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2 major groups
1) Acid soils
2) Salt affected soils (Saline and Alkali soils……..
………………………Other type of problemtaic soil is
3) Calcareous soil
Acid soils occur in those areas where rainfall is higher,
i.e precipitation > evapo-transpiration
Salt affected soils occur in arid and semi arid regions
Calcareous soils occur in semi-arid region which contains
parent material like CaCO3 (pedogenic)
In India Acid soil covers 49.0 million ha, whereas, salt
affected soil covers 8.0 m.ha
Soil acidity is defined as proton (H+) yielding capacity of
soil during its transition from a given soil pH to a
Acid soils have been defined by the soil having pH less than 5.5 in
1:1 soil-water suspension (USDA).
Properties of acid soil
Intermediate texture (Sandy loam to Loam)
Low organic matter content (except hilly region and forest soils)
Low P content but N is variable
High amount of Fe and Al in soil soln.
Soil Acidity and Poverty
Iron range (pH 2.4–3.8)
Very strongly acid
Aluminum range (pH 3.0–4.8)
Cation exchange (pH 4.2–5.0)
Slightly acid to neutral
(all pH values typically >5)
Carbonate (pH 6.5–8.3)
of formation of Acid soil
i) Laterisation of different degrees:
In Tropical region: high rainfall coupled with high
temperature causes laterisation which in turn leads to intense
weathering and leaching of bases like Ca, Mg.
(Orissa, M.P., Bihar, w.B.- Assansol, Medinipur, Bankura, Purulia)
ii) Podzolisation in areas with sub-temperate to temperate
climate, where organic matter content is high. (Low
Temp. and high Rainfall)
(H.P, terrai region, Kashmir, Assam (some parts)
iii) Marshy, Peaty conditions with significant amount of under
decomposed and partly decomposed O.M.
iv) Intense leaching in light alluvial soil in high rainfall
areas having partly decomposed O.M.
(Occurs , N. Bengal, Coochbehar, N and S Dinajpur, Malda (some parts)
Region: Acid sulphate soils: Inundation of sea water in low
land areas (Kerala, Sundarban)
Laterization is a pedogenic process ; found in tropical and subtropical
environments. High temperatures and heavy precipitation result in the
rapid weathering of rocks and minerals. Movements of large amounts of
water through the soil cause eluviation and leaching to occur.
Laterisation is the process of desilication, i.e., removal of silica and
accumulation of sesquioxides ) Iron oxides give tropical soils their unique
reddish coloring. Heavy leaching also causes these soils to have an
acidic pH because of the net loss of base cations.
Podzolization is associated with humid cold mid-latitude climates and
coniferous vegetation. Decomposition of coniferous litter and heavy summer
precipitation create a soil solution that is strongly acidic. This acidic soil
solution enhances the processes of eluviation and leaching causing the
removal of soluble base cations and aluminum and iron compounds from
the A horizon. This process creates a sub-layer in the A horizon that is
white to gray in color and composed of silica sand.
Iron and aluminium are leached down while silica is
present in abundance as evidenced by the characteristic
ash-coloured greyish E-horizon in Podzols.
Most of the silica leaches out of soils while iron and
aluminium get accumulates as oxides.
PEATY AND MARSHY SOIL
Occur in Humid region.
Formed by accumulation of organic matter.
Black in colour.
Highly acidic and heavy.
– Kottayam & Alleppey in Kerala, Coastal Orissa,
Sundarbans of W.B
acidity in Soil
The most common source of H is the reaction of Al
ions with water. The equation for this reaction in very
acid soils (pH<4.0) is:
The species of aluminum ions present vary with pH
The forms of aluminum are mostly exchangeable Al3+
under very acidic conditions (pH<4.5)
to aluminum-hydroxyl ions at higher pH (4.5–6.5) .
Situation 1: In strongly acid condition:
Here the exchangeable Al+3 and exch. H+ are mostly responsible for production of
excess H+ in soil soln.
