Biological properties of soil
Soil is not a dead mass but a habitation of millions of
organisms, which includes crabs, snails, earthworms, mites, millipedes,
centipedes
The soil organisms are of two types: Micro flora and Micro fauna, Bactro Actinomycetes, Fungi and Algae relate to former and Protozoa, Nematodes relate to some of these have symbiosis with other organisms. They act on plant and animal residue and release the food material which in turn used by plants.
The soil organisms are of two types: Micro flora and Micro fauna, Bactro Actinomycetes, Fungi and Algae relate to former and Protozoa, Nematodes relate to some of these have symbiosis with other organisms. They act on plant and animal residue and release the food material which in turn used by plants.
Soil is made partially of organic
and nonorganic elements. Nonorganic elements include minerals and water,
necessary compounds for plants to survive, but native to the earth outside of
organisms. Organic, or biological, elements are organisms and organism components
that plants also need to survive.
Organic matter
Organic matter
constitutes about 5 % of the bulk soil volume or about 10% of the only solid
fraction of the soil. Organic component of the soil consists of substances of
organic origin i.e. living and dead. The soil organic matter consists of a
whole series of products which range from un-decayed animal and plant residues
to fairly resistant decomposition products like humus. Humus is formed as a
result of decomposition of freshly added plant and animal residues. It is an
amorphous, brown to black coloured material which is quite resistant to
microbial decomposition plant relationship
Soil Microorganisms
Microorganisms are bacteria and
other single-celled creatures that live in the soil. Soil hosts many different
types of microbes. Algae and protozoa also call some types of soil home. With
so many different types of microorganisms, they create a variety of effects for
plants that grow in the soil. Some help process nutrients that the plants need,
while others attack and destroy plant tissue.
One gram of topsoil may contain:
- as many as one billion bacteria
- up to 100 million actinomycetes
- one million fungi
- 100 nematodes
Importance of Soil
Organisms
- Responsible for cycling of C, N and other nutrients
- Enhance soil structure
- Relocate and decompose organic materials
- Maintain soil quality and health
- Increase soil aeration and penetrability
- The soil organisms are broadly classified in to two groups viz soil flora and soil fauna, the detailed classification of which is as follows.
A.
Soil Flora
a) Microflora: 1. Bacteria 2. Fungi, Molds, Yeast, Mushroom 3. Actinomycetes, Stretomyces 4. Algae eg. BGA, Yellow Green Algae, Golden Brown Algae.
a) Microflora: 1. Bacteria 2. Fungi, Molds, Yeast, Mushroom 3. Actinomycetes, Stretomyces 4. Algae eg. BGA, Yellow Green Algae, Golden Brown Algae.
·
1. Bacteria is again classified in I)
Heterotrophic eg. Symbiotic & non - symbiotic N2 fixers, Ammonifier,
Cellulose Decomposers, Denitrifiers II) Autrotrophic eg. Nitrosomonas,
Nitrobacterium, Sulphur oxidizers, etc.
·
b) Macro flora: Roots of higher
plants
Soil Fauna
Animals that inhabit the soil. Soil
fauna includes representatives of many groups of terrestrial and aquatic
animals. Protozoans, rotifers, and tiny nematodes (nanofauna) inhabit capillary
and even film water. One sq. m of soil contains from ten to several hundred
animals of the mesofauna group and from several thousand to several hundred
thousand individuals of the micro fauna group. There are thousands of
protozoans in 1 g
of soil.
Soil
fauna occupies mainly the upper horizons, to a depth of 20–40 cm. In arid
localities, only a few species penetrate to a depth of several meters.Soil
fauna, an important factor in soil formation, influences all the properties of
soil, including fertility. Its activity accelerates the humification and
mineralization of plant residues, alters the salt regime and soil pH, and
increases the soil’s porosity and permeability to water and air.
In many countries, to increase soil
fertility, especially of land newly brought under cultivation, the soil fauna
is enriched by the introduction of useful species and by the application of
compost abounding in useful species.
a) Micro fauna: Protozoa, Nematodes
b) Macrofauna: Earthworms. moles,
ants & others.
