organiC Matter: What it is and
Why it’s so iMPortant
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
As we will discuss at the end of this chapter, organic
manures for energy and nutrition, and in the process
matter has an overwhelming effect on almost all soil
they mix organic matter into the mineral soil. In addi-
properties, although it is generally present in relatively
tion, they recycle plant nutrients. Sticky substances on
small amounts. A typical agricultural soil has 1% to 6%
the skin of earthworms and other substances produced
organic matter. It consists of three distinctly different
by fungi help bind particles together. This helps to sta-
parts—living organisms, fresh residues, and well-
bilize the soil aggregates, clumps of particles that make
decomposed residues. These three parts of soil organic
up good soil structure. Organisms such as earthworms
matter have been described as the living, the dead, and
and some fungi also help to stabilize the soil’s structure
the very dead. This three-way classification may seem
(for example, by producing channels that allow water to
simple and unscientific, but it is very useful.
infiltrate) and, thereby, improve soil water status and
The living part of soil organic matter includes a wide
aeration. Plant roots also interact in significant ways
variety of microorganisms, such as bacteria, viruses,
with the various microorganisms and animals living in
fungi, protozoa, and algae. It even includes plant roots
the soil. Another important aspect of soil organisms is
and the insects, earthworms, and larger animals, such
that they are in a constant struggle with each other
as moles, woodchucks, and rabbits, that spend some of
(figure 2.1). Further discussion of the interactions
their time in the soil. The living portion represents about
between soil organisms and roots, and among the
15% of the total soil organic matter. Microorganisms,
various soil organisms, is provided in chapter 4.
earthworms, and insects feed on plant residues and
A multitude of microorganisms, earthworms, and
Photo by Christine Markoe
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chAPter 2 organiC Matter: What it is and Why it’s so iMPortant
insects get their energy and nutrients by breaking down
and starches, are also considered part of this fresh
organic residues in soils. At the same time, much of the
organic matter. These molecules generally do not last
energy stored in residues is used by organisms to make
long in the soil because so many microorganisms use
new chemicals as well as new cells. How does energy get
them as food.
stored inside organic residues in the first place? Green
The well-decomposed organic material in soil,
plants use the energy of sunlight to link carbon atoms
the “very dead,” is called humus. Some use the term
together into larger molecules. This process, known as
humus to describe all soil organic matter; some use it
photosynthesis, is used by plants to store energy for
to describe just the part you can’t see without a micro-
respiration and growth.
scope. We’ll use the term to refer only to the well-
The fresh residues, or “dead” organic matter, consist
decomposed part of soil organic matter. Because it is so
of recently deceased microorganisms, insects, earth-
stable and complex, the average age of humus in soils is
worms, old plant roots, crop residues, and recently
usually more than 1,000 years. The already well-decom-
added manures. In some cases, just looking at them is
posed humus is not a food for organisms, but its very
enough to identify the origin of the fresh residues
small size and chemical properties make it an important
(figure 2.2). This part of soil organic matter is the active,
part of the soil. Humus holds on to some essential nutri-
or easily decomposed, fraction. This active fraction of
ents, storing them for slow release to plants. Humus
soil organic matter is the main supply of food for various
also can surround certain potentially harmful chemi-
organisms—microorganisms, insects, and earthworms—
cals and prevent them from causing damage to plants.
living in the soil. As organic materials are decomposed
Good amounts of soil humus can both lessen drainage
by the “living,” they release many of the nutrients
and compaction problems that occur in clay soils and
needed by plants. Organic chemical compounds pro-
improve water retention in sandy soils by enhancing
duced during the decomposition of fresh residues also
aggregation, which reduces soil density, and by holding
help to bind soil particles together and give the soil
on to and releasing water.
good structure.
Another type of organic matter, one that has gained
Organic molecules directly released from cells of
a lot of attention lately, is usually referred to as black
fresh residues, such as proteins, amino acids, sugars,
carbon. Almost all soils contain some small pieces of
Figure 2.1. A nematode feeds on a fungus, part of a living system of
Figure 2.2. Partial y decomposed fresh residues removed from soil.
checks and balances. Photo by Harold Jensen.
Fragments of stems, roots, and fungal hyphae are all readily used by soil
organisms.
