Lecture 39: Soil Organisms
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Lecture 39: Soil Organisms


Welcome friends to this new lecture of Soil
Science and Technology. And in this lecture, we will be trying to
finish this soil organic matter, and then we will be discussing about the Soil Organisms. So, obviously in the last lecture we if we
remember that we talked about different types of organic matter decomposition, how organic
matter decomposed based on the organic matter quality, then we talked about different pools
of soil, different pools of carbon, and then, you know, how organic matter forms. So, let us starts from here. And in this, in this in this slide, you can
see the soil organic matter distribution. Obviously, if you see the top, you know, at
the top layer of the soil, the concentration or the amount of organic matter is always
higher as compared to the subsoil. So, as we go down from top layer to sub soil
layer, obviously the organic matter percentage of organic matter always goes down. And if you see the, you know, organic matter,
obviously these hydroxyl groups, and you know acidic groups are there, and these groups
basically undergoes dissociation, and basically they attract these positive cations. So, we will discuss in details later on. So, this is just an, you know, this just gives
an idea about how these organic matter attracts different types of cations onto their surface. So, distribution of organic matter depends
from, it varies from one soil to another soil. So, let us talk about the humus. Now, humus is a complex and resistant mixture
of brown and dark brown amorphous and colloidal organic substances that results from microbial
decomposition and synthesis and has chemical and physical properties of great significance
of soil. So, it is we have already discussed that humus
is a more resistant product; it is developed from different types of microbial decomposition. And you can see here, this is a three-dimensional
view of a humus molecule which is very very complex. And one other, you know, another simplified
diagram showing the principal chemical group, which is responsible for high amount of negative
charge on humus colloids. If you remember humus in case of humus, the
CEC is quite couple of folds higher than that of the CEC of clay. So, obviously you can the three groups, we
can see here are basically carboxylic group, sorry, basically carboxylic group, and then
phenolic hydroxyl group, and alcoholic hydroxyl group. So, and also the dissociated forms are given
here. So, you know this dissociated you can see,
they are negatively charged. And as a result, they can attract these positively
charged cations. And also some positively dissociated charge
in which can attract the negative, you know, the anions also, so that is the large, you
know, you can see that is why it can absorb a large amount of plant nutrients for maintaining
the soil fertility. So, what is basically, you know, this humus
is composed of. So, the humic groups. If you if you see humic substances are aromatic
type of structures, and the humic substance are classified. If you break down a humus, we will get basically
three types of humic substances. One is called humic acid, another is called
fulvic acid, another is humin. And these substances are classified based
on their resistance to degradation and solubility in acids and alkali. So, we will see that later on. Again humic acid, fulvic acid, and humin;
so, what is humic acid. The fraction of the humic substances that
is not soluble in water under acidic condition that is pH less than 2, but is soluble in
a higher pH values. And they can be extracted from soil by various
reagents and which is insoluble in dilute acids. So, again remember that humic acid is insoluble
in acidic condition less than pH 2, however it is soluble in alkaline condition. The second important component is fulvic acid. And the fraction of humic substances that
is soluble in water under all pH condition, and they remain in solution after removal
of the humic acid by acidification. So, the fulvic acid is basically soluble under
all pH condition, whereas humic acid is only soluble in the higher pH values. And third one is humin. The fraction of humic substances that is not
soluble in water at any pH values and in any alkaline or in alkali. So, humins are black in colour. So, remember that the low-molecular-weight
fulvic acid, the fulvic acid has low molecular weight. They have high oxygen, but lower carbon content
than high molecular humic acid. And fulvic acid contain more functional groups
of an acidic nature particularly acidic group. And the total acidity of fulvic acid is 900
to 1400 milli equivalent per 100 grams. And are considerably higher than that of humic
acid which has only 400 to 870 milli equivalent per 100 gram. So, another important difference is that while
oxygen in fulvic acid can be accounted largely in known functional groups, that is acidic
group, phenolic group, and carbonyl group, a high portion of the oxygen in humic acids
seems to occur in a structural component of the nucleus. So, these are some important features of these
