Welcome friends to this new lecture of Soil
Science and Technology.
And in this lecture, we will be trying to
finish the cation exchange capacity and anion
exchange capacity, where we left in the last
lecture and then will be trying will be discussing
about a new topic that the adsorption of pesticides.
So, what is the relationship between pH and
CEC?
Obviously, generally CEC increases with the
pH and the permanent charge of 2 is to 1 type
of clay that is pH depended charge of humus
and allophane and some 1 is to 1 type of clay
hold exchangeable ions, you already know that.
So, as the pH is raised obviously, the negative
charge on some 1 is to 1 type of silicate
clays, allophone, humus or even iron aluminium
oxide increases and CEC also increases.
Because, if you remember the pH dependent
charge, when we are increasing the pH that
will basically create the negative charge.
So, creation of the negative charge is basically,
they increasing the CEC because, it is attracting
the cations in the soil surface or clay surface.
So, as the pH is raised obviously for 1 is
to 1 type of clay allophane and even in iron
aluminium oxide, there will be increase in
CEC.
And in alkaline condition CEC reflects the
pH dependent charge as well as the permanent
charge because, you know that permanent charge
is permanent.
However in case alkaline condition just I
told you, it will develop their negative charge
that is pH dependent negative charge.
So, ultimately it will be a total of both
permanent charge as well as pH dependant charge.
So, the determination of CEC is generally
done at pH 7 and or 8.2.
So, exchangeable cations in field soils, you
know the exchangeable ions in the field soil
depend upon the climatic condition.
Obviously, the iron and aluminium and complex
aluminium hydroxy ions and H plus are most
prominent in humid region and calcium, magnesium,
sodium dominate you know soil in low rainfall
areas because, you know in case of highly
leached areas.
Obviously, in case of highly leached, warm
and humid condition obviously, there will
be formation of oxisols.
Now why there will be formation of oxisol?
Because there will be iron aluminium oxides
dominance.
So, you will see that in humid region specifically
in the warm and humid region, there will be
a predominance of iron aluminium ions.
However, in the low rainfall areas; obviously,
in arid region, there will be predominance
of different types of soils, because of high
evaporative demand.
And as a result of high evaporative demand,
these soils will be much more, I would say
dominated by calcium, magnesium and sodium
ions.
So, in a given soil the proportion of the
cation exchange capacity satisfy by a particular
cation is termed as a saturation percentage
of the cation.
So, for example, if 50 percent of the CEC
satisfy by calcium ions the exchange complex
is said to have a calcium saturation percentage
of 50.
So, this terminology is especially useful
in identifying the relative proportion of
sources of acidity and alkalinity in the soil
solution.
So, cation exchage properties are typical
for unamended clay loam surface soil in different
climatic region.
So, if we can see one interesting trend you
can see, we are giving 4 different condition,
one is warm and humid region that is represented
by Ultisol, which is highly weathered one
of the highly weathered soils, and then you
know cool and humid region and then represented
by Alfisols then semiarid region, you know
which is denoted by Ustolls, and then arid
region that is Natrargids.
So, you know if you see a trend the exchangeable
H plus and Al3 plus will be continuously reducing
from this warm humid region to cool humid
to semiarid and arid region.
So, 7.5 percent 75 percent to 28 percent to
0 percent almost; and exchangeable calcium
will show the reverse trend, I mean you can
get 2 percent in the warm and humid region.
However, we are getting 9 percent in cool
even humid region, it is increasing to 17
percent in semiarid region and 13 percent
in the arid region.
And also exchangeable sodium, which is basically
present in alkaline soil, which is another
type of you know which has the high pH.
So, in these basically this alkaline soil
and exchangeable sodium is present in a trace
amount in case of warm and humid regions,
it is very 0, it is very less 0.1 percent
in Alfisol again, 4 percent in Ustolls and
arid soil, which is dominated by different
sodium salts, you will get almost 90 percent.
So, you can see how these, dominance of different
cations are changing for a particular type
of soil, when it is you know for different
climatic conditions.
So, that shows the importance of climate for
soil development as we have discussed in our
earlier lectures of soil.
So, cation saturation and nutrient availability
obviously if the percent saturation of some
ions are high then those ions will be easily
and rapidly displaceable; obviously.
Now influences of complementary ions.
Obviously, the strength of adsorption of common
cations on most colloids is generally follow
this order, what is the order?
That is you know aluminium is strongly adsorbed
followed by calcium then magnesium then potassium,
which is showing the similar adsorption capacity
and which is further you know further more
than sodium.
So, potassium can be easily replaced by Al3
ions in the acid soil and it will be available
for the plants ok.