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chAPter 2 organiC Matter: What it is and Why it’s so iMPortant
bIochAr As A soIL AMendMent
It is believed that the unusual y productive “dark earth” soils of the Brazilian Amazon region were produced and stabilized by incorporation of vast amounts of charcoal over the years of occupation and use. Black carbon, produced by wildfires as well as human activity and found in many soils around the world, is a result of burning biomass at around 700 to 900°F under low oxygen conditions. This incomplete combustion results in about half or more of the carbon in the original material being retained as char. The char, also containing ash, tends to have high amounts of negative charge (cation exchange capacity), has a liming effect on soil, retains some nutrients from the wood or other residue that was burned, stimulates microorganism populations, and is very stable in soils. Although many times increases in yield have been reported following biochar application—
probably a result of increased nutrient availability or increased pH—sometimes yields suffer. Legumes do particularly well with biochar additions, while grasses are frequently nitrogen deficient, indicating that nitrogen may be deficient for a period following application.
Note: The effects of biochar on raising soil pH and immediately increasing calcium, potassium, magnesium, etc., are probably a result of the ash rather than the black carbon itself. These effects can also be obtained by using more completely burned material, which contains more ash and little black carbon.
charcoal, the result of past fires, of natural or human
typical of the tropical forest. Part of this higher fertility—
origin. Some, such as the black soils of Saskatchewan,
the ability to supply plants with nutrients with very low
Canada, may have relatively high amounts of char.
amounts of leaching loss—has been attributed to the
However, the interest in charcoal in soils has come
large amount of black carbon and the high amount of
about mainly through the study of the soils called
biological activity in the soils. Charcoal is a very stable
dark earths ( terra preta de indio) that are on sites of
form of carbon and apparently helps maintain relatively
long-occupied villages in the Amazon region of South
high cation exchange capacity as well as biological activ-
America that were depopulated during the colonial era.
ity. People are beginning to experiment with adding
These dark earths contain 10–20% black carbon in the
large amounts of charcoal to soils—but we’d suggest
surface foot of soil, giving them a much darker color
waiting for results of the experiments before making
than the surrounding soils. The soil charcoal was the
large investments in this practice. The quantity needed
result of centuries of cooking fires and in-field burning
to make a major difference to a soil is apparently huge—
of crop residues and other organic materials. The man-
many tons per acre—and may limit the usefulness of this
ner in which the burning occurred—slow burns, perhaps
practice to small plots of land.
because of the wet conditions common in the Amazon—
Normal organic matter decomposition that takes
produces a lot of char material and not as much ash as
place in soil is a process that is similar to the burn-
occurs with more complete burning at higher tem-
ing of wood in a stove. When burning wood reaches a
peratures. These soils were intensively used in the past
certain temperature, the carbon in the wood combines
but have been abandoned for centuries. Still, they are
with oxygen from the air and forms carbon dioxide. As
much more fertile than the surrounding soils—partially
this occurs, the energy stored in the carbon-containing
due to the high inputs of nutrients in animal and plant
chemicals in the wood is released as heat in a process
residue—and yield better crops than surrounding soils
called oxidation. The biological world, including humans,
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chAPter 2 organiC Matter: What it is and Why it’s so iMPortant
animals, and microorganisms, also makes use of the
and crop growth, soil organic matter is a critical part of a
energy inside carbon-containing molecules. This process
number of global and regional cycles.
of converting sugars, starches, and other compounds into
It’s true that you can grow plants on soils with little
a directly usable form of energy is also a type of oxidation.
organic matter. In fact, you don’t have to have any soil
We usually call it respiration. Oxygen is used, and carbon
at all. (Although gravel and sand hydroponic systems
dioxide and heat are given off in the process.
without soil can grow excellent crops, large-scale sys-
Soil carbon is sometimes used as a synonym for
tems of this type are usually neither economically nor
organic matter. Because carbon is the main building
ecologically sound.) It’s also true that there are other
block of all organic molecules, the amount in a soil is
important issues aside from organic matter when con-
strongly related to the total amount of all the organic mat-
sidering the quality of a soil. However, as soil organic
ter—the living organisms plus fresh residues plus well-
matter decreases, it becomes increasingly difficult to
decomposed residues. When people talk about soil carbon
grow plants, because problems with fertility, water
instead of organic matter, they are usually referring to
availability, compaction, erosion, parasites, diseases,
organic carbon. The amount of organic matter in soils is
and insects become more common. Ever higher levels
about twice the organic carbon level. However, in many
of inputs—fertilizers, irrigation water, pesticides, and
soils in glaciated areas and semiarid regions it is common
machinery—are required to maintain yields in the face
to have another form of carbon in soils—limestone, either
of organic matter depletion. But if attention is paid to
as round concretions or dispersed evenly throughout the
proper organic matter management, the soil can support
soil. Lime is calcium carbonate, which contains calcium,
a good crop without the need for expensive fixes.
carbon, and oxygen. This is an inorganic carbon form.
