Healthy soil and sustainable growing

["TradingPostPaul" <tradingpost@riseup.net>]

Dieter, I posted this some time back when this list was Soilmakers

paul tradingpost@lobo.net
-----------------------

Humus: Still a Mystery
Paul Sachs
http://www.nofa.org/tnf/humus.php

Note: A glossary has been provided, at the end, to define the some of the 
scientific terms used throughout this article.

Humus is like air in a sense. It is abundant, renewable and essential for
life 
to exist on this planet. However, humus is so much more complex than air
that 
even after hundreds of years of research, no one really knows exactly what
it 
is.

The term "Humus" doesn't really describe anything specific. Its like using
the 
word "dog" to describe a German Short Haired Pointer. Humus is sometimes
defined
 as the end product from the decomposition of organic residues. But since
it 
never remains in a static condition, it is hard to refer to it as an end 
product.

Furthermore, the composition of humus in one soil can be so structurally, 
chemically and visibly different from humus in another soil that it is
difficult
 to refer to them both as the same thing.

Over the years, a lot of information has been gathered about humus: Certain

components have been identified; the nature and properties are fairly well
known
 and the factors that control its existence are pretty much accepted as
common 
knowledge. However, to date, an indisputably accurate method of extracting
humus
 from soil has yet to be discovered which, in of itself, severely limits
the 
study of this material.

Attempts to define humus date back to the time of the Romans, but it wasn't

until 1761 when it was first linked to the decomposition of organic matter
by J.
 G. Wallerius. Back then it was thought that plants were able to derive 
nutrients directly from humus but in 1840, Justus von Liebig discovered
that 
plants can only assimilate soil nutrients in an inorganic form and that
plant 
food must be changed into mineral salts first. Liebig believed that this 
occurred from chemical reactions in the soil. About 25 years later,
attention 
was called to the role of microorganisms in the mineralization of nutrients
from
 humus. It was in the early 1900's that most of the significant research on

humus occurred. However, a lot of the information produced became somewhat 
obsolete at the dawn of the chemo-agricultural age in the early 1940's.

Genesis

The formation of humus begins when organic residues of plants and animals
come 
in contact with microbial life in the soil. The carbon compounds contained
in 
the residues, that were synthesized by the plant or animal when it was
alive, 
are protein and energy for the various bacteria, fungi and actinomycetes 
involved in the decay process.

Aerobic microorganisms are the most adept at decomposing organic matter.
They 
need an environment where there is an adequate amount of free oxygen to
live and
 be active. The degree to which free oxygen exists in soil plays a major
role in
 regulating the favorable or unfavorable conditions under which humus is
formed.
 The same is true for moisture, soil temperature and the carbon to nitrogen

ratio of the residues being decomposed.

Where no free oxygen exists (e.g. in water), decomposition of organic
matter 
occurs by anaerobic organisms. This process is much slower than that
conducted 
by aerobic organisms but can, in the long run, produce a greater amount of
humus
 (e.g. muck or organic soil). Humus formed under water is slightly
different 
than its aerobic counterpart due more to the nature of the residues from
the two
 different environments than the process of aerobic vs. anaerobic
humification.
 Most of the contributions of organic matter to organic soils are from
water 
dwelling insects and microbes that have a higher percentage of protein than
the 
plant residues found in forests, fields, or gardens. Other components come
from 
organic residues transported by wind and water currents to a location where
they
 can accumulate and settle. Much of this translocated material may already
be 
humus. Higher percentages of humus are found in soils formed anaerobically 
because conditions are more favorable for humus accumulation and less
favorable 
for its destruction.

At the other extreme is an environment where there is too much oxygen. If 
moisture and soil temperature are also at optimum levels, organic matter
can be 
decomposed so quickly that no accumulation of humus will occur at all (e.g.
in 
tropical environments where high temperatures and moisture levels occur in 
predominantly sandy soils that naturally contain plenty of air).

Soil temperature is another important controlling factor in the formation
of 
humus. As the temperature of a soil increases, there is a corresponding
increase
 in microbial activity. Soils that exist in warmer regions of the earth
tend to 
have lower native levels of humus than soils in colder areas. Figure 1
shows 
that at a temperature of 88 degrees F, in well aerated soil, humus can no
longer
 accumulate.

