the living soil: the basis of ecologically sustainable agriculture by dr. ted c. mendoza
DESCRIPTION
A compilation of the lecture notes of Dr. T.C. Mendoza , Professor of Crop Science , Crop Science Cluster, College of Agriculture, UPLB., Los Banos, Laguna, Philippines. August 2012TRANSCRIPT
The Living Soil: The Basis of Ecologically Sustainable Agriculture*
A compilation of the lecture notes of Dr. T.C. Mendoza , Professor of Crop Science , Crop Science Cluster, College of Agriculture, UPLB. August 2012
1.0. What is soil ?
2.0. Natures Principle relevant to the soil
3.0. Functions of Soil
4.0. What is Soil quality ?
5.0. Composition of soil – the Living Organism
5.1. SOM and its importance
5.2. Functions of soil organisms
5.3. Major groups soil organisms
5.4. Organic Matter and the functions of humus
5.5. Differentiating organic matter and humus
6.0. Building up soil organic matter/humus ( separate lecture module on nutrient cycling )
7.0. Natural plant nutrition
8.0. Nitrogen Fixation
Annex A. Review of Major elements required for plant growth
References:
Alexander, M. 1977. Introduction to Soil Microbiology. 2d ed. John Wiley & Sons. New York, NY.
Bargyla and Gylver Rateaver. The Organic Method Primer.
Carter, V. G. and Dale, T. Top Soil and Civilization (Rev. Ed)
Hendrix, P.F., M.H. Beare, W.X. Cheng, D.C. Coleman, D.A. Crossley, Jr., and R.R. Bruce. 1990. Earthworm
effects on soil organic matter dynamics in aggrading and degrading agroecosystems on the Georgia
Piedmont. Agronomy Abstracts, p. 250, American Society of Agronomy, Madison, WI.
Magdoff F & RR Weil.2004.Soil Organic Matter in Sustainable Agric.CRC Press LLC 398 p.
Paul, E.A. and F.E. Clark. 1996. Soil Microbiology and Biochemistry. 2d ed Academic Press. San Diego, CA.
Scheewe, W. Nurturing the Soil: Feeding the People. Rev., updated & Expd. Ed.
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* This is a compiled lecture notes of T.C. Mendoza , Professor of Crop Science , Crop Science Cluster, College of Agriculture, UPLB. August 2012
1
1. What is soil?
… where you and me or all of us came from (biblical)
… is the transformation product of mineral and organic substances on the earth’s surface under the influence of environmental factors operating over a very long time and having defined organisation and morphology (technical)
….. is a natural medium on the surface of the earth where plants grow.
… growing medium for plants
… bases of life for all plants, animals, humankind
… where our FOOD actually comes…
FOOD = NUTRITION = HEALTH
… where our nutritious food comes from
… the basis of human health
Health is wealth
… soil is the basis of our wealth
… soil is the basis of our happiness
Health is happiness
Soil as the basis of life
FOOD = nutrition = health = wealth = hapiness
How is soil formed?
Soil formation is long and complex, it can take
anywhere from 100 to 10,000 years to create one inch
of topsoil! Soil formation is driven by factors such as
climate, topography, living organisms and the type of
parent materials. Parent materials come from the
break down of underlying rock or from deposits by
streams and rivers, seas and gulfs, hills, wind and
glaciers or organic plant residues. Over time, these
materials are weathered by the effects of freezing,
thawing, wetting, drying, heating, cooling, erosion,
plants, and animals and from chemical reactions with
water, oxygen, organic and inorganic acids, and
organic matter. Eventually the parent material is
divided into smaller and smaller particles and forms
layers called "horizons". The top layer of soil is the
A-horizon is the one containing most of the organic
matter and biological activity. The B-horizon is the
zone of maximum accumulation of materials, and the
underlying C-horizon is mainly the parent material,
but slightly altered. The younger the soil is, the less
developed are these underlying horizons and the
thinner the top horizon.
Evolution of the concepts or misconcepts about soil
Soil is alive -> the living soil. Soil is a living, dynamic matrix that feeds and recycles liquids, gases and solid matter. Hence it is food for microbes, insects, animals and plants.
Soil is not simply a physical object or dust (indicative of non-life). A gram of fertile soil housed….
o 2.5 M – bacteriao 4000,000 – fungio 50,000 – algaeo 30,000 – protozoao 25,000 – nematodeso 500+ - spring tails, worms, insects, mites
Soil as the basis of healthy life
healthy soil -> healthy plant
-> healthy human beings
-> healthy society
Why? Sick soil -> sick plant -> sick society
Impoverished soil
-> impoverished human being
(sick individual, family, society)
Soil as the basis of nations wealth not only health
When soil is gone so is the wealth of the nation !
2
2.0. Nature’s Principle Relevant to the Soil
Principle One: In nature, soil is always covered.
Principle Two: There is always a diversity of
plants.
Principle Three: There is always a cycle of energy
and nutrients.
Principle Four: Plants regulate the uptake of
nutrients on their own.
Principle Five: The vegetation absorbs a maximum
of sunlight.
Principle Six: The soil “plows” itself.
Principle Seven: Life is interconnected.
3.0. Importance/Functions of Soil
Soil is the medium that enables us to grow food
for people or animals, natural fiber, and timber, and
supports wildlife. Around 99 % of global food
supplies (calories) for human consumption come
from land-based food production (FAO, 2007).
Soil is a natural filter that neutralizes certain
pollutants by transforming them or accumulating and
absorbing their toxicity. In addition, soils are a major
factor in purifying water supplies and are a critical
component for regulating flooding through
the storage of rainfall. The sealing and compaction of
permeable soils results in a more rapid delivery of
rainfall to the river network. These are just examples
of the critical ecosystem services provided by soil.
Soil is a biological engine where dead plant
and animal tissues, and other organic wastes, are
decomposed to provide nutrients that sustain life.
Soil plays a crucial role in regulating a
number of life-sustaining natural biological and
chemical cycles (ecosystem services). Carbon,
nitrogen and a range ofessential nutrients are
continuously recycled between the soil and plants,
geological deposits, groundwater and the atmosphere.
The intensity of these biogeochemicalexchanges
varies from place to place and is regulated by soil
characteristics.
Soil protects our buried heritage of
archaeological and historic remains from damage and
depletion. Much of the evidence of past habitats and
human heritage remains buried, awaiting discovery
and study by archaeologists and palaeo-ecologists.
The degree of
preservation of such remains depends on the local
soil characteristics and conditions [2]. Soils that
preserve cultural heritage should also be regarded as
valuable.
Soil provides the foundation on which we
construct our buildings, roads and other
infrastructures. In addition to providing the support
for the vast majority of human infrastructure, soil
provides a range of raw materials such as clay for
pottery and
peat for fuel.
3
Soil has important ecosystems functions
as follows ……..
(a) recycling of organic materials in soils to release
nutrients for further synthesis into new
organic materials;
(b) partitioning of rainfall at the soil surface into
runoff and infiltration;
(c) maintaining habitat diversity of pore sizes,
surfaces, and water and gas relative
pressures;
(d) maintaining habitat stability, including a stable
structure, resistance to wind and water
erosion and buffering of habitat against rapid
changes of temperature, moisture, and
concentration of potentially toxic materials;
(e) storage and gradual release of nutrients and
water
(f) the partitioning of energy at the surface, which
is important in global circulation processes.