fractions. Obviously, again humic acid is insoluble in
acidic condition, soluble in alkaline conditions. Fulvic acid is soluble in all the condition,
insoluble fraction is called humin, and it is black in nature. Fulvic acid contains more oxygen containing
group more acidic in nature. And humic acid contain more carbon content,
and it is more molecular weight, it is having more molecular weight. So, this slide shows that what are the different
features of this humic fraction. So, if you can see, the fulvic acid are light
yellow to yellow brown in colour, humic acids are dark brown to grey black in colour, and
humin is black in colour. So, obviously if you go from fulvic acid to
humic acid, increasing the intensity of the colour you can see here. So, when we are going from fulvic acid to
humin humin, obviously you can see the intensity of the colour increases. Also increase in the degree of polymerization
obviously, fulvic acid is simpler, however humin is much more complex. And then increase in molecular weight obviously,
it is quite understandable, because fulvic acid is low molecular weight, and increase
in carbon content, decrease in oxygen content. So, remember fulvic acid contains more oxygen
containing functional groups. And then decrease in exchange acidity, and
obviously decrease in degree of solubility, because fulvic acid is soluble in all condition. Humic acid is soluble in only alkaline condition,
humin is insoluble. So, this shows a very good comparison of these
three fractions. And obviously, these are humic acids of peaty
sandy soil, and, you know, these are electron-microscope view of these humic acids. So, let us see how we can separate these humic
fractions. So, this soil from the soil organic matter,
we can separate these humic fraction based on their solubility. So, if you start with the soil organic matter,
let us first, you know, treat with the alkali. So, it will separate the humic substances
which will be soluble, and humin, and non-humic matter which are basically insoluble. So, in the first step basically humin and
non-humic matter are separated from humic substances. In humic substances contains both humic acid
and fulvic acid. So, in the second step, when humic substances
is react is mixed with the acid, obviously the fulvic acid will be soluble. However, humic acid will be insoluble, because
we know that humic acid cannot be soluble in acidic condition. So, this fulvic acid which will be soluble,
then if we can adjust the pH to 4.8 obviously that will be separated into fulvic acid which
is soluble, and beta humus which is insoluble and humic acid which was insoluble with acid
reaction. If we reflex that with the alcohol, it will
convert into two major fraction. One is hymatomelanic acid which is soluble,
and another is insoluble humic acid. Now, this insoluble humic acid, when we treat
with neutral salt it will again further divided into soluble brown humic fraction, and insoluble
grey humic fraction. So, you can see how based on the solubility,
these fractions are separated from soil organic matter. So, there are three different foom, you know,
there are different humus formation theories obviously. The lignin theory which was given by scientist
Waksman in 1936; then Kononovas theory, and then polyphenol theory which is the recent
one given by Flaig and Sochtig in 1964. So, these theories basically show basically
tells us about how this humus form formation takes place. So, according to lignin theory, humic substances
are formed due to incomplete degradation of lignin. And Kononovas theory says that humic substance
are formed by cellulose decomposing mycobacteria earlier to lignin decomposition. Whereas, polyphenol theory says that as per
this theory, the humic substances are formed from the condensation of the phenolic compounds. And the polyphenols of lignins are oxidized
to quinones. And these quinones are condensed with low
molecular weight microbial products to form humic molecules. And the microbial product are amino acids,
nucleic acids and phospholipids. So, these basically three theory shows the
pathway through which these humus basically forms ok. So, what is the influence of organic matter
into the soil, why we are studying this organic matter, because, you know, dark, you know,
organic matter is dark in colour. So, dark in colour will facilitate soil warming. And also it will improve the physical condition
of the soil, because divide of organic matters soil will be physically less I mean less I
would say that when the soil contains more amount of organic matter, the soil aggregation
formation is much more. And as a result, soil physical condition is
improved. And also soil, you know, these organic matter
is a reservoir of plant nutrients because of high amount of charge which develops over
its surface. The clay-humus complex has a better buffering
and the exchange capacity and also organic matter can form stable complexes with some
metals, and influences their availability to the plants by forming some chelated compounds. And biodegradation of different chemicals