Microbial Processing

During humification of organic matter, microbes dismantle most of the
sugars, 
starches, proteins, cellulose and other carbon compounds to utilize them
for 
their own metabolism. The assimilation of these nutrients from the original

residues by microorganisms is the first stage in the process of creating
humus.
 Some of the more easily dissolved components of the residues end up being
used 
and re-used over and over, by many different varieties of organisms, and
may 
never actually become humus. However, they provide energy and protein for
the 
life cycles of the organisms involved in its synthesis. The more decay
resistant
 components of the residues are not so much assimilated as they are altered
by 
microbial processing into humic substances.

Much of the nutrient and energy assimilated into the bodies of microbes is 
re-used by other microbes when they die. Some is mineralized back into
plant 
food and some is changed into biologically resistant compounds that
accumulate 
as components of humus. As more and more members of the biomass club
participate
 in the festivities of eating, dying and being eaten, the cycles of soil
life 
are implemented. Plants create organic matter which feeds soil organisms
which 
transform the nutrients from the residues back into plant food, nutrients
for 
other organisms and humus.

The digestion of organic matter in the soil is analogous to the digestive
system
 in animals. Nutrients derived from food ingested by an animal are diffused
into
 its body where they are utilized for energy and production of new cells. 
By-products, such as urea, water, carbon dioxide and other simple compounds
are 
given off. The undigested portion of the food is excreted as feces.

In the soil, organic matter is dissolved and absorbed by microorganisms 
utilizing the nutrients and energy for their own metabolism. Their
activities 
convert most of the organically bound nutrients back into a mineral form
which 
is usable by plants and other microbes. The undigested portion of the
residues 
accumulate as humus. However, humus is not completely immune to
decomposition. 
Microbes will eventually recycle all the elements in humus back to where
they 
initially came from, even if it takes a millennium to do it.

Decay Resistance

Some of the components in organic residues are much more resistant to decay
than
 others. Carbohydrates such as sugars and starches will decompose faster
than 
other carbohydrates such as cellulose and hemicellulose. Fats, waxes and
lignins
 are the most resistant to decay of all the organic components. Proteins
vary in
 decay resistance but are generally more resistant than sugars and starches
yet
 more easily decomposed than all the other components.

Although many of these components exist in humus (in an altered form) (see 
figure 2), the degree to which they exist in the organic residues plays a
role 
in the quantitative accumulation of humus. Materials that contain a high 
percentage of easily decomposed components such as sugars, starches and
proteins
 are, for the most part, assimilated back into the living biomass. Although
the 
energy and protein provided by these residues help in the creation of
humus, the
 ratio of the mass and weight of the residues to the measure of humus
produced 
is relatively high (i.e. only a small amount of humus can be created).

Materials that contain a large percentage of lignins, cellulose or other 
biologically resistant components have less to offer plants in the way of 
recyclable nutrients but contribute significantly more to the formation of 
humus.

Different plants inherently have different ratios of these organic
components 
but variance also appears in the same plants at different stages of their
life.
 Green leaves from deciduous trees, for example, have a very different
analysis 
of proteins vs other components than their dry, fallen counterparts. Figure
3 
shows the changes that occur in rye plants from early growth to maturity.
At the
 young, succulent stage, organic matter from this source would not
contribute 
very much substance for the accumulation of humus but would benefit more
the 
immediate needs of microorganisms and plants. Whereas, near the end of its
life,
 the rye plants would add little to the nutrient needs of plants and soil
life 
but provide more raw materials needed for the formation of humus.

During humification not only are the organic components altered by
microbial 
processing but the ratio that existed in the original residues is changed. 
Figure 4 illustrates how the balance of components in straw changes from 
decomposition. The increase of some components and decrease of others is
not 
magical. The nutrient needs of microbes involved in the decomposition
process 
are taken from any available sources in the soil and reflected in the
analysis 
change from raw material to compost. As the compost humifies further the
changes
 become even more pronounced (see figure 2).

Biologically resistant components such as lignins, fats and waxes are 
structurally and chemically changed by microbial processing. Other
biologically 
resistant carbon compounds are created by microorganisms as by-products of
their
 decay activities. These decay resistant compounds are what humus is made
of. 
This is not to say that humus is immune from further decay, but its
resistance 
to decomposition is at a level that enables it to exist for decades if not 
centuries as a soil conditioner, a habitat for microbial life and a vast 
reservoir of plant and microbial nutrient.