(g) sequestering carbon through the humus
cycle.Under organic management, soils can
provide carbon sequestration of 2 ton/
hectare per year
Cycling of carbon and nutrients is probably
the best known soil function in ecosystems. Carbon
and nitrogen cycles have been measured and
modeled. Nutrient cycling has been studied more in
non cultivated soils (e.g., forests or rangeland). The
levels of cycling activity that can be expected in
specific ecoregions are known.
The partitioning of water at the soil surface is
an equally important function in ecosystems. This
partitioning determines both quantity and quality of
surface and groundwater. Water running over the
surface can carry sediment and other pollutants, and
quickly reaches drainages. Water that infiltrates into
the soil and moves (Source: Warkentin, 1995.
JSWC). Top soil serves as biofilter. It purifies the
water before it infiltrates deep into the ground.
4
4. What is soil quality?
One soil scientist defined soil quality as “the
integration of different chemical, physical, and
biological properties of soil interacting in complex
ways that determine its potential fitness or capacity to
produce healthy and nutritious crops. The concept is
now expanded to include not only soil productivity
but also food quality/safety, environmental quality
and human/animal health.(Fig. 1). Thus, with these
concept, we can now describe a relationship given us:
Healthy soil = healthy crop = healthy society
Healthy animals
The backward arrow depicts a healthy
society nurturing a healthy soil. A healthy crop grows
in a healthy soil. But what is a healthy soil? This
leads us to the characterization of healthy soil. One
soil scientist (Haberern, 1992) introduced the idea of
“Soil Health Index”. Soil Health Index (SHI)
characterize soils capability to produce healthy and
nutritious crops in a sustainable way. SHI explores
the components of soil quality as relate to sustainable
agriculture. How soil quality can be quantified to
indicate the status of soil health and provide an early
warning of soil degradation and the need for remedial
measures.
But exactly, what is a healthy or quality soil
for crop production?
Can we list easily distinguishable physical,
chemical, biological properties?
Physical - thick top soil (> 30 cm)
- Dark brown to black color (red soils are
associated with acid soils and low P) is
associated with high organic matter.
- Friable, loose, porous (low bulk density)
These physical characteristics translate easily to the
following:
high water holding capacity
well-drained and well-aerated
easy to prepare/till for planting
walking on it gives no harm for bare feet, it
is walking on a foam
Chemical - near neutral pH (pH 5.5-6.8)
- High cation exchange capacity (C.E.C.)
or nutrient retention
- Low electrical conductivity (Eo = < 4) or
low saltiness
- contains adequate amount of all the
essential nutrients
- High soil organic matter (OM > 3%).
Singularly, it is one important soil quality
indicator. High SOM is always equated to
fertile or quality soil for crop production.
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Chemical
NPK
Micronutrients
pH, Salinity
Physical Structure
Air, Water, Tilth,
Biological
Organic Matter
Roots/Weeds/Fertility/Insects
Bacteria, Fungi, Insects, Earthworms
Listing all indicators of healthy/quality soil is important as it gives us a picture on the status of the soil
currently use for crop production as shown in the simple illustration below.
Degraded fertile/healthy
poor quality soil quality soil
|__|__|__|__|__|__|__|__|__|__|
0 1 2 3 4 5 6 7 8 9 10
It maybe difficult now to find a soil which has 10
maximum points in the spectral line. Most soils are
perhaps between 3-6 points. What is the relevance of
describing soil as to its quality status? It is related to
the amelioration needed, or management and/or crop
species that must be planted and the adaptive farming
practices that must be done.
Some farmers in Bondoc Peninsula, Quezon no
longer plant corn in adversely eroded/degraded soil,
instead, they plant trees (mahogany, narra, madre de
cacao, coconut) and are seeded with cover crops.
They select the more “fertile” soils for planting
ginger, corn, upland rice and vegetables.
Is Soil health = Soil quality ?
Soil health is used synonymously with soil
quality . The relationship is drawn between and
health and the health of animals and humans eating of
animals and humans eating the crops produced by the
soil (Haberern,1992). The components of soil health
are the biological processes that produce a balance of
6
macro and micro nutrients, traces organics that have
enzymatic functions, and the freedom from plant
diseases and from various pests that attack unhealthy
crops growing in unhealthy soils. Soil health is
achieved by promoting biological activity in the soil
through additions of organic matter, and by avoiding
addition of potentially toxic materials. These ideas
were largely the concern of organic farming as we
learn more about the dependence of our health on the
foods we eat.
. The use of biological activity in concepts
of soil quality or soil health has been a relatively
minor theme in soil science in the last 50 years. Soil
productivity has been related more to additions of
nutrients and management of water. Soil biology was
more prominent in the previous half century, after the
discoveries of soil bacterial functions and nitrogen
fixation by symbiotic and free living bacteria.
Biological processes were not understood until the
latter part of the 1800s, well after physical and
chemical properties of soils were investigated in the
mid years of the century.
In the last 15 years there has been increased
interest in measuring biological parameters to
characterize soil functions. Measurements of
biomass activity measured by respiration, enzymatic
activity, and diversity of organisms are all becoming
much more important (Doran et al.). These
biological parameters are being related to soil
management – to determine the effects, for example,
of different tillage practices on soil biota.
Fig.1 Attributes of Soil Quality
Soil Productivity
Human/ Animal Food Quality/
Health Safety
Environmental
Quality
7
Fig.2 Interrelationships of principal factors Affecting Soil Quality
SOIL ATTRIBUTES
Physical
Chemical
Biological
CLIMATE SOIL QUALITY LAND
Rainfal l Vegetation
Temperature Geology
Storms Drainage
Run-off
HUMAN
MANKIND
Land use
Mgt. Practices , Costs of inputs , Ownership
Marketability , Farm policy
Indicators of Soil Quality:
Soil Quality Index (SQI)
SQI = f (SP, P, E, H, ER, BD, FQ, MI)
Where: SP = Soil Properties (Physical, Chemical, Biological), ER = Erodibility
P = Potential Productivity BD = Biological Diversity
E = Environmental Factors FQ = Food Quality/ Safety
H = Health (Human, Animal) MI = Management Inputs
3.0. Composition of Soil - the Living Organism
Lady Eve Balfour coined in 1943 the notion
of “the living soil” and concluded that one has to
“feed the soil and let the soil feed the plants.” The
tradition of organic agriculture sees the soil as an
organism, which contains countless organisms. Soil
is alive: decomposition processes are driven by a
mass of soil microorganisms.
A living soil contain around 50 percent clay
and sand particles (solids), 25 percent water, and 25
percent air. The solids can contain around 5 percent
(but often less) organic matter that is composed
largely of humus. Humus originates from various
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forms of organic matter and is essential for the
fertility and structure of a soil. Although it represents
only a little share of the soil, it determines how the
soil looks like and its corresponding fertility. (Humus
will be discussed in more detail later).In a good or
fertile soil, a huge variety of micro-animals and
microorganisms are active in making up humus, to
convert it later into plant nutrients, and to provide
additional services to the plants growing on the
surface of the soil. There are earthworms, termites,
nematodes, and many other micro-animals (about 50
to 100 different species and varieties) involved in
these activities. Further, there are algae, fungi
(similar to molds), bacteria, actinomycetes, and
protozoans. A handful of soil may contain more than
10 billion microorganisms (Torsvik and Ovreas,
2002), the majority of which are bacteria —
comparable to the number of people on Earth! In
addition to the huge amounts of bacteria, 1 m3 of
fertile topsoil will contain hundreds of kilometers of
fungal hyphae, tens of thousands of
protozoa,thousands of nematodes, several hundred
insects,spiders and worms, and hundreds of metres of
plant roots. The total weight of microorganisms in the
soil below a hectare of temperate grassland can
exceed that of a medium-sized elephant — five
tonnes — and often exceeds the above-ground
biomass. This biota is involved in most of the key
functions of soil, driving fundamental nutrient
cycling processes, regulating plant communities,
degrading pollutants and helping to stabilise soil
structure. Soil organisms also represent a crucially
important biotechnological resource, with many
species of bacteria and actinomycetes providing
sources of antibiotics and other medicines.