Al3+ +H2O ⇋ Al(OH)2++ H+
Al(OH)2+ +H2O ⇋ Al(OH)2++ H+
Al(OH)2+ +H2O ⇋ Al(OH)3 + H+
Al(OH)3 +H2O ⇋ Al(OH)4- + H+
The Al3+ hydrolysis with the formation of insoluble Al(OH)3 and H+ and Al(OH)3
is pptd. As a result more Al+3 will comeout from the exch.complex (clay) to the
soil soln to maintain an equilibrium and these process continued. Due to Al +3
hydrolysis more H+ are formed in soil soln which leads to more acidity to the
The species of aluminum ions generates hydrogen ions through a series of
hydrolysis reactions shown above (Lindsay, 1979):
Situation 2: In moderately acid condition:pH ..... 5.5
Adsorbed H+ does not play any important role
Al(OH)2+ +H2O ⇋ Al(OH)2++ H+
Al(OH)2+ +H2O ⇋ Al(OH)3 + H+
Situation 3: In slightly acid condition:pH ..... 6.0
All ions like Al+3, Al(OH)2+ + Al(OH)2+ and also H+ disappers from the soil soln.
Dissociation of H from –COOH, phenolic-OH, and –NH2 group is responsible for
H+ dissociates at the broken bond at the edges and corners of the crystal
A lewis acid is one which accepts a pair of electrons and a Lewis base
is one which donates a pair of electrons
It is known that large charge to radius ratio (i.e. ionic potential, z2/r,) for
cations results in an increase in hydration energy. Closely related to hydration
is the process of hydrolysis.
In case of hydrolysis, the acidity ( i.e. ionic potential) is so great as to rupture
of H-O bond with ionisation of hydrate to yield H3O+ ions ( proton).
Al3+ + (H2O)6 ═ [Al(H2O)6]3+
[Al(H2O)5OH]2+ + H3O+
Cations that hydrolyse are either small (Be+2) and /or highly charged (Al3+,
Fe3+, Sn+4) . A bare H+ should have the highest charge to radius ratio and
reveals that cations having a very large charge to radius ratio tend to
polarize water sufficiently strongly to promote hydrolysis. This can
be thought of as hydration
taken to the extreme, where the "ionic potential," z2/r, of the ion is
large enough to rupture 0 - H bonds.
the greater the charge density of an ion, the more heavily hydrated it
Anions are hydrated less than cations because of lesser charge
Cations are heavily hydrated because of their higher charge density,
and the process can be demonstrated as follows:
Commonly, two processes take place when a metal salt is added to water:
1. Hydration (H2O molecules adsorb onto the ions)
2. Hydrolysis (degree to which adsorbed H2O dissociates
to satisfy ion electronegativity).
the overall reactions of the metal ions with water into several plausible steps.
1. The metal ion is solvated by water molecules (we can also call this hydration);
typically metal ions are surrounded by about six water molecules in what is known
as the primary hydration sphere. Most of the heat liberated in the above reaction is
due to the enthalpy of hydration. These hydrated cations are also called aqua
complexes (or aqua acids).
2. The hydrated ion can now undergo step-wise hydrolysis in which it delivers one
or more H+ to the bulk solvent. This is illustrated in the graphic below for the aluminum ion,
though the same scheme applies for any element, differing only by degree.
The 1st step in this diagram shows how a strong attraction of the cation for an oxygen
of one of the directly attached water molecules results in the splitting-off of one H+:
Since these are Brønsted equilibria, we can
write a standard hydrolysis constant for any
step in this reaction:
metal hydroxide usually starts to precipitate
when the pH of the solution is approximately
equal to the pKa of the metal ion.
Structure of the free aqueous aluminum [Al(H2O)3+6 ] ion.
Nordstrom and May (1996),
2. Aluminium and iron polymers
The exchnaheable Al3+ ions displaced by cations from clay
minerals are hydrolysed to monomeric and polymeric
hydroxyaluminium complexes. The high charge to radius
ratio i.e., high acidity of Al3+ ion enables it to rupture O-H bond
in [Al(H2O)6 ]+3 Hydrate,causing ionisation of hydrate thereby
yielding protons. The step wise hydrolysis of monomeric Al3+
forms, occurring progressively at higher pH. This rn will be
facilitated by the presence of proton acceptor in soil soln.