like pesticides through the interaction with organic matter is an important phenomenon
in relation to the human and animal health, we have already discussed it in our previous
lectures. So, the organic matter is huge important as
far as the soil physical, chemical, and biological properties are concerned. So, how we measure the soil organic matter,
obviously we talked about this Walkley and Black method in our soil testing lectures. And, but remember that this Walkley and Black
method, we can measure up to 6 percent of soil organic matter. When it is more than 6 percent of soil organic
matter, generally we prefer two methods. One is called LOI or loss on ignition method,
we call it Loss on Ignition method given by two scientist called Nelson and Sommers in
1996, which basically, you know, tells the what is the what is the, you know, it basically
gravimetrically measures the loss of carbon when we put a known weight of soil within
an oven, and, we know, we oxidize all the carbon at high temperature. So, when the all the carbon which is present
in the soil get oxidized to carbon dioxide obviously, there will be difference in the
weight. And this weight difference will be measuring
and gravimetrically, it will calculate the soil organic matter. And finally, CN analyser this is an advanced
method Carbon Nitrogen Analyser. And generally in advanced, you know, soil
testing lab, we use that and it is very very precise. So, the so the working principle of CHN analyser
in a there is a sample holder. And there is an ignition chamber, we incorporate
oxygen through it. So, the sample is injected here there is a
pre-packed column. And ultimately the sample the carbon, which
is present within the sample get to co oxidized to carbon dioxide, and this carbon dioxide
move through these GC or gas chromatography column. And ultimately it is detected through TCD
detector a thermal conductivity detector, and it will give the spectrum of the compounds,
and from that we can calculate the concentration of carbon, hydrogen, and nitrogen ok. So, this is the principal of the carbon CHN
or both are same CHN and CN analyser. So, how to manage the soil organic matter? Now, there are several ways to manage the
soil organic matter. First of all, the conservation tillage is
one of the important aspect. Then providing the cover crops, then crop
rotation, crop residues applying, and then nutrient management, organic amendment, commercial
humates; all these are different management strategies for maintaining the soil organic
matter. Obviously, conservation tillage helps in maintaining
the soil organic carbon, because you know they less disturb the soil. Then cover crops, crop rotation, crop residues,
all these add organic matter into the soil. And then organic amendments like different
types of organic manure, when you apply different types of bulky organic manure, concentrated
organic manure when you apply that also adds to the soil organic matter. Now, what is the implication of soil organic
matter in the climate change. Now, if you see here, this is the proportion
of the gases responsible for the global warming or greenhouse gas effect. So, obviously carbon dioxide is responsible
for a large portion of it apart from nitrous oxide, methane, and fluorine gases. So, soil of basically cultivated land is the
major source of these gases, because when we cultivate the soil, it exposes the soil. And whatever carbon is present that will exposed,
and that will be oxidized by, you know, by the oxygen which is present in the air. And ultimately, this carbon will, you know,
will be released into the atmosphere in the form of carbon dioxide. So, it is clear that among the many strategies,
sequestering more carbon into the soil under crop lands and grazing lands and restoring
formerly drained wetlands are the three most important. So, again sequestering carbon, so this is
very important. Carbon sequestration what is carbon sequestration,
carbon sequestration means we will prevent the movement of carbon from soil to atmosphere. So, how could you do that? So, another one way is to conservation tillage. So, in the conservation tillage, we are not
exposing the soil. So, as a result the carbon oxidation and formation
of carbon dioxide is reduced. So, it is a one of the strategy of, you know,
of this carbon sequestration. And that fortunately, these measures are quite
feasible as their implementation would not only reduce greenhouse gases, but also they
improve the soil quality and provide the benefits of enhanced soil function and productivity
to the owners. So, these are some strategies, and remember
that that is why soil carbon sequestration is very very important nowadays ok. So, we have finished this soil organic carbon
lecture. Let us go ahead, and start soil organism guys. In this lecture, we will be covering these
following concepts. What are the different types of soil organisms,
and then role the roles of these soil organisms, then classification of soil organisms, and
different types of soil organisms. So, soil organisms are the, you know, the