It is difficult to see soil biological features.
But many living organisms thrive in
soil as in .
. .
2.5 million Bacteria
1 gram of fertile 400, 000 fungi
agri-soil
(Erlich, et. Al. 1977) 50, 000 algae
30, 000 protozoa
These organisms compete but the prevailing
mode of existence is mutual dependence. One
member of the ‘soil community’ is in one way or
another dependent on many other members. Most
members of the soil community are living either from
dead organic matter or depend on energy provided by
plants through their roots. The interrelations between
the soil organisms are so manifold that it is
impossible to understand them completely. We know
very little about the control and adjustment processes
between the different soil organisms. (Winfried
Scheewe, 2000. Feeding the People: An Introduction
to Sustainable Agriculture)
9
4. Soil Organisms and their Functions
Organisms and the soil animals can be
distinguished according to their respective functions
in the soil.
1. The first group is
composed of the decomposers or ‘humus
makers.’ Many soil animals belong to this
group as well as many species of bacteria,
fungi, and actinomycetes - food preparers.
2. Another group breaks
down the soil humus making the nutrients
accessible for the plant roots done by bacteria,
actinomycetes, and fungi food cookers.
3. Several species of
bacteria, algae, and actinomycetes fix gaseous
nitrogen and make it available to plants - food
synthesizers
4. Many soil animals and
microorganisms improve soil structure through
their natural activities - soil structural builders
5. Many organisms
discharge some kind of enzymes, which can
stimulate plant growth, and the activities of
other organisms. Some organisms secrete
substances like antibiotics that inhibit the
development of various organisms.
6. Several organisms and
soil animals also exude substances in the soil
environment adverse to some crops and other
plants.
Major Groups of Soil Organisms
A diverse biological community in soils is
essential to maintaining a healthy environment for
plants. There may be over 100,000 different types of
organism living in soils. Of those, only a small
number of bacteria, fungi, insects, and nematodes
might harm plants in any given year. Diverse
populations of soil organisms maintain a system of
checks and balances that can keep disease organisms
or parasites. Some fungi kill nematodes and others
kill insects. Still others produce antibiotics that kill
bacteria. Protozoa feed on bacteria. Some bacteria
kill harmful insects. Many protozoa, springtails, and
mites feed on disease-causing fungi and bacteria.
Beneficial organisms, such as the fungus
Trichoderma and the bacteria Pseudemonas
fluorescens, colonize plant roots and protect them
from attack by harmful organisms.
Microorganisms - are very small forms of life that
can sometimes live as single cells, although many
also form colonies of cells. A microscope is usually
needed to see individual cells of these of these
organisms. Many more microorganisms exist in
topsoil, where food sources are plentiful, than in
subsoil. They are especially abundant immediately
next to plant roots, where sloughed off cells and
chemicals released by roots provide ready food
sources. These organisms are important primary
decomposers of organic matter, but they do other
things, such as providing nitrogen through fixation to
help growing plants. Soil microorganisms have had
another direct importance for humans - they are the
origin of most of the antibiotic medicines we use to
fight various diseases.
1
Considering the biomass that each group in the
soil presents, microorganisms are the most abundant.
The following groups are common in the soil:
a) Bacteria.
One of the simplest and smallest forms
of life known. They possess an unlimited
capacity to increase their numbers in the soil by
growth and division as they are single-cell
organisms
Many bacteria are able to produce
spores or similar resistant bodies that allow
them to endure unfavorable conditions. Soil
bacteria are either autotrophic or heterotrophic.
Autotrophs obtain their energy from the
oxidation of mineral parts in the soil. Most
bacteria are heterotrophic; their energy and
carbon requirements come directly from
organic matter.
Bacteria live in almost any habitat. They
found inside the digestive system of animals, in
the ocean and fresh water, in compost piles
(even at temperatures over 130oF), and in soils.
They are very plentiful in soils; a single
teaspoon of topsoil may contain more than 50
million bacteria. Although some kind of
bacteria live in flooded soils without oxygen,
most require well-aerated soils. In general,
bacteria tend to do better in neutral soils than in
acid soils.
Bacteria as a group participate in all of
the organic transactions in support of higher
plants. They dominate processes like nitrogen
oxidation, sulfur oxidation and nitrogen
fixation. In addition to being among the first
organisms to begin decomposing residues in
the soil, bacteria benefit plants by increasing
nutrient availability. For example, many
bacteria dissolve phosphorous, making it more
available for plants to use.
Bacteria are also very helpful in
providing nitrogen to plants. Although
nitrogen is needed in large amounts by plants,
it is often deficient in agroicultural soils. You
may wonder how soils can be deficient in
nitrogen when we are surrounded by it - 78
percent of the air we breathe is composed of
nitrogen gas. Yet plants as well as animals
face the dilemma of the Ancient Mariners, who
was adrift at sea without fresh water: "Water,
water, everywhere nor any drop to drink."
Unfortunately, neither animals nor plants can
use nitrogen gas (N2) for their nutrition.
However, some types of bacteria are able to
take nitrogen gas from the atmosphere and
convert it into a form that plants can use to
make amino acids and proteins. This
conversion process is known as nitrogen
fixation.
b) Fungi.
Similar to most bacteria, they depend
on the soil organic matter for their energy and
carbon needs or relieve it from growing plant
roots. The most common group of fungi in the
soil are molds which can grow in all kinds of
soil environment; acidic neutral, or alkaline,
but have a preference for slightly acidic soil
environment. The diameter of fungal hyphae
ranges from two to ten micrometres. In one
gram of dry soil, one can find 100,000 up to
one million individuals. This would result in
biomass of 800 to 8,000 kg/ha.
1
Fungi are very versatile in their
ability to decompose organic residues. They
decompose cellulose, starch, gums, lignin, and
other substances. In decomposing plant
residues, fungi are usually more effective than
bacteria. They turn more biomass into humus
and produce less “by-products” like carbon
dioxide and ammonium. Up to 50 percent of
the substances decomposed by molds become
organic tissue compared to 20 percent in the
case of bacteria. Fungi can, however, not
perform some tasks which the bacteria can, like
fixing nitrogen or oxidizing metals.
Many plants develop a beneficial
relationship with fungi that increases the
contact of roots with the soil. Fungi infect the
roots and send out root-like structures called
hyphae. The hyphae of these mycorrhizal
fungi take up water and nutrients that can then
feed the plant. This is especially important for
phosphorous nutrition of plants in low-
phosphorous soils. The hyphae help the plant
absorb water and nutrients and in return the
fungi receive energy in the form of sugars,
which the plant produces in its leaves and
sends down to the roots. This symbiotic
interdependency between fungi and roots is
called a mycorrhizal relationship. All things
considered, it's pretty good deal for both the
plant and the fungus. The hyphae of these
fungi help develop and stabilize soil aggregates
by secreting a sticky gel that glues mineral and
organic particles together.
c) Soil Algae are generally chlorophyll-bearing
organisms and, like higher plants, are
capable of performing photosynthesis.
They are largely restricted to the parts of
the soil penetrated by sunlight. Thus, the
soil surface and larger cracks in the soil
are the major zones of algae activity.