Sources of acidity in Soil
2. Rainfall / leaching:
Soils become acidic due to leaching of the basic cations such
as Ca2+, Mg2+, K+, and Na+ down the soil profile by excess rains.
This situation is common in high rainfall areas, where
precipitation exceeds evaporation, and leads to leaching.
CO2 + H2O H2CO3
H+ + HCO3The released hydrogen ions replace the calcium ions held by soil
colloids, causing the soil to become acid. The displaced calcium
(Ca++ ) ions combine with the bicarbonate ions to form calcium
bicarbonate, which, being soluble, is leached from the soil. The
net effect is increased soil acidity.
Ca+2 + 2HCO3- Ca(HCO3-)2
Use of adequate amounts of nitrogen fertilizer is fundamental
for higher yield of crops under all ecosystems. Urea and
ammonium sulfate are dominant nitrogen carriers used for
crop production around the world. The acidification of soils
by using the ammonium form of nitrogen fertilizers can be
explained by the following equation:
Ca (NO3)2, Mg (NO3)2
Highly soluble,Ca, Mg depleting from
in soil above equation is known as
The oxidation of NH4+
nitrification and hetrotrophic and autrotrophic bacterias can
carry it out. The most important autrotrophic genera of
bacteria are Nitrosomonas and Nitrobacter
Reaction of Nitrogenous fertilizer containing N in ammonium form
(NH 4 )2SO 4
then hydrolyzed into NH4 + OH
SO4 + 2H
NH4 OH (weak base)
H2 SO4 (strong acid)
As NH4 OH is a weak base, though the are produced in equivalent amount, NH 4 OH will slowly dissociate than
H2 SO4 producing less OH than H producing acidity in soil.
The NH4 will be adsorbed on the soil colloids replacing the exchangeable bases like Na, Ca, Mg etc
Ca -[soil + (NH 4 )2SO 4
[soil + CaSO 4 ( soluble, causing leaching of bases)
the released bases will react with the corresponding anion , i.e., SO 4-,
Cl-, and NO3- to form respective salts, e.g., CaSO4, CaCl2, Ca(NO3)2 etc.,
the Ca-salts, thus, produced may be lost from the surface soil by
leaching. In this way, there will be some loss of exchangeable bases
from the salts.
Under well-drained and aerated condition, this adsorbed NH4+ will be
liberated in soil solution and nitrified by microbes
NH4+ + 1½ O2
NO2- + H2O+ 2H+
NO2- + ½ O2
NO3- + 18 kcal (Ntrobacter bacteria)
+ 66 kcal (Nitrosomonas bacteria)
4. Use of legume crops continuously or in rotation can increase soil
The reason for generating acidity is associated with higher absorption of
basic cations by these crops and the release of H+ ions by the root of
legume crops to maintain ionic balance
5. Soil acidification is also caused by the release of protons
(H+) during the transformation and cycling of carbon, nitrogen,
and sulfur in the soil–plant–animal system
6. Soil acidification is caused by acid precipitation (acid rain),
the result of industrial pollution
7. O.M. decomposition: organic acids ionized as follows :
R-COO- + H+
respiration: CO2 + H2O ----> H2CO3 = H+ + HCO3Humus contains Carboxylic and phenolic (OH) groups that behave as weak
Active acidity (acidity produced due to H+ and Al3+ ions held in soil soln)
Reserve acidity/ Potential acidity
Exchangeble acidity :
Acidity which is produced due to
presence of exchangeable Al3+ and H+
from exchangeable complex.