basically if we consider the soil, the soil is having abundant and millions and millions
of living organisms which makes it a living and dynamic system. And these organism not only help in development
of the soil, but also carry out a number of transformation facilitating in the availability
of the nutrient to the plant. As you have I have we have already discussed
in case of nitrification process. Say for an example, in case of nitrification
process, it is mediated through different types of microorganisms. The ammonification is mediated by microorganism. The decomposition of organic matter is mediated
by microorganisms, because, you know, it requires extracellular enzymes for oxidations. And, you know, for oxidation into carbon dioxides,
so if you remember those things. So, all these are very very important. Now, this role of microorganism in soil fertility
is first of all if they helps in decomposing the organic matter, and also plant nutrient
transformation through for through maintaining different cycles like nitrogen, sulphur, all
these cycles. And then soil organisms are also useful for
the preparation of different bio-fertilizers and compost, we will discuss compost too. And these bio fertilizers and compost will
help in building, the soil fertility or increase the soil fertility for better crop growth. So, these are very very important, these without
these microorganisms, the soil would be literally a dead system. So, what are the class what is the classification
of soil organism. So, organism in soil can be broadly classified
into soil flora and soil fauna. Now, soil flora and soil fauna, we should,
there should be a discontinuity here. So, basically organisms are, you know, soil
flora. The soil flora is again divided into micro
flora and macro flora. And within the micro flora and macro flora,
the in case of macro flora, that is the plant roots and macro algae are there. And in case of micro flora, bacteria, actinomycetes,
fungi, and algae are important. And in case of soil fauna, there are some
macro fauna and micro fauna. And in case of macro fauna, obviously earthworm,
termites, ants, grubs, slugs, snails, centipedes, millipedes are important. And in case of micro fauna obviously, protozoa,
nematodes, and rotifers are important. So, you can see here basic overview of the
soil organism classification ok. So, what is the size, relative size differences
of these organisms? So, you can see if we consider sand, silt,
and clay obviously, the sand is the coarser portions. So, vertebrates here, so earthworms are in
the sand, arthropods, obviously nematodes. Then protozoans, you know, comes in between
then plant roots, then fungi, algae, bacteria, archaea, and viruses. So, remember that viruses are the most minute
fraction, and in the soil micro flora, obviously bacteria are the smallest organisms ok. So, algae and algae comes in between bacteria
and fungi, and in between algae and bacteria, there is also actinomycetes. So, let us start with the macroorganisms in
the soil. So, the macro organisms in the soil include
different different organisms like acari, like collembola, like, you know, like isopoda,
then amphipoda, earthworm and so on so forth. So, a population estimation is highly difficult
as they are not uniformly distributed. So, the population is lesser than the microorganism. Remember that, in the case of macro organism
their population is lesser than the microorganism. And why they are very very useful? They are very very useful, because they help
in mixing, churning, and fragmentation of the plant materials which accelerates the
decomposition process, because for decomposition we need to break down the soil organic matter
first. Secondly, they forms burrows and tunnels which
increases soil aeration and drainage, so that is also important, because without the, you
know, presence of oxygen, this oxygen, you know, organic matter decomposition does not
take place. And they also ingest the soil into the guts
of the earthworms, and you know and they converted into a worm cast, we call them mull humus,
other name is vermicompost we will discuss that. And they also feed on microorganisms including
plant pathogens. So, these are some of the, you know, utility
of these macro organisms. So, let us start with the earthworms. Now, earthworms can be of two type. One of this one is called epigeic type of
earthworm, another is endogeic earthworms. Epigeic earthworms are surface feeders, so
they remain on the surface feed on the surface litters. Endogeic are feed on the different materials
within the soil. The geophagus species of earthworms, you can
see this is an epigeic earthworm, and these are the burrows made within the soil by endogeic
earthworms. So, the geophagus species of earthworms is,
you know, ingest materials per day which is 5 to 36 times of their body weight. And the worm cast, they create is rich in
nitrogen, phosphorus, calcium, and the casting rate is 2600 tons per hectare per year. So, it is a huge amount of, you know, cast
worm cast they produce per year, which further helps in building the soil fertility status. So, earthworm-worked soils are generally have