Among the four groups of algae, it is the
blue-green algae that is abundant and of
ecological importance in tropical soils.
Algae release carbonic acids that can
accelerate the weathering of minerals. They
also produce large amounts of extra cellular
polysaccharides which can act as soil
aggregating agents. This may be especially
important on bare soils. Some algae have the
capacity to carry out non-symbiotic and
symbiotic nitrogen fixation. This ecological
role is important in flooded rice fields where
algae form a symbiosis with the fern azolla.
d) Actinomycetes resemble molds in their
morphology as they are filamentous,
often profusely branched, and produce
fruiting bodies in the same way. On the
other hand, they are similar to bacteria:
they are composed of one cell and are of
similar size. Actinomycetes produce
reproductive spores that allow them to
endure adverse situations. They develop
best in moist, well-aerated soil but can
stand also drier conditions. They are
relatively sensitive to acid soil condition.
At pH values below 5.0, their activities
cease. Most actinomycetes are free-
1
living saprophytes and are able to
decompose a tremendous array of carbon
rich substrates, like lignin, chitin, and
cellulose. Some actinomycetes produce
quickly evaporating substances called
geomins which contribute to the soil
much of its earthy smell, especially
apparent during and soon after rainfall.
Many actinomycetes exude antibiotics
like streptomycin in support of their
species. Antibiotics also possibly
hamper pathogens in general. Ironically,
some soil borne pathogens come from a
group of actinomycetes. One example is
the sweet potato pox caused by
Streptomyces ipomoeae.
Soil macrofauna
a) Earthworms (oligochaetes) and
the smaller ‘pot’ worms (belonging to the
family of the enchytraeid) are the most cited
members of the soil fauna. They present the
bulk of the soil animals. They are actively
decomposing soil organic matter. They
increase soil aeration and penetrability as well
as the aggregation of soil particles. The
channels created may also influence the
movement of nutrients and enable greater
infiltration of rainwater. Their most important
contribution, however, is the mixing of soil
materials. Worms need a moist but well-
aerated environment to thrive. They need
rough organic matter like leaves as their feed.
Normally in arable soil, their number can vary
from 30 to 300 per square meter.
Earthworms are important as Charles Darwin
believed more than a century ago. They are
keepers and restores of soil fertility. Different
types of earthworms, including the night-
crawler, field (garden) worm, and manure (red)
worm have different feeding habits. Some feed
on plant residues that remain on the soil
surface, while other types tend to feed on
organic matter that is already mixed with the
soil.
The surface-feeding night-crawlers fragment
and mix fresh residues with soil mineral
particles, bacteria, and enzymes in their
digestive system. The resulting material is
given off as worm casts. Worm casts are
generally higher in available plant nutrients,
such as nitrogen, calcium, magnesium, and
phosphorous than the surrounding soil and,
therefore, make an important contribution to
the nutrient needs of plants. They also bring
food down into their burrows, thereby mixing
organic matter deep into the soil. Earthworms
feeding on debris already below the surface
continue to decompose organic materials and
mix them with the soil minerals.
A number of types of earthworms, including
the surface-feeding night-crawler, make
burrows that allow rainfall to easily infiltrate
into the soil. These worms usually burrow to
three feet or more under dry conditions. Even
those types of worms that don't normally
produce channels to the surface help loosen the
soil, creating channels and cracks below the
1
surface that help aeration and root growth. The
number of earthworms in the soil ranges from
close to zero to over a million per acre. Just
imagine, if you create the proper conditions for
earthworms, you can have 800,000 small
channels per acre which conduct water into
your soil during downpours.
They aerate the soil pores for water passage by
tunneling through its depth, mixing in organic
materials as they go. The castings in the soil or
on its surface, are enriched by humus extra
minerals: 5 times as much nitrate nitrogen
surrounding soil, 2 times as much calcium, 2
½ times as available magnesium, 7 times as
much available phosphorus, and
exchangeable potassium..
An earthworm can produce in 24 hours enough
castings of its own weight, up to ½ pound a
year. 10 millions of 5 tons of castings a day,
and 12 tons of topsoil per year. Castings are
not affected by rain because they are colloidal.
The digestive processes of the earthworms kill
many harmful microorganisms of the soil,
while not destroying the useful actinomycetes
multiplying the content of the latter 7 times as
they pass through the worm.
Earthworms multiply very rapidly. There are
many and each type prefers a certain habitat
and cannot be induced to stay at home
anywhere else. It is best to transfer eggs so
when they hatch, the new worms will consider
their current location as home and will be
willing to stay there. Mature worms tend to go
back from where they came. It sometimes
helps to take them up with some of the soil
around them. They leave when organic matter
diminishes, but come from all directions when
there is a good supply of garbage, manure or
sludge, especially attracted to coffee grounds,
cornmeal, mash, and all kinds of carbohydrates.
Cold and heat earthworms cannot take; they
simply go far down into the soil or move away
to a more comfortable spot. Light hurts their
sensitive skins, so when exposed to it they
make every effort to squirm back into the soil.
To sound waves also they are sensitive,
vibrations and noises make them move far
down into the soil.
b) Nematodes or eelworms are
well known for their parasitic role in some field
crops. These non-segmented worms, usually
about one millimeter long and less than 50 mm
in diameter, can often reach population
densities of millions per square meter.
Nematodes need an aerobic habitat and feed off
the tissue of plants (under certain
circumstances they can become a pest) as well
as algae and other decomposing
microorganisms. Thus, they are affecting
primary production and decomposition in the
soil as well. A number of nematodes are
parasites as they feed on other members of the
soil microfauna, particularly protozoa and also
other nematodes. The activities regulate
secondary decomposition in the soil.
Nematodes live mostly in water films which
allow them high mobility. They can survive as
cysts in adverse conditions for longer times.
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Some types of nematodes feed on plant roots
and are well known plant pests. Diseases such
as pythium and fusarium, which enter feeding
wounds on the root, sometimes cause more
damage than the feeding itself. However, most
nematodes help in the breakdown of organic
residues and feed on fungi, bacteria and
protozoa as secondary consumers. In fact, as
with the protozoa, nematodes feeding on fungi
and bacteria also helps convert nitrogen into
forms for plants to use. As much as 50 percent
or more of mineralized nitrogen comes from
nematode feeding.
c) Termites represent another
group of decomposers in tropical soil. The
quantities of materials they deposit often
compare to the surface casting of earthworms.
The effect of termites on soil productivity,
however, I less beneficial than that of
earthworms chiefly because of the organisms in
the guts of the termites. These organisms
require energy in the form of carbon which
could otherwise serve different soil organisms
or be turned to humus. Termite deposits
commonly have a lower organic matter content
than the surrounding topsoil because termites
mix subsoil with low organic content into their
mounds. They accelerate the decomposition of
dead areas and grasses but disrupt crop
production by the fast growth of their nests or
mounds.
d) Springtails or Collembola are wingless
insects that live near or on the surface of soils.
The species living on the surface posses
springing organs, compound eyes, and
elongated antennae. None are more than a few
millimeters in length and those normally living
beneath the soil surface are even smaller. Like
mites, springtails are very numerous. They
show varying degrees of tolerance to different
environmental factors. Springtails feed on
leaves and other organic matter as well on the
microflora.
d) Arthropods composed another
large family among the soil animals and is
composed of many classes and species of
invertebrate animals, like insects, arachnids,
and crustaceans. They include saprophagous
animals like millipedes which feed on dead
plant materials as well as the centipedes which
are preying on soil animals. It is unclear if the
species decompose organic matter with their
own enzymes or if they depend on the
microflora in their guts. In general, arthropods
comprise a small part of the soil animal
biomass. Ants play an important role in
aerating the soil. They help improve the crumb
structure of the soil and transport a lot of
biomass. In many forest sites, ants represent
50 percent of the arthropods found in the soil.