So, in the acid soil aluminium will replace
this potassium and this potassium will be
available to the plants.
So, there are some nutrient antagonism.
So, this is another important aspect, there
are some nutrient antagonism that in certain
soil cause inhibition of uptake of some cations
by plants.
For example, sometime high potassium levels
are known to limit the uptake of magnesium.
So, that is means potassium and magnesium
has a some nutrient antagonism even when significant
quantities of magnesium are present into the
soil.
So, that shows the different interaction or
influence of different complementary cations
in the soil system.
So, here you can have a better idea in this
diagram.
So, basically it is a soil colloid; basically
in our case, that is clay and this is a root
extending H plus and to exchange cations on
the colloid and in the cations in the colloid
surface, you can see there are several types
of cations like sodium and then potassium.
And they are basically loosely held complimentary
ions with large oscillations zones.
So, if we compare potassium and sodium, sodium
has more large oscillation zones.
So, this is called the oscillation zone and
in this oscillation zone, you know they are
loosely held this sodium.
So, sodium is easily replaceable than that
of potassium.
So, root you know and in another condition.
Obviously, this aluminium and potassium are
present.
However, this aluminium are more tightly held
complimentary ions with small oscillation
zone than that of potassium.
So, in these 2 condition, you are seeing basically
the difference in oscillation zones and ultimately,
difference in their affinity to the clay colloid.
So, the half circles are loosely held with
the soil.
So, these are the half circles, which are
basically loosely held with the soil and the
root will take the cations from the soil in
exchange with H plus ions.
So, in the left figure the loosely held sodium
plus will be easily taken up by the plant
and in the right figure, the k plus will be
easily taken up by the plant.
And you know the k plus will be comparatively
more vulnerable to be replaced and sent to
the soil solution and k plus will be more
available to the plant uptake and leaching.
So, this is how the interaction or between
complementary cations influence their uptake
by different plants ok.
So, this is now let us consider another important
very very important topic, that is percent
base saturation or BS.
Now percent base saturation on the CEC occupied
by it is basically, the percentage of CEC,
which is occupied by the basic cations.
So, we have calculated already the CEC.
Now if you can calculate what is the percentage
of the total CEC, which has been occupy, which
is contributing, which we which is being contributed
by the basic cations, that is calcium magnesium
and potassium then we can calculate the percent
base saturation.
So, basic cations are distinguished from the
acid cations H plus and Al3 plus.
So, if we can distinguish basic cations will
be calcium, magnesium and potassium whereas,
acid cations will be aluminium and and H plus.
So, at an approximately soil pH of 5.4 or
less Al3 plus is present in a significantly
high concentration then that hinders the growth
of most of the plant species and the lower
the soil pH, the greater the amount of toxic
Al3 plus.
So therefore, soil with high percent base
saturation are generally more fertile, because
when there will be high dominance will of
Al3 plus; obviously, that will create the
nutrient toxicity or Al3 or aluminium toxicity.
So, we do not need that condition, we need
a fertile soil.
That means, that can support the plant growth
that can give that can support the plant growth
that can supply the required nutrient to the
plant.
So obviously, the soil with high percent base
saturation will have more fertility and they
have little or no acid cations Al3 plus that
in the toxic to the plant.
So, you know this high you know percent base
saturation soil, they have little or no acid
cations like Al3 plus that is toxic to the
plant growth.
So, soil with high percent base saturation
have a higher pH obviously.
Therefore, they are more buffered against
the acid cations from plant roots and soil
processes that acidify the soil like nitrification,
acid rain, etcetera we will discuss nitrification
in our coming lectures.
And also they can contain greater amounts
of essential plant nutrients cation like potassium,
calcium, magnesium for they use by the plants.
So, these are very important what is the formula
percent base saturation?
The percent base saturation is basically,
calcium plus magnesium plus potassium over
total CEC multiplied by 100.
So, depending on soil pH the soil base saturation
maybe a fraction this is very important.
So, depending on the soil pH the soil base
saturation or percent base saturation maybe
a fraction of CEC or approximately equal to
the CEC.
So, in general if the soil pH is below 7 the
base saturation is less than CEC, because
there must be some acidic cations; however,
at pH 7 or higher soil clay mineral and organic
matter surface are occupied by basic cations
and thus base saturation will be equal to
the CEC in that condition.
So, what are the effect of different colloid
types?
Difference exists in the tenacity with which
several types of colloids will specific cations.
So, at a given percentage base saturation
smectites, you know at given percent base
saturation smectite, which have a high charge
density per unit of colloid surface hold calcium
more tightly or more strongly than that of
kaolinite.