acidity produced due to dissociation of H+ from Al-OH,
Si-OH at the broken edges or external surface of the
and also H+ dissociate from -COOH , -OH phenolic
and –NH2 group
Source of negative charge:
Unsatisfied valences originated due to broken bond at the edges and
corners of crystal
Dissociation of the H+ from the OH of the Octahedral layer at definite
Isomorphous repalcement by a action of similar size with lower
Source of positive charge:
The protonation or addition of H+ to hydroxyl groups on edge of these
The exchange of the OH group for other anions such as Phosphate
〉 Al − OH + H 2 PO 4 − ⇔〉Al − H 2 PO 4 + OH −
The negative OH- being replaced from positively charged Al+3 ions in the
Creation of Clay Colloid Charge
• The replacement of one ion for another
of similar size within the crystalline
structure of the clay
takes eons –
doesn’t change rapidly
Octahedral sheet neutral
Net negative charge
Edge Effects are pH Dependent
Negative charges arises due to
unsatisfied valences at the
broken edges of the silica and
At high pH, the hydrogen of
these hydroxyls dissociates
slightly and the colloidal
surface is left with a negative
charge carried by the oxygen
Diagram of a broken edge of a Kaolinite crystal, showing O2 as the source of
negative charge. At high pH values the H ions tend to be held loosely and can
be exchanged for other cations
Adsorption of cations by
The charge on humus colloids is pH dependent.
Under strongly acid conditions H is tightly bound and not
easily replacebale by other cations. The colloid therefore exhibits
a low negative charge.
With the addition of bases and the consequent rise in alkalinity,
first the H from the carboxyl groups and then the H from the
phenolic groups ionizes and is replaced by Ca,Mg and other
Negative charges on humus
ENORMOUS external surface area! (but no
internal surface – all edges)
Central unit of a
(mostly C and H)
Influence of pH on the CEC ( a
measure of the negative
charge) of montmorillonite
Note that below pH 6.0 the
charge for the clay mineral is
relatively constant. This
charge is considered
permanent and is due to ionic
substition in the crystal unit.
Above pH 6.0 the charge on
the mineral colloid increases
because of ioniosation of
hydrogen from exposed O-H
groups at crystal edges.. In
contrast to clay, essentially all
of the charges on the organic
colloid are considered pH
Held more strongly
Held more weakly
H+ ≥ Al3+ > Ca2+ > Mg2+ > NH4+ = K+ > Na+
This is referred to as the “Lyotropic series”
Strength of adsorption proportional
to valence ÷ hydrated radius
Active acidity is very very low than Reserve Acidity
Total acidity = Reserve acidity + Active acidity
Active acidity is measured by pH of the soil..
Total acidity measurement: Soil suspension titrated with strong alkali (0.1N NaOH,
0.1N KOH, 0.1 M Ca(OH)2 at pH 7.0 . The amount of alkali reqd. measures the total
Q.1 : 5 g soil, alkali consumed 10 ml 0.1N NaOH, to increase pH 5.0 to 7.0
1 ml 1 (N) alkali ≡ 1 meq of H+
10 ml 0.1 N alkali ≡ 10* 0.1 *1 = 1 meq of H+
5 g soil contain 1 meq H+
100 g soil contain 20 meq H+ (total acidity)
pH..5.0.(5 g soil, 10 cc water)... 10-5 g eq H+ / litre;
1 litre suspension contains 10-5 g eq H+ ; 10 cc contains 10-7 g eq H+
5 g soil contains 10-7 g eq H+ ; 100 g soil contains = 20 x 10-7 g eq= 20x10-4 meq
Reserve acidity= 20 meq- 20x10-4 meq
How to measure the exchange acidity:
Unbuffered salt(KCl, NaCl) soln mixed with soil and shake. K+ will replace H+, Al+3
on the soil echange complex surface. Then the remaining soln containing H+, Al+3 is
titrated by NaOH. The quantity of NaOH indicates the magbitude of exchangeable
Any resistance to change in pH of the soil or soln. by an
external agency (alkali) is known as buffering capacity.
–Direct correlated with CEC of a soil, a high CEC is associated with a large
number of exchange sites
Mont > Illite > Kaolinite
Soils having higher Humus
shows higher bc
–High buffered soils are organic soils, and 2:1clay soils.
–Low buffered soils are low organic matter soils and 1:1 clay soils.