Energy

Humus is essentially a massive storage battery containing energy that was 
originally derived from the sun. Researchers in England discovered that an
acre 
(furrow slice) of soil with 4 percent organic matter contains as much
energy as 
20-25 tons of anthracite coal. Another researcher in Maine equated the
energy in
 that amount of organic matter to 4000 gallons of #2 fuel oil. This organic

energy, which is stored as carbon compounds, was originally derived from
the sun
 by autotrophic organisms such as plants that can extract carbon from 
atmospheric carbon dioxide. About one percent of the energy from the sun
that 
reaches plant leaves is used to photosynthesize carbon compounds. During
the 
plants' life much of the energy that is absorbed from the sun is utilized
for 
growth, foliage production, flowering, seed production and other functions.

About 10 percent of the absorbed energy, initially from the sun, is left 
available to a consumer, e.g. an animal that eats and digests the plant.
Like 
the plant, the animal uses most of the energy it consumes for functions
such as 
growth and sustenance but can offer about 10 percent of the energy it
derived 
from plants to the next consumer in the food chain.

Subsequent digestions through the food chain continue the rapid depletion
of 
available energy from one trophic level to the next. The final consumers of
this
 energy reside in the soil. In figure 5 an arbitrary quantity of energy has
been
 used as an example to show its flow and use. In this case, the 1 million 
calories of energy offered by the sun is reduced to 1 calorie of available 
energy by the time it flows through the food chain to soil saprophytes.
However,
 during the season when plants are active about 20% of the carbon that they

absorb from the atmosphere is exuded through the roots as photosynthesized 
carbon compounds and utilized by organisms living on, near or within the
root 
surface. This phenomena provides a direct and constant flow of plant
synthesized
 energy for many soil microorganisms.
Obviously, the level of energy available from plant residues is higher than
what
 can be offered by the remains of herbivores which is higher that what is 
available from the residues of carnivores. The various energy levels of 
different residues stimulate populations of different soil organisms that 
perform different functions in the soil. Their populations are controlled
by the
 amount and type of residues introduced into the soil and the production of

humus is controlled by these organisms.

Carbon Cycle

Throughout this digestion and assimilation process, from the consumption of
the
 sun's energy by the plant to the decomposition of all residues in the
soil, 
carbon is released back into the atmosphere as carbon dioxide. The
evolution of 
carbon dioxide from organic matter is an integral part of the life cycle.
If CO2
 did not evolve, atmospheric carbon would not be available, the earth would
be 
buried in humus and life could not exist. Figure 6 examines the cycle of
carbon 
from the atmosphere, through the food chain and back into the atmosphere.
Plants
 and other autotrophic organisms (producers) need carbon dioxide in the 
atmosphere to live. Without it, no heterotrophic organisms (consumers),
which 
depend on the producers for energy, could exist either.
Over a one year period and under average conditions, about 60% to 70% of
the 
carbon in organic residues is recycled back to the atmosphere as carbon
dioxide.
 Five to ten percent is assimilated into the biomass and the rest resides
in new
 humus.

Colloidal Properties

Colloidal refers to the attraction certain soil particles have for cations
which
 are positively charged ions of soil nutrients. Whether organic (humus) or 
mineral (clay), the colloid is very small and carries a negative 
electro-magnetic charge that can hold cation nutrients in a manner that
allows 
plant roots access to them. This phenomena is called cation exchange.

When decomposed organic matter reaches a certain level of "maturity" and
can be 
referred to as humus it gains colloidal properties which react, in terms of

cation exchange, almost identically to mineral colloids. However, humus can
have
 a far greater capacity to adsorb cations than clay, especially in a soil
with a
 near neutral pH.

In the soil scientists' quest to isolate and define humus, many terms such
as 
humic acid, fulvic acid, humates, humins and ulmins were developed to help 
literalize their findings. Some of these terms have relatively complicated 
definitions and are used to identify the different compounds produced by
various
 chemical extraction methods. Most are too general and ill-defined for the 
complexities of humus. However, it is helpful to be familiar with two of
these 
terms, i.e. humates and humic acid, to understand the colloidal properties
of 
humus.