5.0. Organic Matter and the Formation of Humus
All organic matter that remains in the field
will eventually fall to the ground and decay. The first
decomposers rapidly invade the organic debris. The
useful substances are converted into the body
materials of the decomposers. Larger soil animals
also produce feces or excrements which serve as
feeds for other life forms. Additional waves of
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decomposers follow and feed on whatever remained
from the debris.
Under the participation of many different
species and innumerable individuals of soil animals
and microorganisms, organic matter is turned into
humus. In a natural systems, it is an ever-repeating
process: leaves, straw, or stems of plants fall on the
soil after they have died and are decomposed. The
following illustrations provide a brief description of
the formation of humus.
1. Usually, the dead organic matter
already contains microorganisms like
fungi (molds) on its surface when it falls
down to the ground. The withering
continues on the surface of the soil. The
microorganisms make the leaves or straw
soft in texture.
2. After some weeks it becomes
digestible to various soil animals which
start feeding on it. Among these soil
animals are millipedes, mites, insect
larvae, springtails, and worms.
3. By passing through the intestines
of the soil animal, like earthworms, the
organic matter is turned into a medium
that is suitable for the growth of certain
bacteria, fungi, and algae.
4. The leaf or straw litter and the
feces of the animals are now suitable for
the growth of other microorganisms.
The microorganisms create the feed for
other species of springtails, mites, and
small worms.
5. The feces of these soil animals
and the remaining fragments of leaves
and straw are providing the feed for
algae, nematodes, and other
microorganisms that decay further the
organic matter. Near the end of this
cycle, the original organic matter is no
longer visible. Everything is converted
into humus. In this stage, humus can
persist for a long time.
The roles of the soil animals and microorganisms in
decomposing organic matter are complementary and
interrelated. The conversion of organic matter into
humus is a gradual process. It relies on the activities
of many different organisms and soil animals. The
intensity of this process depends mainly on the
quantity and quality of organic matter, the
temperature and the level of moisture in the soil. 5.1.
Organic matter and its importance
Soil organic matter, and the soil organisms
that live on it, are critical to many soil processes.
Organic matter allows high crop yields and reduced
input costs. Up to 15% of soil organic matter is fresh
organic material and living organisms. Another third
to one half is partially and slowly decomposing
material that may last decades. This decomposing
material is the active fraction of soil organic matter.
The active organic matter, and the
microbes that feed on it, are central to nutrient
cycles. Many of the nutrients used by plants are held
in organic matter until soil organisms decompose the
material and release the nutrients to plants. Organic
matter is especially important in providing nitrogen,
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phosphorus, sulfur, and iron. A soil with 3% organic
matter contains about 3,300 kg of nitrogen per ha
(225 kg/mu). Depending on the rate of
decomposition, 10 to 50 kgs may become available to
plants in a year.
The Changing Forms of Soil Organic Matter
1. Additions. When roots and leaves die, they
become part of the soil organic matter.
2. Transformations. Soil organisms
continually consume plant residue and other
organic matter, and then create by-products,
wastes, and cell tissue.
3. 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.
4. Stabilization of organic matter. Eventually,
soil organic compounds become stabilized
and resistant to further changes. This
compound is knows as “humus”, is the end
result of organic matter decomposition by
microbes.
5.2. Functions of Organic Matter
1. Nutrient cycling Increases the nutrient holding
capacity of soil (CEC). Is a pool of nutrients for plants. Binds nutrients, preventing them
from becoming permanently unavailable to plants.
Is food for soil organisms from bacteria to worms. These organisms hold on to nutrients and release them in forms available to plants.
2. Water dynamics Improves water infiltration.
Decreases evaporation. Increases water holding capacity.
Stabilized organic matter acts like a sponge and can absorb six times its weight in water. Water held by organic matter can make the difference between crop failure or success during a dry year
3. Structure
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Reduces crusting, especially in fine-textured soils.
Encourages root development. Improves aggregation,
preventing erosion. Prevents compaction, allowing
air to access the roots.
4. Other effects of soil organic matter Dark, bare soil may warm more
quickly than light-colored soils, but heavy residue may slow warming and drying in spring.
Many of the effects of organic matter are related to the activity of soil organisms as they use soil organic matter.
Plant residues and other organic material may support some diseases and pests, as well as predators and other beneficial organisms.
5.3 Determinants of Soil Organic Matter Levels
The amount of organic matter in soil is changed through:
the addition of organic matter (roots, surface residue, manure, etc.),
and the loss of organic matter through decomposition.
Five factors affecting both additions and losses:
1. Management. Practices that increase crop yield will increase the amount of roots and residue added to the soil each year. On the other hand, 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 if small aggregates are broken apart. In addition to changing the amount of soil organic matter, tillage practices also affect the depth of soil organic matter.
2. Vegetation. 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 previously forest.
3. 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.
4. Climate. 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.
5. Landscape position. Low, poorly-drained areas have higher organic matter levels, because less oxygen is available in the soil for decomposition. Low areas can accumulate organic matter that erodes off hill tops and steep slopes.
Differentiating Organic Matter and Humus
Humus is the product of the decomposing and synthesizing activities in the soil and exists in a dynamic state. Humus is composed of relatively stable organic substances. It can endure in the soil for longer time, but microbes continuously convert humus to provide energy and nutrients for plants. In natural systems there would also be a more or less continuous creation of new humus from plant debris.
Basically, three kinds of humus or humic compounds can be distinguished. They are differently resistant to degradation and solubility through acidic and alkali substances.
a) Fulvic acid has the lightest color and the lowest molecular weight. It is soluble in both acid and alkali and most susceptible to microbial attack.
b) Humic acid is medium in molecular weight and color. It is soluble in alkali but not in acids and is much more resistant to degradation.
c) Humin is highest in molecular weight and darkest in color. It is insoluble in both acid and alkali and is most resistant to decomposition by microbes. Even fulvic acid can last for several years or decades in the soil, depending on the environmental
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condition. The humus forming compounds are much more resistant to microbial decomposition than freshly applied crop or plant residues.
A similar distinction can be made between stable humus and friable humus (fertile mould). The former is long lasting and mainly found in the clay-humus complex and supports the soil structure. The latter can easily be decomposed by soil organisms and serves as plant nutrients.
Humus has distinct properties that make it different from the mineral soil material and from the less complex substances that emerge during its synthesis:
The presence of humus gives the topsoil a dark or brownish color. If the soil looks yellow or red, the humus content in the soil and the fertility is most likely very low.
Humus contains usually about 55 percent carbon and about four to five percent nitrogen. The ratio between carbon and nitrogen (C:N) is about 10:1.
Humus content in topsoils varies usually between 1.5 and 4.5 percent. Soils in rainforests contain 3 to 5 percent humus, tropical soils under annual cropping with no special measures have only 1 to 1.6 percent humus. The higher the clay content of a soil the greater usually is the humus content.
Humus serves as a stock of nutrients for higher plants. The higher the content of humus in the soil, the better the nourishment of the plants on it’s surface. Humus based plant nutrition makes inappropriate dosages impossible and losses of nutrients are negligible. Below a level of 1.5 percent, however plants can no longer be adequately nourished. Humus is not necessarily degraded to minerals, as the adherents of the mineral theory believe. Organic substances can be directly absorbed by plants and play an important role in their nutrition. Besides, humus provides several active agents, plant hormones, antibiotics, and increases the biological activity. This limits adverse organisms.