X ppm= x mg/kg = x. /(eq.wt) meq/kg =
For Ca……x/20 meq/kg=x/20 m mol/kg=x/200 cmol/kg
Factors which influence the relationship between pH and
1. Nature of the Colloidal material of the soil
Soil A (dominated by Humus) vs Soil B (dominated by Silicate clay minerals)
Binding energy of H+ with Humus <
binding energy of H+ with silicate clay
pH will be lower in soil A (release of H+ is easy, losoely held)
pH will be higher in soil B
2. Nature of clay minerals:
Mont. Release H+ quickly
pH( soil A) < pH ( soil B )
3. Nature of adsorbed cation ( Ca, Na )
Soil B ( exch. Base 5 meq and CEC 10 meq)
(Ca+2 3.0, Mg+2 1.0, Na+ 0.5, K+ 0.5meq.) (Ca+21.0, Mg+2 1.0, Na+ 2.0, K+1.0 meq.)
pH( soil B) > pH ( soil A )
Problems of acid soil
Adverse effects of Acid soils on plant growth may be due to following reason
1. Toxicity of elements (Al, Mn, Fe)
2. Deficiency of bases (Ca+2, Mg+2)
3. Imbalance of P,S and Mo
4. Poor microbial activity
1.a) Toxic effects of Al :- Al inhibits the root growth of the plants, interferes with
the various physiological process of the plants like cell-division, respiration and
DNA synthesis; restricts the uptake of Ca+2, P and H2O
b) Mn–toxicity: In soils having pH below 5.0, excess Mn accumulates in all the
tissue of the plant, the normal metabolism of the plant is seriously affected.
c) Fe-toxicity: iron concentration in soil increases with the decrease in pH,
increase in O.M. content and the intensity of soil redn.
Under waterlogged condition of rice cultivation the soil undergoes reduction which
reduces Fe+3 iron to Fe+2 iron which is more soluble and sometimes the concn. of
Fe+2 in waterlogged rice soils becomes high and hence toxic to rice plants. This
creates what is known as physiological diseases of rice(browning disease).
2. Deficiency of bases:
The amount of exchangeable Ca, Mg is lower in acid soils. Percentage
base saturation is also low as the most exchangeable sites are occupied
by Al and H. Ca and Mg are secondary essential elements as far as plant
nutrients are concerned. Plant (Leguminous plant) require high amount of
Ca and Mg. Due to lack of Ca and Mg yield will be hampered. But Rice,
wheat do not require Ca, Mg, so not seriously affected.
Ca, Mg improve the structure of the soils and so their deficiency will give
rise to poor structure of the soil and thus they inhibit proper aeration.
Microbial activities are also decreased due to the insufficiency of bases.
So, mineralisation will be adversely affected .
3. Imbalance of Nutrient elements:
a) Phosphorus: i) P in soil is precipitated due to formation of AlPO4 and FePO4. In
acid soil Al and Fe concn is high. So availability of P in acid soil is very low.
ii) Plant generally take up P from soil in the form of H2PO4- and HPO4-2. Chemical
adsorption on the surface of the colloidal material and soil dominated with
kaolinite clay mineral adsorbs more P. Therefore, P is not released from the
surface and P availability will be low.
b) Sulphur: most of the S in soils present in organic form. These
organic S are mineralised by some soil microbes. But in acid
condition, they can’t function well to mineralise the S. these
micro org. can’t grow well in low pH. Unless these S are
changed to inorganic form, plant can’t absorbs S as nutrients .
c) Molybdenum (Mo): generally micro nutrients are more soluble in
acid soil but Mo is the exception. Mo is less soluble in low pH
and thus becomes less available to the plants. In acidic
condition it produces insoluble molybdates. Lack of Mo
4. Poor microbial activity: most of the microbes prefer
neutral pH. So in acid condition, their activity will be
Effect of pH on the
nutrients important in
plant growth and of
microorganisms. As the
band for a particular
nutrient or microbe
widens, the availability
of the nutrient or
activity of the microbes
is greater. For example,
with K the greatest
availability is from pH
From Brady (1984),
Fungi prefer acidic soil reaction;
Bacteria and actinomycetes perform better in
intermediate pH ranges.