The organic matter content of agricultural topsoil
Even in humid climates, when limestone is found very
is usually in the range of 1–6%. A study of soils in
close to the surface, some may be present in the soil.
Michigan demonstrated potential crop-yield increases
WHY SOIl ORgANIc MATTER IS SO IMPORTANT
of about 12% for every 1% organic matter. In a Maryland
experiment, researchers saw an increase of approxi-
A fertile and healthy soil is the basis for healthy plants,
animals, and humans. And soil organic matter is the
mately 80 bushels of corn per acre when organic matter
very foundation for healthy and productive soils.
increased from 0.8% to 2%. The enormous influence
Understanding the role of organic matter in maintain-
of organic matter on so many of the soil’s properties—
ing a healthy soil is essential for developing ecologically
biological, chemical, and physical—makes it of critical
sound agricultural practices. But how can organic matter,
importance to healthy soils (figure 2.3). Part of the
which only makes up a small percentage of most soils,
explanation for this influence is the small particle size
be so important that we devote the three chapters in this
of the well-decomposed portion of organic matter—the
section to discuss it? The reason is that organic matter
humus. Its large surface area–to–volume ratio means
positively influences, or modifies the effect of, essentially
that humus is in contact with a considerable portion of
all soil properties. That is the reason it’s so important to
the soil. The intimate contact of humus with the rest of
our understanding of soil health and how to manage soils
the soil allows many reactions, such as the release of
better. Organic matter is essentially the heart of the story,
available nutrients into the soil water, to occur rapidly.
but certainly not the only part. In addition to functioning
However, the many roles of living organisms make soil
in a large number of key roles that promote soil processes
life an essential part of the organic matter story.
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chAPter 2 organiC Matter: What it is and Why it’s so iMPortant
add
increased biological activity
organic
(& diversity)
matter
decomposition
reduced
soilborne diseases,
aggregation
parasitic nematodes
increased
pore structure
improved
nutrients
humus and other
released
growth-promoting
substances
improved tilth
harmful
and water storage
substances
detoxified
HEALTHY PLANTS
Figure 2.3. Adding organic matter results in many changes. Modified from Oshins and Drinkwater (1999).
Plant Nutrition
or mineral forms that plants can easily use. This process,
Plants need eighteen chemical elements for their
called mineralization, provides much of the nitrogen
growth—carbon (C), hydrogen (H), oxygen (O), nitrogen
that plants need by converting it from organic forms.
(N), phosphorus (P), potassium (K), sulfur (S), calcium
For example, proteins are converted to ammonium
(Ca), magnesium (Mg), iron (Fe), manganese (Mn),
(NH +
–
4 ) and then to nitrate (NO3 ). Most plants will take
boron (B), zinc (Zn), molybdenum (Mo), nickel (Ni),
up the majority of their nitrogen from soils in the form
copper (Cu), cobalt (Co), and chlorine (Cl). Plants obtain
of nitrate. The mineralization of organic matter is also
carbon as carbon dioxide (CO
an important mechanism for supplying plants with such
2) and oxygen partially as
oxygen gas (O
nutrients as phosphorus and sulfur and most of the
2) from the air. The remaining essential
elements are obtained mainly from the soil. The avail-
ability of these nutrients is influenced either directly
or indirectly by the presence of organic matter. The
WhAt MAKes toPsoIL?
elements needed in large amounts—carbon, hydrogen,
Having a good amount of topsoil is important. But
oxygen, nitrogen, phosphorus, potassium, calcium, mag-
what gives topsoil its beneficial characteristics? Is it
nesium, sulfur—are called macronutrients. The other
because it’s on TOP? If we bring in a bulldozer and
elements, called micronutrients, are essential elements
scrape off one foot of soil, will the exposed subsoil
needed in small amounts. (Sodium [Na] helps many
now be topsoil because it’s on the surface? Of course,
plants grow better, but it is not considered essential to
everyone knows that there’s more to topsoil than its
plant growth and reproduction.)
