
give the soil a good structure.
Organic molecules directly released from cells of
fresh residues, such as proteins, amino acids, sugars, and starches,
are also considered part of this fresh organic matter. These molecules
generally do not last long in the soil because so many microorganisms
use them as food.
The well-decomposed organic material in soil, the
"very dead," is called humus. Humus is a term sometimes
used to describe all soil organic matter. Some use it to describe
just the part you can't see without a microscope. We'll use the
term to refer only to the well-decomposed part of soil organic matter.
The already well-decomposed humus is not a food for organisms, but
its very small size and chemical properties make it an important
part of the soil. Humus holds on to some essential nutrients, storing
them for slow release to plants. Humus also can surround certain
potentially harmful chemicals and prevent them from causing damage
to plants. Good amounts of soil humus can both lessen drainage or
compaction problems that occur in clay soils and improve water retention
in sandy soils.
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| Figure 2.2
Partially decomposed fresh residues (the "dead")
removed from soil. Fragments of stems, roots, fungal hyphae,
are all readily used by soil organisms. |
Organic matter decomposition is a process that is
similar to the burning of wood in a stove. When burning wood reaches
a certain temperature, the carbon in the wood combines with oxygen
from the air and forms carbon dioxide. As this occurs, the energy
stored in the carbon-containing chemicals in the wood is released
as heat in a process called oxidation. The biological world, including
humans, animals, and microorganisms, also makes use of energy inside
carbon-containing molecules. This process of converting sugars,
starches, and other compounds into a directly usable form of energy
is also a type of oxidation. We usually call it respiration.
Oxygen is used and carbon dioxide and heat are given off in
this process.
A multitude of microorganisms, earthworms, and insects
get their energy and nutrients by breaking down organic residues
in soils. At the same time, much of the energy stored in residues
is used by organisms to make new chemicals as well as new cells.
How does energy get stored inside organic residues in the first
place? Green plants use the energy of sunlight to link carbon atoms
together into larger molecules. This process, known as photosynthesis,
is used by plants to store energy for respiration and growth.
Soil carbon is sometimes used as a synonym for organic
matter. Because carbon is the main building block of all organic
molecules, the amount in a soil is very strongly related to the
total amount of all the organic matter --the living organisms plus
fresh residues plus well decomposed residues. However, under semiarid
conditions, it is common to also have another form of carbon in
soils limestone either as round concretions or dispersed evenly
throughout the soil. Lime is calcium carbonate, which contains calcium,
carbon, and oxygen. This is an inorganic carbon form. Even
in humid climates, when limestone is found very close to the surface,
some may be present in the soil. So, when people talk about soil
carbon instead of organic matter, they are usually referring to
organic carbon. The amount of organic matter in soils is
about twice the organic carbon level.
Sources
Brady, N.C., and R.R. Weil. 1999. The Nature and Properties of
Soils. 12th ed. Macmillan Publishing Co. New York, NY.
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