When humus particles called micelles are formed the chemical composition is

predominantly carbon, hydrogen, nitrogen and oxygen (see figure 7). The
hydrogen
 ions that reside in compounds on the surface of the micelle can be
displaced by
 other cations such as calcium, magnesium, potassium or sodium (see figure
8). 
By chemical definition, any compound that contains displaceable hydrogen is
an 
acid...hence humic acid.

If the hydrogen ions are displaced with base cations such as potassium,
calcium 
or magnesium the new compound is considered chemically to be the salt of
humic 
acid or humate.

By weight, hydrogen accounts for only a small percentage of humic acid but 
because the atoms of this element are the smallest and lightest of all
elements 
its numbers are overwhelmingly higher than any other. Each location of a 
hydrogen ion on the surface of the micelle can potentially become an
exchange 
site for a base cation. The tremendous amount of surface area of humic
particles
 coupled with the high number of exchangeable H+ ions can significantly
increase 
the cation exchange capacity (CEC) of any given soil.

In soil conditions where hydrogen ion activity is high (i.e. low pH), humus

becomes saturated with adsorbed hydrogen ions and is called humic acid
(HA). HAs
 have the ability to react with mineral particles in the soil liberating
base 
ions such as potassium, magnesium and calcium. As more and more bases are 
released and are adsorbed to the humic colloid, replacing H+, HA is
chemically 
changed into humates.

Humates, are essentially organic colloids that are saturated with base
cations. 
A rich soil with a near neutral pH would contain a high level of humates. 
Whereas the same soil with a low pH would be replete with humic acids. 
Unfortunately, many humic substances can dissolve and leach to lower soil 
horizons in a low pH environment such as in a New England forest.

Soil Conditioning

Humus is an amazing soil conditioner. Only 5% humus will transform lifeless
rock
 dust into rich loam. It has abilities to both bind sand and granulate
clay.

In sandy soils, plant and microbial mucilages from humus clog up the porous

environment, increasing the moisture holding capacities and slowing down
the 
percolation of soil water, with all the dissolved nutrients it contains. As
the 
moisture content increases, more plants and microbes can inhabit the 
environment, accelerating the creation of more humus. Under ideal
conditions, 
the advancement of humus in sand eventually will develop the most preferred
type
 of loam for plant production. Unfortunately, conditions for the
development of 
humus in sand are not always ideal. In tropical environments, for example,
where
 moisture and temperature are optimum for populations of decomposition
bacteria,
 organic matter is quickly assimilated back into the biomass. Coupled with
the 
abundance of oxygen in a porous sand, it is difficult if not impossible for

humus to accumulate.

In clay soils, humus forms an alliance with clay particles. Both particles
are 
colloidal; i.e. they have an electro-negative charge capable of attracting
and 
holding cation nutrients. Complexes are formed in the soil between the two 
particles which not only increases the soil's overall CEC but also
mitigates the
 cohesive nature of clay, causing granulation.

The accumulation of humus is naturally easier in clay soil than in sand
because 
the environmental conditions for decay bacteria are often not as ideal.
Moisture
 levels in clay soils often reach the saturation point leaving little room
for 
oxygen needed by aerobic life. Soil water also acts as a buffer for
temperature 
changes keeping the much needed heat level for microbial activity at a
minimum, 
In addition, the evaporation of moisture from the surface actually has a
cooling
 effect on the soil (just as evaporation of perspiration from the skin
cools the
 body).

Clay can also assist in the stabilization of humus. The clay-humus
complexes 
formed in the soil can further inhibit bacterial decomposition and can
increase 
the lifespan of humus to over a thousand years. Soil scientists calculate
that 
in Allophanic soils (a volcanic clay soil) the mean residence time of humus

ranges from 2000 to 5000 years.

Over time and under the best of conditions, humus can eventually change
both 
clays and sandy soils into media that are visible similar.

Accumulation/Destruction

In an uncultivated, natural environment, humus accumulates in accordance
with 
the favorable or unfavorable conditions of the region. Unless global or
regional
 conditions change, the level of humus accumulation reaches an equilibrium
with 
the factors that destroy it and it becomes a relatively fixed component of
that 
environment.