Activity of Nitrosomonas and Nitrobacter reduced
Azotobacter population is less in acid soils.
Activity of BGA and Rhizobia is also less.
Ca and Mo deficiency adversely affects nodulation
How to overcome the deleterious effect
i) Agronomic approach –grow acid tolerant crops
ii) Chemical approach- use liming material to increase the soil pH
LIME REQUIREMENT(LR)The lime requirement of an acid soil is the amount of a liming
material that must be added to raise the soil pH to a desired
level.( usually in the range of 6.0 to 7.0)
The lime requirement ranges from 3.5 to 15 t/ha.
Common Liming materials:
Calcium Carbonate (CaCO3) ,
Dolomite (CaCO3,MgCO3), Burnt lime (CaO),
Slaked lime (Ca(OH)2
Marl, Oyster shell,
Basic slag (CaSiO3), Paper Mill Sludge,
Reactions of lime in acid soil :
1. CO2 + H2O H2CO3
2. H2CO3 + CaCO3 (insoluble)
3. Ca(HCO3)2 Ca+2 + 2 HCO3H
(Clay) + Ca+2
Ca----(Clay) + 2H+
5. H+ + HCO36.
H2CO3+ CaCO3 (insoluble) Ca(HCO3)2
Calcium Carbonate Equivalent (CCE):
Defined as the acid neutralizing capacity of an agricultural
liming material expressed as a weight percentage of Calcium
The molecular constitution is the determining factor in the
neutralising value of chemically pure liming materials;
CaCO3+ 2HCl →CaCl2 + H2O + CO2
MgCO3 + 2HCl →MgCl2 + H2O + CO2
MW : 100
MW : 84
84g MgCO3 =100g CaCO3
100 g MgCO3 = 119 g CaCO3
Name of Liming
(Neutralizing value )
How much quantity of lime is required
The titration curve of soil A, with a low buffer capacity, and soil B, with
a high buffer capacity. The lime requirements of these soils are
indicated (in units of centimoles of base per kilogram of soil) assuming
that they are to be limed to pH 6.0.
Factors on which the rate of reaction of liming material depends:
Soil moisture : The greater the amount of moisture the more rapid is the
rate of Rn. (Higher the moisture, soil will develop gradually more anaerobic
condition giving rise more CO2 which forms H2CO3. This acid is solubilising
agent of CaCO3)
Temperature : Limestone reacts more rapidly at higher temp. this effect is
probably related to diffusion rates of end products
Fineness of the material: A more finely ground limestone exposes more
surface area than a coursely ground lime stone. (40 mesh vs 10 mesh)
To get maximum benefits from liming or for improving crop yields,
liming materials should be applied in advance of crop sowing
thoroughly mixed into the soil to enhance its reaction with soil
Fineness is measured by the proportion of processed agricultural lime which
passes through a sieve with an opening of a particular size.
A 60-mesh sieve, which is the standard for comparisons of lime fineness
and efficiency rating of 100%, is assigned
Beneficial effects of Liming:
1. Liming raises soil pH, base saturation, and Ca and Mg contents,
and reduces aluminum concentration in acid soil
i) Ca and Mg react with H+ on the exchange complex and H+ is replaced
by Ca2+ and Mg2+ on the exchange sites (negatively charged particles of clay
or organic matter), forming HCO3-.
ii) HCO - reacts with H+ to form CO and H O to increase pH.
2.Reducing phosphorus immobilization
Increase in availability of P in the pH range of 5.0 to 6.5 was
associated with release of P ions from Al and Fe oxides, which were
responsible for P fixation (Fageria, 1989).
At higher pH (>6.5), the reduction of extractable P was
associated with precipitation of P as Ca phosphate (Naidu et al., 1990).
These increases in extractable P or liberation of this element in the
pH range of 5.0–6.5 and reduction in the higher pH range (>6.5) can be
explained by the following equations:
The liming acid soils result in the release of P for plant uptake;
this effect is often referred to as ‘‘P spring effect’’ of lime
(Bolan et al., 2003).
The base forming cations are the sources of hydroxyl ions.