A high humus content enables an adequate supply of nitrogen, partially released from the humus. Besides, energy from humus supports nitrogen-fixing microorganisms that supply additional nitrogen to the crop.
Humus enhances the physical and chemical properties of soil. Its cation exchange capacity is higher than that of most silicate clays. This property is very important for poor soils.
The water holding capacity of humus exceeds that of clay four to five times (on weight basis). Humus can absorb water in equivalent of 80 to 90 percent of its weight.
Stable humus forms are involved in the formation of soil aggregates with day particles. The humus particles are very small and have a consequently high surface area. Thus, they act like glue linking mineral soil particles to so-called clay-humus complexes. Humus contributes greatly to the stability of soil aggregates of various sizes. Ideally, the soil consists of many small crumbs that allow plant roots to grow through. A soil with a good structure is well aerated and acts like a sponge that can absorb plenty of water. This is also very important for sandy soils. In clay soils, humus improves the aeration.
Humus reacts with metal cations to form complexes. Some are soluble, others not. Complexes involving iron (Fe3+) and aluminum (Al3+) are firm. Humus interacts also with oxides of iron and aluminum to form stable aggregates. Thus, an increased humus content can reduce toxic metal concentrations.
Humus serves as a buffer system for the pH value in the soil. The higher the humus content the more the soil tends to have an optimal pH value (between pH 6-7). This enhances the activities of bacteria and actinomycetes which are hampered in an acidic environment. Mot plant nutrients are also more readily available compared to acidic conditions.
Humus gives soil optimal condition and structure. The humus content of the soil influences the activities of the soil flora and fauna to a large extent. The higher the humus content of the soil the greater the dynamics of microbial processes. The more biologically active the soil, the higher is its fertility, the better is its structure. Humus enhances the capacity of the soil to supply mineral nutrients since the nutrients are stored in compounds which bacteria can break up to make them available to plant roots. Minerals contained in inorganic soil compounds are more difficult to access.
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One can consider humus a stock of nutrients. Taking into account that humus is constantly decomposed or degraded to provide nutrients to crops and energy or soil organisms, it is obvious that some agricultural practices have to replenish the reserves. Research results show that tropical soils lose about 4,000 kg humus per hectare within three years of cultivation. An average of fallow would replenish 250-600 kg per hectare, thus it would take 7 to 16 years to compensate for the losses.
Some years ago, German scientists suggested to keep a humus inventory or balance sheet for each field. Through long-term experiments they found out that different crops deduct differently from the humus
reserve. Organically grown crops draw 30 to 50 percent more humus than crops raised with synthetic fertilizer that substitutes humus to some extent. Several root crops, like potatoes, and vegetables like squash, cucumber and cabbage, for instance, use up more humus than carrots or tomatoes. Cereals, radish, and onions belong to the group of ‘low humus contents.’ The latter would require about one ‘humus unit’ (HU) per hectare. One humus unit is equivalent to one ton humus containing 50 kg nitrogen and 580 kg carbon. Potatoes require almost three HU while tomatoes, for example, need two HU per hectare in organic farms.
5.5. Building up soil organic matter / Humus
To build organic matter levels in topsoil, more
organic matter must be added than is lost to
decomposition and erosion. Increasing organic matter
is about changing the balance between how much
energy goes in and how much is burned off.
Another way to think of soil is like a giant wood
stove. You continually add organic matter (wood),
and it burns to release energy and nutrients that will
be used by plants and microorganisms. Ideally, you
want a slow, steady burn that releases nutrients to
plants as needed.
Intensive tillage aerates the soil and is like
opening the flue or fanning the flames.
Decomposition is desirable because it releases
nutrients and feeds soil organisms. But if
decomposition is faster than the rate at which organic
matter is added, soil organic matter levels will
decrease. It is just as important to increase the
amount of organic matter added into the soil.
Building organic matter is a slow process.
First, the amount of residue and active organic matter
will increase. Gradually, the species and diversity of
organisms in the soil will change, and amounts of
stabilized organic matter will rise. It may take a
decade or more for total organic matter levels to
significantly increase after a management change.
Fortunately, the beneficial effects of the changes
appear long before organic matter levels rise.
Why does it take so long for organic matter
levels to increase? An acre of soil six inches deep
weighs about 1000 tons, so increasing the proportion
of organic matter from two to three percent is
actually a 10 ton change. However, you cannot
simply add 10 tons of manure or residue and expect
to measure a one percent increase in soil organic
matter. Only ten to twenty percent of the original
material becomes part of the soil organic matter.
Much of the rest is converted over several years into
carbon dioxide.
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Figure 4. An illustration of soil organic matter losses and gains in response to tillage.
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.
Steps in Building up soil organic matter / Humus
Step # 1. Add organic matter
Grow more organic matter. Plant a high
residue rotation that includes sod crops that
leave lots of roots in the soil (small grains or
forages), crops that leave a lot of surface
residue (e.g., grain corn) or cover crops that
supply both.
Apply livestock manure. Manure is an
excellent way to build organic matter.
Step # 2. Reduce organic matter losses
Reduce tillage. Merely maintaining soil
organic matter levels is difficult if soil is
intensively tilled (such as with annual use of
a moldboard plow.) Reducing tillage means
leaving more residue, and tilling less often
and less invasively than conventional tillage.
No-till is the most extreme version of
reduced tillage, but is not easy to practice for
some farmers. As you reduce tillage, some of
the nutrients in manure or legumes will go
into building soil organic matter levels and
not into your crops.
Control erosion. The soil that erodes from
the surface of your land is the soil with the
highest concentration of organic matter.
Erosion is especially detrimental where
topsoil organic matter is shallow.
To maintain fertility and soil structure in a
sustainable organic farming system:
Recognize and respect soil as a
complex living system in which soil
organisms play an important role in digesting
organic matter, leaving soil soluble minerals
and CO2.
Identify farming practices that either
impair or enhance soil life.
Recognize soil life as the focus of
our efforts to practice good management.
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6.0. Natural Plant Nutrition
Under natural conditions, plants direct their
nutrition themselves in a manner quite different from
the concept of agricultural chemistry. During the last
decades, research revealed that the so-called
rhizosphere (root zone) is a zone of complex
interactions between plant root and soil organisms.
The growing roots constantly discharge various kinds
of exudates (basically carbohydrates) which supply
many soil organisms near the roots with humus or to
produce acidic substances that help to dissolve
nutrients from minerals.
During their growth plants continue to extend
their roots. Through the tips of their roots, plants
secrete mucus (slime). This serves as energy source
for bacteria and other organisms and encourages
them to break down humus into substances that
plants can absorb.
During their growth plants establish new
roots, while older roots die. Thus, during its growth
plants produce organic matter that is available to the
soil biota. Cereal plants, for example produce around
four times more roots during their development than
are left at the end of their cycle.
The microorganisms in the soil work together
with the roots of the plants for the growth of the
vegetation. The mucus secreted by the tips of the
roots encourages soil organisms to convert organic
substances (humus) into nutrients for the plants. The
plants absorb minerals like nitrogen and phosphate,
and other more complex substances. This natural
process is only possible by the interaction of plant
roots and soil organisms. This process provides the
plants with the proper quantities of nutrients. The
application of chemical fertilizer cannot replace this
system. It will disturb or destroy many soil
organisms. Further, the plants will suffer from
unbalanced nutrition which makes them susceptible
to pests and diseases.