So, calcium percentage will have to be increase
up to a certain percentage to satisfy the
need of plant in case of smectites and kaolinite
can supply calcium at relatively lower percentage
of base saturation, and obviously the need
to add limestone to the 2 soils will be somewhat
differently partly.
Because of this factor we will discuss why
we add limestone, in why when we will discuss
the soil acidity.
Now, let us discuss anion exchange, anion
exchange is also same anion it is basically,
the ability of the soil to exchange anions
from it is surrounding medium and anion held
in 2 major ways.
Firstly, they are held by anion adsorption
mechanism similar to the responsible for similar,
which are those responsible for cation exchange.
Secondly, they may actually reactive surface
oxides or hydroxides forming more definitive
inner sphere complexes.
Now in the last lectures, we have discussed,
what are inner sphere, inner sphere complex
and outer sphere complex.
So, I am not going to discuss them in details.
So, anion adsorption mechanism the basic principles
of anion exchange are similar to those of
cation exchange and the charge on the colloids
are positive.
And the exchange is among negatively charged
anions, the positive charge develop due to
the pH dependent charge you know that, and
a positive charge is associated with the surface
of kaolinite iron aluminium oxides and allophone,
and attract anions such as sulphate and nitrate.
You can see here you know the nitrate is first
adsorbed to the soil colloid and which is
at that getting replaced by another cation
that is chlorite.
So, just in case of cation exchange equivalent
quantities of ammonia, nitrate and chloride
are exchange here.
So, the reaction can be reversed and nutrient
anions release can be absorbed by the plant.
So, AEC generally decreases with the increase
pH.
Obviously, when there will be increase pH,
there will be negative charge development.
So, when there will be negative charge development
due to the pH dependant charge; obviously,
the AEC or anion exchange capacity will increase.
So, this is quiet you know with the increase
of the , I am sorry with the increase of soil
pH, there will be development of negative
charge.
So, when there will be development of negative
charge obviously, the anion exchange capacity
will decrease and cation exchange capacity
will increase because, it will attract more
cation and it will repel more, you know anions
and attract more cations.
So, this graph basically shows that relationship.
So, what is the relationship between weathering
and CEC and AEC level?
Obviously, you can see there are three different
conditions mostly 2 is to 1 type of clays
you know you can see here three different
conditions.
In the first condition is the mild weathering
condition, there is a intermediate weathering
condition and strong weathering condition,
mild weathering condition obviously, 2 is
to 1 type of clays intermediate weathering
condition, 1 is to 1 type of clays and strong
weathering condition mostly iron aluminium
oxide clays, we have already discussed that.
So, increasing weathering is basically in
this direction so and which we are an increasing
the weathering obviously the cation exchange
capacity will decrease.
Obviously, you will see most cation exchange
capacity in case of 2 is to 1 type of soil,
which will further reduced to 1 is to 1 type
of soil and it will reduce to almost 0.
In case of iron aluminium oxides and in and
for anion exchange capacity, we will see the
reserve strength.
So, as we are increasing from mild to intermediate
to strong weathering condition, the anion
exchange capacity will further increase ok.
So, we have completed this cation exchange
capacity topic, let us start another important
topic that is sorption of pesticides in the
soil.
So, we will be covering the importance of
pesticides sorption then distribution coefficient
then binding of biomolecules and to clay and
humus.
So, importance of sorption of pesticides in
soil, you know that soil can adsorb charge
organic ions by either AEC or CEC because,
either it is positive or negative depending
on the positive and negative nature of the
charge organic ions, it will be adsorbed and
sorption can reduce the movement of the groundwater
because, when it will be sorbed.
Obviously, it will not leach down to the ground
water and it can allow time for soil microorganism
to break the chemical down to less toxic byproducts.
So, when it will be adsorb by the clay colloids,
it will be you know the chance of further
leaching down to the ground water will be
reduced.
And there will therefore, it will get more
time for the microorganism to degrade this,
and also it can also produced inner sphere
complexes.
So, it is common for organic compounds to
be aborbed within the soil organic colloids
by a process called partitioning and the hydrophilic
part will not be adsorbed by the moist clays.
Now, what is partitioning?
Now this is a very good picture of partitioning
process.
Now this method is also known as the extraction
methods an extraction means drawing a compound
out of a mixture using a solvent.
So, solvent partitioning is you know we can
we can call it solvent partitioning is more
specific.
So, it means compound of a choice of 2 solvents
that they can dissolve in some compound, dissolve
in one solvent, and some compound dissolve
in the other solvent that way the compound
in the mixture become separated in 2 groups.