H and Al ions are replaced on the exchange complex by Ca+2 ions
when an acid soil is limed, thereby lowering the concn. of H + and
Al3+ and raising the concn. of OH- ion and hence soil pH
3. Improving soil structure
Calcium in liming materials helps in the formation of soil
aggregates, hence improving soil structure. The lime-induced
improvement in aggregate stability manifested through the
effect of liming on dispersion and flocculation of soil
particles (Bolan et al., 2003).
Liming is often recommended for the successful
colonization of earthworm in pasture soils.
The lime-induced increase in earthworm activity may
influence soil (Springett and Syers, 1984) structure and
macroporosity through the release of polysaccharide and the
burrowing activity of earthworm
4.Improving nutrient use efficiency
The improvement in efficiency of these nutrients was
associated with decreasing soil acidity, improving their
availability, and enhanced root system
Disadvantages of Overliming
Overliming effects are more pronounced on
coarse-textured soils than on fine-textured soils.
Fine-textured soils have high buffer capacity; hence,
changes in chemical properties due to liming are not as
pronounced as for coarse-textured soils.
Overliming can create deficiencies of micronutrients like
Fe, Mn, Zn, Cu, and B. These nutrients are usually adsorbed
onto sesquioxide soil surfaces
Why gypsum is not considered as liming material?
Gypsum is not considered as liming materials because on its application to
an acid soil it dissociates into Ca+2 and SO4-2 ions:
↔ Ca +2 +
SO 4 -2
The accompanying anion is sulphate and it reacts with soil moisture
produces mineral acid (H2SO4) which also increases soil acidity instead of
reducing soil acidity
SO 4 -2 + H 2 O →H 2 SO 4
Basic slag can also be used as liming material when it is
applied to an acid soil the following chemical reaction takes
+ 2CaSiO3 + 3H2O = Soil
+2H2SiO3 + Al(OH)3
The metasilicic acid is weakly dissociated, much less so that the clay adsorbed
H+ ions and the pH of the soil raised
Q. If sandy loam soil having CEC, 20 cmol(p+)kg-1 and 30% base
saturation at pH 4.5. Calculate the theoretical amount of lime (CaCO3)
required per ha of land (0-15 cm depth) for raising BS to 60%.( wt of
soil 1 ha furrow slice(15 cm )=2.2x106 kg
Ans. PBS = [meq of basic cations (S) / Total exch cations (T)] x 100
BS i =30%,
BSf= 60% ,
(S i ) =30*20/100 =6 meq /100g soil
(S f )=60*20/100= 12 meq / 100g soil
So meq of basic cation will be required to raise the base
saturation at 60% = 12-6 =6 meq
1 meq of CaCO3 weighs 50 mg (Eq. wt of CaCO3 = 100/2 = 50)
6 meq of CaCO3 weighs 50 x 6 mg= 300 mg
100 g soil requires 300mg = 300x 10-6 kg CaCO3
1 kg soil requires = 300* 10-5 CaCO3
2.2 *106 kg requires = 300 x 2.2x 106 x 10-5 = 6600kg= 6.6 t
To titarte 100 g of soil of initial pH 4.5 to a specified
pH of 6.5 with N/10 NaOH, 50 ml of N/10 NaOH was
required. Calculate the amount of CaCO3 to be applied in
1 ha 30 cm layer of soil. Assuming, all the CaCO 3 present
in the material undergoes dissociation.
1 ml 1N NaOH ≡ 1 meq of bases
50 ml N/10 NaOH = 5 meq of bases
5 meq of NaOH ≡ 5 meq of CaCO3 = 5 x 50 mg CaCO3 = 250
100 g soil requires 250 mg CaCO3
1 kg soil requires 250*10 mg=2500mg=2.5 g CaCO3
4.5 x 106 kg soil requires= 11.25 t/ha
Some important crop species, pasture species, and plantation
crops tolerant to soil acidity in the tropics
Practices to be considered during
Lime application should be done split doses.
Smaller doses with frequent application.
Lime applied in furrows at the time of sowing.