7.0. Nitrogen fixation
Nitrogen is the most abundant gas in the
atmosphere of the earth. Since the first occurrence of
life, it has been essential for plant growth, as it is
necessary in synthesizing protein molecules. In
natural systems, most of the nitrogen is recycled
through the organic matter. Agricultural products
take considerable quantities of nitrogen out of the
system. Thus, nitrogen is usually the most limiting
factor for plant growth. Besides, even soils covered
with natural vegetation lose some nitrogen tot e
atmosphere which has to be replaced by other
mechanisms. For agriculture and related sciences,
there is still q question not fully solved: how does
atmospheric nitrogen become available to plants and
especially to crops in the fields? Plants themselves
lack the capacity to absorb nitrogen directly from the
air. They depend on other organisms to supply it in a
form they can incorporate into their metabolism.
Farmers knew that planting leguminous
plants improved the growth of other crops as well.
The Romans used to sow lupines in rotation with the
main crops around 2,000 years ago. In the 15 th
century Dutch farmers rediscovered this method.
Chinese and Japanese farmers are using clover and
soybeans to improve the fertility of their fields for
almost 4,000 years.
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The wisdom of these practices fell into
oblivion with the development of modern sciences
that paved the way for agricultural chemistry. With
the introduction of synthetic nitrogen, nature’s ways
of providing the essential nutrient seemed outmoded.
That natural systems and optimized cropping systems
can supply sufficient nitrogen to achieve yields
similar to those obtained by use of synthetic inputs.
Biological nitrogen fixation is mediated by
organisms that can use atmospheric nitrogen as their
sole source of nitrogen. They are called diazotrophs:
‘Diazo’ refers to dinitrogen, the form of nitrogen
which occurs usually in the atmosphere. Gaseous
nitrogen occurs as a molecule consisting of two
nitrogen atoms (N2) connected by triple bond.
Because of the triple bond, it requires relatively high
amount of energy to break up the dinitrogen
molecule.
Scientists distinguish basically between
symbiotic nitrogen fixation and nonsymbiotic
nitrogen fixation. Both ways have in common that
bacteria or actinomycetes incorporate nitrogen, or
more exactly dinitrogen, from the air. Plants as well
as all other organisms need nitrogen either bound
with oxygen (NO2) or hydrogen (NH4). The nitrogen
fixing organisms require energy in the form of
carbohydrates derived from plants and organic matter
to break up the N2 molecule. The energy requirement
differ significantly among the various species and
genera. An important factor in the process is the
enzyme nitrogenase which the organisms has to
protect against oxygen. As energy source glucose is
converted into adenosine-triphosphate (ATP). The
glucose molecules are either derived from
photosynthetic products of the host plant or from
decomposed organic matter. An exception is the
group of cyanobacteria, which are capable of
photosynthesis.
7.1. Symbiotic Nitrogen Fixation
Symbiotic nitrogen fixation in nonleguminous
plants. Several kinds of shrubs and trees as well as
many grasses including rice and sugarcane varieties
possesses root nodules with nitrogen fixing capacity.
The Frankia symbiosis is common among
nonleguminous shrub and trees. It can also be found
in rice. Frankia is a filamentous organisms closely
related to actinomycetes. It infects root hairs or
invades cells where it forms nodules. The nodules
are perennial, modified lateral roots with lobes up to
five centimeter in length.
Another symbiotic relationship exists
between the aquatic fern azolla and the bacteria
Anabaena. The bacteria occur mainly in
specialized cells called heterocysts. The aquatic
fern azolla is a common green manure used in
Vietnam and China for rice production (see
section: Nitrogen fixation in flooded rice fields).
Symbiotic nitrogen fixation in leguminous plants
Nitrogen fixation in legumes depends
upon a highly coordinated sequence of
interactions between plants of the family
Leguminosae and soil microorganisms belonging
to the genera Rhizobium and Bradyrhizobium.
The symbiosis results in the formation of
nodules, usually on the roots. Most of the
leguminous plants form nodules. An exception is
Sesbania rostrata, a green manure crop that can
grow in flooded area. It develops nodules also on
the lower stem. The plants supply carbohydrates
2
from their leaves for the bacteria which in turn
fix atmospheric nitrogen and supply amino acids
for both organisms. The root nodule bacteria
may become antagonistic to the plant if the plant
experiences stress due to drought or at the time of
flowering or setting of seed.
Nodules can only fix nitrogen actively if the
plant is adequately supplied with all nutrients for
active growth. Obviously, the plant must be in good
condition to benefit optimally from the symbiosis.
Of the total respiration of soybean plants, for
example 57 percent was from the top, 18 percent
from the roots and 25 percent from the nodules. The
tolerance of acidity depends on the Rhizobium strain.
Rhizobium nodules appear to be more
efficient fixers of nitrogen than the nonsymbiotic
bacteria. The latter needs 2.5 times more
carbohydrates than the former to fix one gram of
nitrogen.
7.2. Nonsymbiotic nitrogen fixation
To the group of free-living organisms belong
the cyanobacteria that resides on the soil surface.
Microbes like Azotobacter, Azospirillum, and
Beijerinckia associate loosely with the roots of
certain plants. The last mentioned group depends on
carbon supplied from either decomposing organic
matter or from plant roots. There are genera of
bacteria that live in aerobic condition. Other genera
prefer the anaerobic environment, like the genera
Clostridium and Enterobacter. Some genera relate
only to one plant species like the Azobacter paspali
which associates with the tropical grass Paspalum
notatum (Bahia grass). Bahia grass shows good
growth even on N-poor soils. Several sugarcane
varieties contain in their internal tissues the
bacterium Acetobacter diazotrophicus. Due to its
location, the organisms captures a significant part of
the sugar to drive nitrogen fixation. It is, however,
free of competition from other organisms. Some
cultivars may derive 100 to 150 kg per hectare from
this symbiosis. The contribution of the free living N-
fixers to the N reserve of the soil varies between 10
and 90 kg per hectare per year.
7.3. Nitrogen fixation in flooded rice fields
Of great agricultural importance is the
Azolla-Anabaena symbiosis. Azolla is a floating fern
that has long been used as nitrogen source for
lowland rice. Anabaena, a blue-green algae is
contained in ‘pockets’ of the fern. The rate of
nitrogen fixation (83-125 kg/ha) is comparable with
that of legumes.
Most kinds of nitrogen fixation occurring in
rice fields, however, are non-symbiotic. Usually,
they contribute most of the nitrogen. Nitrogen
fixation is associated with the decomposition of straw
and can take place in the root zone (rhizosphere) of
the rice plant. In addition, blue-green algae and
various heterotrophic organisms fix nitrogen.
Rhizosphere associated fixation
Among the bacteria which settle on the
root surface are also diazotrophs. They comprise
about 10 percent of the microbial activities in the
rhizosphere and obtain their energy mainly from
root exudates. Their contribution to nitrogen
supply is relatively low with estimates ranging
between 1 to 7 kg per hectare and crop. Several
nitrogen-fixing bacteria have also been found to
colonize in the interior of the roots of rice and
other grass plants. They persist within or
between the cells of the plant tissue. Rhizobium
2
which enters into symbiosis with legumes has
been found to associate with rice. The bacteria-
rice relationship, however, does not lead to the
formation of nodules.