As soil inhabit several diverse
groups of microorganisms, but the most important amongst them are: bacteria,
actinomycetes, fungi, algae and protozoa..
Organic Nutrients
·
Organic nutrients are the broken-down particles
of decomposed organisms that have been separated out by the bacteria and
creatures that live in the soil. Carbon, nitrogen, sulfur
·
and phosphorus are all important organic
nutrients that plants need to survive but cannot find without actions by
microorganisms to alter the elements into accessible compounds.
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Chemical Properties of
Soil
Soil pH
. Soils may have either
an acid or an alkaline reaction, or may be neutral. The measure of the
chemical reaction of the soil is expressed by its pH value. The pH value
varies from 0 to 14, with pH = 7 indicating that
the soil has a neutral reaction. Values smaller than 7 indicate acidity and
values greater than 7 indicate alkalinity. The further from the neutral
point, the greater the acidity or the alkalinity.
Soil
pH and plant growth
Nutrient Availability
Increasing
soil pH: Liming materials (pure calcium carbonate or dolomitic
lime) will increase soil pH.
Wood
ashes are another product to raise soil pH. They also are a source of K, Ca,
and Mg. Some composts also can increase soil pH.
Gypsum is calcium sulfate. It is not a substitute for lime,
and has little effect on soil pH. Gypsum only improves structure in soils
that have extremely high sodium contents (rare in the NW).
Decreasing soil pH: Some plants thrive under acidic
conditions (ex. rhododendrons, blueberries, and azaleas). Elemental sulfur is
often recommended (50 lb S per 1000 sq. ft). Ammonium and ammonium-forming N
fertilizers will also result in a decrease in soil pH.
Soil Colloids
The clay fraction of the soil contains particles less than 0.002
mm in size. Particles less than 0.001 mm size possess colloidal properties
and are known as soil colloids.
Soil colloids are very
important in agronomy: they absorb ammonium, potassium, calcium, magnesium,
and phosphate ions from soil solutions, and promote the formation of soil
structures.
Soil colloids consist of
three groups of compounds—organic, mineral, and organomineral (complex). Most
of the organic matter in the soil is in dispersed colloidal form. Mineral
colloids consist primarily of clayey minerals. The composition of the
colloidal mineral fraction is different in different types of soil. The
organomineral soil colloids are represented primarily by compounds of humus
substances with clayey and other secondary minerals. Under natural conditions
colloidal particles form in two ways—condensation and dispersion
The
colloidal state refers to a two-phase system in which one material in a very
finely divided state is dispersed through second phase.
The examples are:
Solid
in liquid - Clay in water (dispersion of clay in water)
Liquid in gas -Fog or clouds in atmosphere.
General
Properties of Soil Colloids
1. Size: The
most important common property of inorganic and organic colloids is their
extremely small size. They are too small to be seen with an ordinary light
microscope. Only with an electron microscope they can be seen. Most are
smaller than 2 micrometers in diameter.
2. Surface area: Because of their small size, all soil
colloids expose a large external surface per unit mass. The external surface
area of 1 g of colloidal clay is at least 1000 times that of 1 g of coarse
sand. Some colloids, especially certain silicate clays have extensive
internal surfaces as well. These internal surfaces occur between plate like
crystal units that make up each particle and often greatly exceed the
external surface area. The total surface area of soil colloids ranges from 10
m2/g for clays with only external surfaces to more than 800 m2/g for clays
with extensive internal surfaces. The colloid surface area in the upper 15 cm
of a hectare of a clay soil could be as high 700,000 km2/g
3. . Surface charges: Soil colloidal surfaces, both external
and internal characteristically carry negative and/or positive charges. For
most soil colloids, electro negative charges predominate. Soil colloids both
organic and inorganic when suspended in water, carry a negative electric
charge. When an electric current is passed through a suspension of soil
colloidal particles they migrate to anode, the positive electrode indicating
that they carry a negative charge. The magnitude of the charge is known as
zeta potential. The presence and intensity of the particle charge influence
the attraction and repulsion of the particles towards each other, there by
influencing both physical and chemical properties.
The
negative electrical charge on clays comes from
i) Ionizable hydrogen ions and ii) Isomorphism substitution.