So basically, you can see here in this first
condition two compounds the dissolved in a
some in a solvent and now we are adding a
second solvent, which is denoted by this circles
hollow circles.
So, we are basically mixing the 2 different
solvents as well as 2 compounds together in
here.
And finally, when the solvent separates again
the compounds go into 1 solvent on the other
based on their polarity.
So, based on their polarity, they will separate
into 2 different solvent.
So, this is called partitioning process ok.
So, this is very important for pesticides
option, how?
We will see.
So, this partitioning can be quantified using
a partitioning coefficient or Kp, which is
basically concentration on solid over concentration
in the solution.
So obviously, you can see there are this shows
this graph shows the relationship between
Ceq, which is equivalent concentration or
per and concentration on solid.
So, Ceq is basically concentration in solution
and here it is q is basically, concentration
on solid.
So basically, this line shows the higher Kp
and this line shows the lower Kp.
So, high Kp means higher sorption; obviously,
and low Kp means weaks options.
So, high Kp means hydrophobic compounds on
organic matter when hydrophobic compounds
like the organic pollutants like organic pesticides,
they are hydrophobic in nature.
So, they will not you know they will not mix
well with water.
So, they will mix more with the you know organic
matter into the soil and you know water soluble
compounds, which are hydrophilic that prefers
to stay in the solution will responsible for
this low Kp.
So, let us move ahead and see what are the
organoclays?
Now, organoclays is very important aspect,
the hydrated metal cations like calcium that
are adsorbed on the surface of the smectites
can be replaced with large organic cations
and giving rise to what we are termed as organoclays
that is smectite organoclays specifically.
Now, such clay surface are more friendly towards
the applied organic compounds, making it possible
for the clay to participate in partitioning.
Obviously, when these clays are more friendly
towards the applied organic cordon, you know
compounds it will take part more into the
portioning processes.
So, let us see an example of sorption.
So, this is a very good example of sorption
procession experiment.
Now this experiment was done by a gentian
violet colour solution and in a sandy loam
soil with moderate CEC and it is sandy soil
with negligible CEC.
So, these are 2 you know different soil, which
is sandy loam soil with moderate CEC and this
is sandy soil with negligible CEC.
So, after we leach these 2 soil with these
gentian violet solution, a clear solution
and we collect the leach it at below in a
beaker.
So, after leaching we will see a clear solution
was found in case of sandy loam soil and in
contrast water drained from the sandy soil
was still purple in colour in case of you
know, in case of sandy soil with the lower
CEC so that means, higher CEC or moderate
CEC because, in the sandy loam soil there
was some amount of silt or organic matter.
However, in case of you know sandy soil; there
is low amount of organic matter and due to
the presence of organic matter.
In case of sandy loam soil you know these
organic matter will adsorb, all the gentian
blue solution.
So ultimately, it will you know the clear
solution will leach.
However, in the second case the total.
So, there is no change.
So, further the purple colour solution was
leach down.
So, that shows the import of sorption by different
by organic matter which is present into the
soil.
Now, this is a picture of an African field
and you can see after applying.
So, in this field scientist are applied you
know the farmers are applied herbicides and
the field is still full of weeds, what is
the reason behind this?
There is a bizarre condition, where applying
the herbicides to kill the weeds, but we still
finding that this is full of weeds.
So, what happens the soil has a upper A horizon,
you know what is A horizon with a low CEC
and a clay subsoil with high CEC, argillic.
Now you know, what is argillic?
Argillic is dominated by clay clay mineral.
So, this soil have upper A horizon with low
CEC and lower argillic horizon with high CEC.
So, when they applied all the pesticide, all
the pesticides was adsorbed by subsoil clay.
So, that there is no longer available to be
taken out by the roots of the weeds.
So, as a result of that, weeds further grow.
So, that also shows the importance of sorption
of different organic pesticides by clay and
different other component, which are present
into the soil.
Now, what is distribution coefficient?
The tendency of a pesticide or other organic
compound to leach into the groundwater is
determined by the solubility of the compound.
And by the ratio of amount of chemical solved
by the soil to remain into the solution and
this ratio is basically known as the distribution
coefficient or Kd.
So, Kd is basically, you know milligram of
chemical sorbed per kg of soil over milligram
of chemical per litre of soil solution.
So, the unit of Kd obviously it is litre per
kg and it depends upon the nature of the soil;
obviously, the variation is related mainly
to the amount of organic matter or organic
carbon into the soil.
So, it can also be represented by using a
similar ratio, we call it Koc or other name
is organic carbon distribution coefficient.
So, Koc is basically, mg of chemical sorbed
per kg of organic carbon and mg of chemical
per litre of solution.