Decomposition of straw
The application of straw to the surface or
subsurface layers of flooded soils result in
intense microbial activity. This creates a
conducive condition for various nitrogen-fixing
bacteria. They use the carbohydrates from
decomposed straw as their source of energy. The
nitrogen-fixing bacteria include the
heterotrophic. Azotobacter, Azospirillum
lipoferum, and Pseudomonas diazotrophicus a
well as phototrophic bacteria which can utilize
light energy. It is estimated that straw-associated
nitrogen fixation can contribute 2-4 kg nitrogen
per ton of rice straw. Given a supply of five tons
rice straw, 20 kg could be fixed. Another source
estimates the possible nitrogen fixation of about
25 kg nitrogen per hectare within 30 days.
Blue-green algae (Cyanobacteria)
Free-living nitrogen fixing cyanobacteria
are abundant in flooded rice fields. Due to the
photosynthetic nature of these organisms, the
quantity of fixed nitrogen is independent from
the supply of energy from the soil. Some species
liberate extracellular ammonia (NH4), Blue-green
algae can fix up to 70 kg nitrogen per hectare per
cropping. The inoculation with cyanobacteria
increased rice yields by 337 kg grains per
hectare, indicating a gain of about 10 kg nitrogen
per hectare. The activities of the blue-green
algae appear to decrease if the ammonium
content in the water is increased.
Heterotrophic nitrogen fixation
This group is comprised of free-living
bacteria. Some of these live in association with
roots and other submerged portions of the rice
plant. Most members of this group, however,
depend on the supply of organic matter. They
live predominantly on organic debris. The
bacteria need between 19 to 117 gram of carbon
to fix one gram of nitrogen. They can add an
average of 7 kg nitrogen per hectare per
cropping. The quantity, however, varies
significantly among varieties. The ability of the
rice plant to optimize this associated is controlled
by several genes.
2
Annex A. Review of Major elements required for plant growth
Nitrogen (N), phosphorus (P), and potassium (K) are the 3 most important soil nutrients required for plant growth. In all, plants require sixteen elements, each of which has one or more special function in the plants growth and development. Some elements like calcium, magnesium and sulfur (macronutrients) are required in relatively large amounts.
Element Function Symptom of Deficiency
Nitrogen (N)
Essential for plant growth. Necessary for protein production
by the plant. Necessary for many critical plant
functions (photosynthesis, cell division and plant growth).
Adequate N produces a dark green color in the leaves, caused by a high concentration of chlorophyll. Nitrogen deficiency causes a yellowing of the leaves, which first starts on older leaves. Nitrogen deficient plants tend to be stunted, grow slowly, and produce fewer tillers than normal.
Phosphorus (P)
Essential for plant growth and is especially vital to early growth.
Promotes early root formation and growth
Improves the quality of many fruits, vegetables and grain crops.
The first sign of P deficiency is an overall stunted plant. A purple or reddish color is often seen on young plants especially at low temperatures. With severe P deficiency, dead areas may develop on the leaves, fruits and stems.
Potassium (K)
Essential for protein synthesis and cell division.
Decreases water requirement of plants
Important in fruit formation. Helps plants survive winter Helps improve stalk strength and
resistance to lodging
Potassium deficiency symptoms show up as scorching or firing along the margins of older leaves in most plants, especially grasses. The leaves may later turn brown. Deficient plants grow slowly and have poorly developed root systems. Stalks are weak, lodging is common and seed and fruits are small and shriveled. In grass/legume forages the legume will not persist in the mixture when K is deficient.
What are the important sources of nutrients?
Table 2: Macronutrients required by plants and their sources*Supplied by Air and Water Supplied by Soil Supplied by Precipitation
Carbon (C) Nitrogen (N2)Hydrogen (H2) Phosphorous (P4) Sodium (Na)Oxygen (O2) Potassium (K) Chlorine (Cl)
Calcium (Ca)Magnesium (Mg)
Sulfur (S8)Silicon (Si)Iron (Fe)
Aluminium (Al)*Other nutrients are required in small amounts (known as micronutrients) and are obtained from the soil.
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Nitrogen: Animal and human manures, nitrogen-fixing plants (clover, alfalfa, peas, etc.) and soil microbes are the major sources of N on ecological farms. N can also be supplied by chemical fertilizers and small amounts are dissolved in rainwater or produced by lightning. Nitrogen mineralization is a major source of nitrogen to the soil. Mineralization is the process of decomposition of organic matter. The rate of nitrogen mineralization depends largely on the temperature and moisture content of the soil. Soil tillage accelerates the mineralization process because it accelerates soil warming and increases aeration. Efforts to build up soil organic matter in the cropping rotation need to be made to support mineralization processes to release N for crops that require a N supply.
Phosphorus: The most common source of P outside of farm manures, is phosphate rock from which various commercial fertilizers are made. Most ecological farmers prefer to rely on natural decomposition and soil processes to recycle P and make insoluble P fertilizers and fixed soil P become available. This
process is more efficient on ecological farms that have a “living” soil where soil microbes, earthworms fungi and root acids work to make P available to the plant.
Potassium uptake in plants is also greatly enhanced through the development of a “living” soil. Most ecological farmers emphasize strategies that make the vast natural reserves of potassium in soil become available for plant growth especially on loam and clay soils. Deep rooted green manure crops, soil microorganisms, earthworms and root acids play a major role in making potassium more available to plants. Improving soil structure and the soil organic matter content improves potassium uptake by improving the soils cation exchange capacity (CEC) (see below) and enabling more extensive and deeper rooting of plants. Farmers then need to manage the supply of potassium in the farm cycle to optimize its availability for crops with high demands. For example many farmers will apply manure to potatoes because it has a high potassium demand.
Manure/compost is an excellent way to supply trace elements to crops and will generally prevent deficiencies from occurring!
Common Soil Amendments having minimal negative impact on soil life these include:
Calcitic limestone Increases pHCompost Storehouse for plant nutrientsDolomitic limestone increases pH (may cause excessive magnesium imbalance)Gypsum Adds CalciumLeaves & Leaf Mold Humus, Ca, Mg, N,P,KManure N,P,K (varies with type)
Rock PhosphatePhosphorus and Trace Minerals
Sawdust Mulch, High Carbon for CompostSeaweed Potassium and Trace MineralsSul Po Mag Adds PotassiumStraw Carbon source for compost, aerates soil
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How nutrients move within the soil?
Nitrogen:
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Figure 2. the cycling of nitrogen in the environment. Nitrogen comes from organic matter => NH4 (ammonium) => NO2 (nitrite)=> NO3 (nitrate)=> and then back into the plant, or organic matter. (http://muextension.missouri.edu/xplor/envqual/wq0252.htm)
In the nitrate form, nitrogen is the most mobile nutrient in the soil and subject to leaching. N in the organic ammonium form is not subject to leaching and will stay stuck to the soil particles. The availability of N to crops depends on the conditions for the breakdown of organic matter. Urea fertilizer converts to ammonium when it is applied to the soil, and then is converted to nitrate. The silty and porous loess soil with low organic content cannot hold or bind nitrogen fertilizer and it can be readily leached into ground water and water courses. http://www.geog.ouc.bc.ca/physgeog/contents/9s.html
Phosphorus: Phosphorus sticks so tightly to the soil, that it is safe to say that it doesn't move unless the soil does. Unfortunately large losses of P through erosion can occur because the concentration is highest on the soil surface.
Potassium: It also sticks to soil particles, although not quite as tightly as phosphorus. In some very coarse sandy or gravelly soils with low organic matter, there may have been some loss of potassium through leaching.
Micronutrients: A healthy soil microbial community is essential for release of nutrients from organic matter. “Nutrient cycling” ensures that, when a plant dies, all its nutrients end up recycled back into the soil. Bacteria and fungi break down the plant tissue and make available the nutrients which can be once
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