4. Adsorption of cations: As soil colloids possess negative
charge they attract the ions of an opposite charge to the colloidal surfaces.
They attract hundreds of positively charged ions or cation such as H+, A13+
Ca2+ , and Mg2+. This gives rise to an ionic double layer.
The
process, called Isomorphous substitution and the colloidal particle
constitutes the inner ionic layer, being essentially huge anions; with both,
external and internal layers that are negative in charge. The outer layer is
made up of a swarm of rather loosely held (adsorbed) cations attracted to the
negatively charged surfaces. Thus a colloidal particle is accompanied by a
swarm of cations that are adsorbed or held on the particle surfaces.
5. Adsorption of water: In addition to the adsorbed cations, a
large number of water molecules are associated with soil colloidal particles.
Some are attracted to the adsorbed cations, each of which is hydrated; others
are held in the internal surfaces of the colloidal particles. These water
molecules play a critical role in determining both the physical and chemical
properties of soil.
5. Cohesion: Cohesion is the phenomenon of sticking
together of colloidal particles that are of similar nature. Cohesion
indicates the tendency of clay particles to stick together. This tendency is
primarily due to the attraction of the clay particles for the water molecules
held between them. When colloidal substances are wetted, water first adheres
to the particles and then brings about cohesion between two or more adjacent
colloidal particles.
6. Adhesion: Adhesion refers to the phenomenon of
colloidal particles sticking to other substances. It is the sticking of
colloida1 materials to the surface of any other body or substance with which
it comes in contact.
7. Non permeability: Colloids, as opposed to crystalloid, are
unable to pass through a semi-permeable membrane. Even though the colloidal
particles are extremely small, they are bigger than molecules of crystalloid
dissolved in water. The membrane allows the passage of water and of the
dissolved substance through its pores, but retains the colloidal particles.
Soil salinity
Soil Salinity and Interpretation
Cation-Exchange
Capacity
A
cation is a positively charged ion. Most nutrients are cations: Ca2+, Mg2+, K
+, NH4 +, Zn2+, Cu2+, and Mn2+. These cations are in the soil solution and
are in dynamic equilibrium with the cations adsorbed on the surface of clay
and organic matter. CEC is a measure of the quantity of cations that can be
adsorbed and held by a soil.
CEC
is dependent upon the amount of organic matter and clay in soils and on the
types of clay. In general, the higher OM and clay content, the higher the
CEC.
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- Temperature (70°-100°F most active microbes)
- Moisture (Field capacity is optimal)
- Aeration (want a nice mix of pores filled with water and air)
- pH (optimal pH is 6-7)
- Soil organic matter
Soil
Microorganisms: Bacteria
- Most numerous in soil
- Most diverse metabolism
- Can be aerobic or anaerobic
- Optimal growth at pH 6-8
- Examples: Nitrosomonas and Nitrobacter in nitrification processes, N2 fixers, fire blight is caused by a bacterium
Soil Microorganisms: Actinomycetes
- Transitional group between bacteria and fungi
- Active in degrading more resistant organic compounds
- Optimal growth at alkaline pH
- 2
important products:
– produce antibodies (streptomycin is produced by an actino)
– produce geosmin - Negative impact - potato scab (Streptomyces scabies)
Soil Microorganisms: Fungi
- Dominate the soil biomass
- Obligate aerobes
- Can survive desiccation
- Dominate in acid soils
- Negative
impacts:
– Apple replant disease (Rhizoctonia, Pythium, Fusarium, andPhytophtora)
– Powdery mildew is caused by a fungus - Beneficials:– Penicillium
Nematode-trapping Fungus
Plant
root / Soil / Microbial Interactions
Beneficial
- Symbiotic associations such as that found with Rhizobia (N2 fixing bacteria, ex. legumes)
- Fungi-mycorrhizal associations: important for water and P uptake; also improves soil structure
- Earthworm channels: improve permeability and aeration
Deleterious
- Agrobacterium (bacteria) cause gall formation in plants
- Fungi causing soil-borne plant rot diseases
- Rhizoctonia and Pythium (involved with replant disease)
Soil Nutrient Cycling
- Materials are broken down by macro and meso-fauna
- Nutrients are taken up and converted by lower life forms in the soil
- They convert these nutrients to organic forms within the cell or to inorganic forms released to soil
- These organisms die and are decomposed by other organisms
- This also releases inorganic ions for plant or other microbe uptake and…
- The cycle starts all over
Nitrogen Cycle: Nitrogen is the nutrient needed in
largest amounts by plants and is the most commonly applied fertilizer. Excess N
can have negative affects on plant growth and crop quality as well as harming
the environment, especially water quality.