high porosity, high water holding capacity, then water infiltration rate, water stable
aggregates, and different nutrients you can see. The burrows which are made and these are helpful
for water movement, and also they helps in aggregation of the soil. And the earthworm increases the surface area,
and the availability of organic matter for microbial action by mixing it thoroughly within
the soil. So, these are some activities of the earthworms,
beneficial activities of the earthworms. Another important is vermicompost, we will
discuss vermicompost details in the next lecture. So, earthworms are being used for hastening
the decomposition of the farm produce and waste for composting, and the product is called
the vermicompost. And the species which are generally used for
vermicompost are Eisenia foetida, Eudrilus engeniae, and Perionyx excavates. Eisenia foetida is reported to be the more
efficient than others. So, you can see this is the vermicompost which
is made through the earthworms, we will be discussing this vermicompost later on. Termite, termite also disturb the soil while
preparing the nest. So, they are also important for water and
air movement within the soil. Plant roots, they are macro flora. Plant root exert a physical pressure on the
soil particles, you know, influencing their aggregation. And also the environmental, you know, remember
that rhizosphere, this is very important term. So, this rhizosphere is an environment under
the influence of the roots. So, this is very important habitat for microorganisms. And also the root produce different types
of chemicals like exudates, secretions, mucigels, lysates, which are also like helpful for different
types of nutrient transformation, at different and accelerating different bio-chemical processes. So, because of these chemicals a different
types of niche is formed in the rhizosphere. Remember rhizosphere is very very chemically
active and biologically active zone, where different types of nutrient transformations
are going on. So, let us start with the bacteria. Now, bacteria is the smallest and most numerous
of the organism present in the soil. And they are having different types of shapes
like spherical, known as cocci, you know, you know, spherical, cocci, rod-shaped, you
know, bacillus, and spiral-shapes are known as spirilla. And common bacterial genera found in soil
are, you know, pseudomonas, then arthrobacter, clostridum, bacillus, achromobacter, micrococcus
and agrobacterium. Remember that I have highlighted this bacillus,
because this is the most abundant soil bacteria you can see here bacillus here. And followed by pseudomonas, pseudomonas is
the second most abundant bacteria in this soil, and this clostridum is strictly anaerobic. Now, apart from this soil bacteria also, the
soil also contains some other bacterias like bdellovibrio, myxococcus, and polyandium. And these genera help to maintain the biological
population by keeping a check on the growth of the other bacteria. So, this is very very important for maintaining
the biological equilibrium within the soil. So, what are the roles of bacteria in the
soil, obviously you know that nitrogen fixation is an important role. You have already, you know, we have already
discuss about the rhizobium, and how they fix atmospheric nitrogen, then phosphate solubilisation,
organic matter decomposition, synthesis of humus, nitrification, denitrification in water-logged
soil, protein decomposition ammonification, transformation of various macro, micro nutrients
in the soil. So, all these are very, you know, mediated
through bacteria in the soil. So, these are these are very very important,
you know, activities of the bacteria. The second important is actinomycetes. Now, taxonomically actinomycetes are in the
evolutionary face between bacteria and fungi, because they are called fungi like bacteria,
because they are like bacteria which possess aerial hyphae like fungi. So, you can see here strands of actinomycetes. Now, they are generally found in dry soils,
and common genera found in soils are Streptomyces, micromonospora, nocardia, thermo-actinomycetes. Remember that the population of actinomycete
is high in compost pits. In fact, they are the highest, you know, microorganism
highest population microorganism, which are present in compost pit. And they are nutritionally heterotrophic and
tolerant to high temperature. And they play the most important role in humus
formation, and pigmentation of the humus. And they cannot compete with bacteria and
fungi, so they grow on such substances which are not decomposable by bacteria and fungi. So, these are some important points of actinomycetes
remember that streptomycetes. Streptomyces is a most important soil actinomycetes,
and they are the most abundant soil actinomycetes. So guys, so let us wrap up here in this soil
microorganisms. And in the next lecture, we will start we
will we will try to finish this soil microorganisms as well as we will talk about the composting
methods also. Thank you.

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