In cultivated environments, humus is an important asset which, like most
other 
assets, is easier to maintain than it is to replace. Unfortunately, the
value of
 humus is, oftentimes, not fully realized until it is severely depleted and
its
 benefits are no longer available.

Old, stable humus is biologically resistant. Depending on the environmental

conditions under which it exists humus can sit in the soil for centuries,
even 
millenniums, with only a minimal amount of decomposition occurring. However

slight, decay still occurs and eventually even old humus will cycle back
from 
where it came. The formation of new humus is critical to maintaining a
stable 
presence of this asset in the soil.

Current agricultural and horticultural practices have little effect on old,

stable humus. However, many of those methods of cultivation can destroy new

humus in the formative stages when it is more vulnerable to decay than its
older
 counterpart.

Figure 9 shows a typical response of organic matter introduced into the
soil. It
 is important to note that even under the best of conditions, a relatively
small
 amount of humus is created in comparison to the level of organic matter 
initially introduced.

If conditions exist that further accelerate the decomposition of organic
matter,
 even less humus will eventually be created. In extremes such as tropical 
environments where moisture, heat and soil oxygen are abundant, a lot of
carbon 
dioxide is evolved but not much in the way of humus.

Aeration from the plow or rototiller is probably the most significant
factor in
 the depletion of native humus levels of cultivated topsoil. That, coupled
with
 mono-cultural practices and the absence of organic carbon in fertilizer 
materials has cause a greater than 50 percent decline in native humus
levels 
over the years on many of the farms throughout the U.S. This represents a
loss 
that probably, will never be recovered. Even old humus complexes that are 
normally very resistant to decay can be fractured by cultivation and made
more 
vulnerable to biological processes.

Excessive applications of lime can significantly accelerate the
decomposition 
process of humus. The low pH in acid soils inhibit the activities of
bacteria. 
As the pH is raised by applications of lime, bacteria populations grow and
a 
relative increase in decomposition occurs. Experiments done back in 1920
show a 
marked increase in CO2 evolution (a measure of organic matter
decomposition) as 
varying amounts of lime were added to soil. (See fig. 10) A small amount of

calcium can stimulate plant growth to the point where the increased amount
of 
residues added to the soil balance the loss from greater microbial
activity. 
However, excessive lime applications can hasten the destruction of humus at
a 
pace greater than the plant residues can accumulate it.

Excess nitrogen applied to crops is another culprit. The effect of adding
too 
much nitrogen to the soil is very similar to what happens when it is added
to a 
pile of slowly composting carbonaceous organic matter such as dry leaves or
saw 
dust. The temperature of the pile is raised immediately, large volumes of
carbon
 dioxide are released and the whole process of decomposition is accelerated

exponentially. The reaction occurs regardless of the type of nitrogen added

(i.e. organic or inorganic).

Humus in the soil has more real value than money, real estate, stocks or
bonds. 
Its value doesn't fluctuate; it doesn't become scarce in a recession; it's
worth
 can't be depleted by inflation and it can't be stolen. It is the direct or

indirect source of sustenance for all life on earth. It can sometimes be
lost by
 environmental changes but more often, its demise is from either the apathy
or 
the inadvertent errors of the steward who tends it.

Humus is a renewable resource. Its presence in the soil can be maintained 
indefinitely. Unfortunately, many agricultural and horticultural practices
are 
essentially mining humus. Like other mined products, such as coal, minerals
and 
oil, the natural resource can eventually be exhausted.

Summary

The popular scientific definition of humus is "A more or less biologically 
stable, dark, amorphous material formed by the microbial decomposition of
plant 
and animal residues". It is difficult to visibly differentiate humus from 
organic matter in other stages of decay. Compost, well rotted manures and
peat 
are not necessarily humus. However, at some hard-to-define point, all of
these 
organic materials will contribute immeasurably to the humus content in the
soil.

It is not realistic to think that one can quantify or qualify humus
production 
from contributions of organic matter to soil. There are too many factors
that 
control its formation and existence in the soil environment. One can only
assume
 that cultural practices that both minimize the depletion and contribute to
the 
formation of humus will maintain the best possible level of soil humus for
each 
individual environment.