location on the soil surface. Most of the properties
Nutrients from decomposing organic matter.
we associate with topsoil—good nutrient supply,
Most of the nutrients in soil organic matter can’t be used
tilth, drainage, aeration, water storage, etc.—are there
by plants as long as those nutrients exist as part of large
because topsoil is rich in organic matter and contains a
organic molecules. As soil organisms decompose organic
huge diversity of life.
matter, nutrients are converted into simpler, inorganic,
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chAPter 2 organiC Matter: What it is and Why it’s so iMPortant
plant. A number of essential nutrients occur in soils as
positively charged molecules called cations (pronounced
cat-eye-ons). The ability of organic matter to hold on
to cations in a way that keeps them available to plants
crop residues
growing crops
and
is known as cation exchange capacity (CEC). Humus
animal manures
has many negative charges. Because opposite charges
attract, humus is able to hold on to positively charged
nutrients, such as calcium (Ca++), potassium (K+), and
magnesium (Mg++) (see figure 2.5a). This keeps them
from leaching deep into the subsoil when water moves
soil
through the topsoil. Nutrients held in this way can
organic matter
be gradually released into the soil solution and made
available to plants throughout the growing season.
However, keep in mind that not all plant nutrients occur
Figure 2.4. The cycle of plant nutrients.
as cations. For example, the nitrate form of nitrogen is
micronutrients. This release of nutrients from organic
negatively charged (NO –
3 ) and is actually repelled by the
matter by mineralization is part of a larger agricultural
negatively charged CEC. Therefore, nitrate leaches easily
nutrient cycle (see figure 2.4). For a more detailed
as water moves down through the soil and beyond the
discussion of nutrient cycles and how they function in
root zone.
various cropping systems, see chapter 7.
Clay particles also have negative charges on their
Addition of nitrogen. Bacteria living in nodules
surfaces (figure 2.5b), but organic matter may be
on legume roots convert nitrogen from atmospheric gas
the major source of negative charges for coarse and
(N2) to forms that the plant can use directly. A number
medium-textured soils. Some types of clays, such as
of free-living bacteria also fix nitrogen.
those found in the southeastern United States and in the
Storage of nutrients on soil organic matter.
tropics, tend to have low amounts of negative charge.
Decomposing organic matter can feed plants directly,
When those clays are present, organic matter may be
but it also can indirectly benefit the nutrition of the
the major source of negative charges that bind nutrients,
Ca ++
Ca ++
-
Ca ++
Mg ++
- - - -
Ca ++
-
-
- -
-
-
-
-
-
- - - -
-
K +
-
Zn ++
Mg ++ - Ca ++
K +
-
a) cations held on
b) cations held on
c) cations held by
humus
clay particle
organic chelate
Figure 2.5. Cations held on negatively charged organic matter and clay.
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chAPter 2 organiC Matter: What it is and Why it’s so iMPortant
even for fine-textured (high-clay-content) soils.
Protection of nutrients by chelation. Organic
orGAnIc MAtter IncreAses the
molecules in the soil may also hold on to and protect
AvAILAbILItY of nutrIents . . .
certain nutrients. These particles, called “chelates”
Directly
(pronounced key-lates) are by-products of the active
• As organic matter is decomposed, nutrients are
decomposition of organic materials and are smaller
converted into forms that plants can use directly.
than the particles that make up humus. In general,
• CEC is produced during the decomposition process,
elements are held more strongly by chelates than by
increasing the soil’s ability to retain calcium, potas-
binding of positive and negative charges. Chelates work
sium, magnesium, and ammonium.
well because they bind the nutrient at more than one
• Organic molecules are produced that hold and
location on the organic molecule (figure 2.5c). In some
protect a number of micronutrients, such as zinc
soils, trace elements, such as iron, zinc, and manga-
and iron.
nese, would be converted to unavailable forms if they
Indirectly
were not bound by chelates. It is not uncommon to find
• Substances produced by microorganisms promote
low-organic-matter soils or exposed subsoils deficient in
better root growth and healthier roots, and with a
these micronutrients.
larger and healthier root system plants are able to
Other ways of maintaining available nutri-