So basically, we are you know this is the
formula of Koc that is Kd over gram of organic
carbon per kg of soil.
So, let us see Kd and Koc are used for herbicides
and metabolites, you know one soil which has
higher Kd and Koc values will absorb more
pesticides obviously.
And these high values indicates that chemicals
are strongly adsorbed by the soil and less
susceptible to leaching.
So, these are some Kd and Koc values for several
widely used herbicides like atrazine, diethyl
atrazine, metachlor, metolachlor.
And all this thing, you can see what are the
variation of their Kd and Kc values and these
are the very important or the adsorption in
the soil.
Now, binding of biomolecules and to clay and
humus is very important.
Now the bond between the biomolecules and
the colloid is often quite strong.
So, that the biomolecules cannot be easily
removed by washing or exchange reactions and
the initial attraction maybe between charge
colloidal surface and positively or negatively
charged functional groups on biomolecules
and these type of reaction has 2 environmental
impact, obviously the first the bound.
Firstly, the bound chemical remains for a
long period of the long period longer period
in the soil as the microorganism cannot recognize
and react with their target sites.
And secondly, some chemicals remain active
after adsorption to so toxins remains toxic
to the susceptible organism, enzyme continuous
to catalyze the reactions.
So, these are 2 types of impacts environmental
impacts, you can get from this sorption of
biomolecules, you know to the clay and humus.
Let us see some example Bt toxin, you already
know that.
Now, Bt toxin is basically a toxin produced
by a soil micro organism called Bacillus thuringiensis,
and basically it is used for organic farming
for protecting crops from insect damage.
So, it is adsorbed by soil.
So, the 2 is to type of clay montmorillonite
soil, you can see in this graph.
Obviously, the 2 is to 1 type of soil like
montmorillonite can adsorb the toxin up to
30 to 80 percent of it is mass.
So, 1 is to 1 type of obviously it is not
that efficient, because of low charge development.
And obviously, the adsorption reaction completed
within a minute, in case of montmorillonite.
Second let us see DNA, now it this is a scanning
electron micrographs or SEM image, SEM image
of DNA from Bacillus subtilis bound on kaolinite
clay, which is represented in the left picture
or montmorillonite clay, which is shown in
the right picture.
So, the arrow point stands for basically,
the stands bound to DNA stands of bound DNA.
Now, DNA bound to clay or humus is protected
from decomposition, but retains the capability
of transferring genetic information to living
soil.
So, this is implication of DNA sorption in
the soil antibiotics.
So, antibiotics also adsorb by soil colloids
by the process of CEC, it also has a very
high Kd values, after sorption it develops
positively charged sites in the soil.
And increasingly research shows that even
though sorption to soil colloids, may reduce
their efficacy somewhat the soil bound antibiotics
still works against bacteria and there is
evidence that at least some antibiotics can
be taken up from soil by food crops.
So, enter into the human food supply.
So, it will increase the resistance against
the life saving drugs into the body.
So, this is the implication.
And let us see one example of antibiotic,
that is chlorotetracycline, it is the first
antibiotic which has been you know developed.
Now adsorption isotherms illustrates, the
retention of chlorotetracycline or CTC by
montmorillonite and kaolinite as a function
of ionic background cation.
So, you can see the montmorillonite or smectite.
Obviously, montmorillonite is showing more
you know adsorption you know more adsorption
than that of kaolinite and calcium CTC retention
decrease in the presence of calcium nitrate
than that of sodium nitrate; obviously, because
calcium is more competitive with CTC than
sodium.
So, CTC is adsorbed by the soil via CEC.
So, you can see in case of presence of sodium
nitrate, the adsorption is higher.
However, in case of presence of calcium nitrate
the adsorption is somewhat lower because,
calcium is more competitive with the CTC than
sodium.
So, these are some complex interaction between
the different molecules, which is present
into the soil, which we apply into the soil
and the components, which are already present
into the soil and you know you can search
some literature.
Obviously, consult some literature to gain
a in depth knowledge of these sorption characteristics.
These are very interesting and I would suggest
you to go and discuss and to consult some
literature to gain more in depth knowledge
of this sorption process, which will be giving
you more practical perspective of learning
soil science and technology.
Thank you guys we are completing this week
5 of lectures.
So, we will be starting the week 6 of lectures
from our next lecture.
Hope you have understood and learnt something
new in this week, we have discussed you know
several important topics of soil chemistry,
and we will be also discussing some other
important topics of soil chemistry related
to the soil silicate clays in the week 6.
Thank you, and let us meet and in our next
weeks lecture.
Bye.