Glossary

Actinomycetes - Decay microorganisms that have a fungus-like appearance
but, 
like bacteria, do not contain a well defined nucleus.
Adsorption - The adherence of one material to the surface of another via 
electro-magnetic forces, e.g. dust to a television screen.
Aerobic - Needing oxygen to live.
Anaerobic - Needing an environment with little or no oxygen to live.
Assimilation - Digestion and diffusion of nutrients by an organism for
growth 
and/or sustenance.
Autotrophs - organisms that can synthesize carbon compounds from
atmospheric 
carbon dioxide utilizing energy from light or chemical reactions.
Base cation - A positively charged ion, historically belonging to the earth

metal family e.g. potassium, magnesium, calcium, etc.
Biomass - The accumulative mass of all living things in a given
environment.
Carbon : Nitrogen ratio - a ratio measured by weight of the number of parts
carbon to each part nitrogen e.g. 10:1, 50:1, etc.
Carnivores - Organisms that consume animals or insects for sustenance.
Cation - An ion of an element or compound with a positive electro-magnetic 
charge.
Cation exchange capacity - The total amount of exchangeable cations that a
given soil can adsorb.
Colloids - Very small soil particles with a negative electro-magnetic
charge 
capable of attracting, holding and exchanging cations.
Faunal - Pertaining to microscopic or visible animals.
Floral - Pertaining to plants or bacteria, fungi, actinomycetes, etc.
Free oxygen - Gaseous oxygen not bound to other elements as in oxides, 
hydroxides or water for example.
Furrow slice - Plow depth of approximately 6 - 7 inches.
Hemicellulose - A carbohydrate resembling cellulose but more soluble; found
in 
the cell walls of plants.
Herbivores - Organisms that consume plants for sustenance.
Heterotrophs - Organisms capable of deriving nutrient for growth and
sustenance 
from organic compounds but incapable of synthesizing carbon compounds from 
atmospheric carbon dioxide.
Humification - The biological process of converting organic matter into
humic 
substances.
Humology - The study of humus.
Lignin - A biologically resistant fibrous compound deposited in the cell
walls 
of cellulose whose purpose is for strength and support of stems, branches, 
roots, etc.
Micelle - (Micro-cell) A negatively charged (colloidal) soil particle most 
commonly found in either a mineral form (i.e. clay) or organic form (i.e. 
humus).
Mineralized - The biological process of transforming organic compounds into

non-organic compounds (minerals) e.g. mineralization of protein into
ammonium.
Monoculture - The cultural practice of growing only one variety of crop in
a 
specific area every season without variance.
Mucilage - compounds synthesized by plants and microbes that swell in
water, 
taking on a gelatinous consistency, that function to maintain a moist 
environment.
Saprophyte - an organism that can absorb nutrient from dead organic matter.
Taxonomy - Science of classification.
Trophic levels - Levels of consumers within a food chain in relation to 
producers of organic nutrient such as plants e.g. producers - primary
consumers 
- secondary consumers - tertiary consumers - decay organisms.

References

Albrecht, W.A. 1938, Loss of Organic Matter and its Restoration. U.S.
Department
 of Agriculture Yearbook 1938, pp 347-376.
ASA# 47. 1979, Microbial - Plant Interactions. American Society of
Agronomy. 
Madison, WI
Bear, F.E. 1924, Soils and Fertilizers. John Wiley & Sons, Inc. New York,
NY
Brady, N.C. 1974, The Nature and Properties of soils. MacMillan Publishing
Co. 
Inc. New York, NY
Huang, P.M. and M. Schnitzer 1986, Interactions of Soil Minerals with
Natural 
Organics and Microbes. Soil Science Society of America, Inc. Madison, WI
Makarov, I.B. 1986, Seasonal Dynamics of Soil Humus Content. Moscow
University 
Soil Science Bulletin, v41 #3 pp 19-26.
Parnes, R. 1986, Organic & Inorganic Fertilizers. Woods End Agricultural 
Institute. Mt. Vernon, ME
SSSA 1987, Soil Fertility and Organic Matter as Critical Components of 
Production Systems. Soil Science Society of America, Inc. Madison, WI
Waksman, S.A. 1936, Humus. Williams and Wilkins, Inc. Baltimore, MD