Nitrogen
is present in one of five forms in soil:
- Organic N: 90% of N is in organic form. It must be mineralized to become available.
- Ammonium N (NH4+): Inorganic, soluble form
- Nitrate (NO3-): Inorganic, soluble form
- Atmospheric N (N2): 80% of atmosphere but unavailable to most plants except N-fixers
- Nitrite (NO2-): only under anaerobic conditions. This form is toxic to plants and normally will not be present in significant amounts in soil.
3
Essential Nutrients
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Primary Nutrients
Nitrogen
(N)
Phosphorus (P) Potassium (K) |
Micronutrients
Zinc
(Zn)
Iron (Fe) Copper (Cu) Manganese (Mn) Boron (B) Molybdenum (Mo) Chlorine (Cl) |
|
Secondary Nutrients
Sulfur
(S)
Calcium (Ca) Magnesium (Mg) |
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Trick
to remember nutrients:
“See
(C) HOPKiNS (name) CaFe Managed By Mine CuZins, Mo and Claude”
C H O P K N S CaFe Mg B Mn CuZn Mo Cl
Plants
receive Carbon, Hydrogen and Oxygen from water and air.
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Nutrient Deficiencies
- N, P, and K are required in the largest amounts and are commonly deficient (especially, N)
- Deficiency
symptoms for any element depend primarily on two factors:
– the functions of the element
– whether or not the element is readily translocated from older leaves to younger leaves
Nutrient Toxicities
- Nutrients applied as a result of over fertilization or at the wrong time can have deleterious affects on plant growth.
- Want to balance fertilizer application with plant needs and environmental concerns
- Excess N, for example, can harm plants and contaminate surface and ground water possibly making drinking water unsafe.
Soil
Microorganisms
Microorganisms constitute < 0.5% (w/w) of the
soil mass yet they have a major impact on soil properties and processes. 60-80
% of the total soil metabolism is due to the microflora. In numbers, soil
microorganisms beat out all other organisms. One gram of topsoil may contain:
- as many as one billion bacteria
- up to 100 million actinomycetes
- one million fungi
- 100 nematodes
Importance of Soil
Organisms
- Responsible for cycling of C, N and other nutrients
- Enhance soil structure
- Relocate and decompose organic materials
- Maintain soil quality and health
- Increase soil aeration and penetrability
- Involved in disease transmission and control
Soil
Fauna (or zoo)
- Macrofauna: Mice, moles, etc.; Earthworms and other worms; Ants, beetles, termites, spiders
- Mesofauna: Nemaodes, arthropods (mites, centipedes, and springtails), molluscs
- Microfauna: Protozoa
Soil Macrofauna: Earthworms
- Important in mixing and redistributing OM
- Enhances soil physical properties
- Neutralize soil pH
- Increase the availability of many nutrients
- Stimulate microbial populations
- May reduce levels of harmful nematodes
Soil Mesofauna: Nematodes
- Microscopic non-segmented roundworms
- Ecologically diverse
- Found in all habitats
- Overall, 10-20 million/m-sq are found
- Major consumer group
- Both free-living and parasitic groups exist (predatory nematode pictured)
Soil Microfauna: Protozoa
- Important in mineralization and immobilization of N, P, and S
- Most numerous soil fauna
- Prey on microbes (especially bacteria)
- Enhance nitrification rates
- Suppress bacterial and fungal pathogens
- Can be agents of plant disease
The
Soil Flora (or Garden)
- Macroflora: Vascular plants, Mosses, etc.
- Microflora: Bacteria, Actinomycetes, Fungi, Algae
Influences on Microbial
Activity
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