Applied in alternate year till the soil pH is brought to normal range.
• Organic matter content
• Types of clay present
What are acid sulfate soils?
Soils formed from the
weathering of sulfide-bearing
which results in extremely low pH
(commonly < 3.0)
and precipitation of sulfate salts.
Potential acid sulfate soil
Potential acid sulfate soil contains
sulfidic soil material that contains pyrite
but has not oxidized to an extent that the
soil-pH dropped to a value below 3.5
Formation of pyrite
Fe2O3 + 4SO42- + 8CH2O + 1/2O2 = 2FeS2 + 8HCO3- + 4H2O
Iron must be present
Sulfur must be present
Anaerobic condition must prevail to reduce SO42- & Fe3+
Organic matter as energy source for the microbes
The process increases pH
Location of pyrite in the landscape
In delta regions and lagunes where
sea water is meeting fresh water.
Inland wetland areas which are enriched with
ferro iron and sulfate from higher parts of the
Soil material with high content of pyrite is called
sulfidic soil materials
Oxidation of pyrite
If the soil is drained pyrite will be oxidized:
4FeS2 + 15O2 + H2O -> 2 Fe2(SO4)3 + 2H2SO4
pH drops significantly and not only ferro iron but also
ferri iron will be mobile.
Soils which become very acid due to oxidation of
pyrite are classified as actual acid sulfate soils
actual acid sufate soils
Low soil pH
Salinity (from sea water)
(precipitation of aluminiumphosphates)
H2S toxicity if flooded
N-deficiency due to slow microbial activity
Engeneering problems as soil acidity attacks
steel and concrete structures
Oxidation of pyrite might form
a sulfuric horizon
• Definition of sulfuric horizon
• A sulfuric horizon must:
• have a soil-pH < 3.5 (in 1:1 water suspension); and
– yellow/orange jarosite [KFe3(SO4)2(OH)6] or yellowish-brown
schwertmannite [Fe16O16(SO4)3(OH)10.10H2O] mottles; or
– concretions and/or mottles with a Munsell hue of 2.5Y or more
and a chroma of 6 or more; or
– underlying sulfidic soil materials; or
– 0.05 percent (by weight) or more of water-soluble sulphate; and
• have a thickness of 15 cm or more.
Root system affected
Poor maize crop
in an acid field
Source : http//www.rag.org.au/phildickiestories …..
• Cation exchange- the interchange between a cation in
solution and another cation on a soil surface
• Cation exchange capacity (CEC)- the total sum of
exchangeable cations that a soil can adsorb.
pH independent charge (permanent)
Isomorphic substitution: substitution of one element for another in ionic
crystals without changing the structure of the crystal
Substitution of Al+++ for Si++++ in tetrahedral
Mg++, Fe++, Fe+++ for Al+++ in octahedral
Leaves a net negative charge (permanent)
pH dependent charge: positive charge developed at low pH and excess
negative charge formed at high pH
Gain or loss of H+ from functional groups on the surface of soil solids.
pH dependent charge (cont.)
Only 5-10% of the negative charge on 2:1 layer silicates is pH dependent whereas 50%
or more of the charge developed on 1:1 minerals can be pH dependent.
1. Kaolinite: 1:1
At high pH, these H+ are weakly held and may be exchanged.
At low pH, H+ are held very tightly and are not exchanged.
Deprotonation or dissociation of H+ from OH- groups at the broken edges of clay particles
is the prime source of negative charge in the 1:1 clay minerals. High pH values favor
this deprotonation of exposed hydroxyl groups.
This creates some confusion since high pH is seldom associated with weathered
soils. Fifty percent or more of the charge developed on 1:1 clay minerals can be
In a weathered soil, hydrous oxides (Fe and Al oxides) are a more important
source of pH dependent charge. Si (as silicic acid H4SiO4) is weathered and
leached. 2:1’s become 1:1’s become Fe-Al oxides
Where does the negative charge come from in an acid soil?
2. Organic Matter
Most soils have a net negative charge due to negative charges on layer silicates and
organic matter, however, some highly weathered soils dominated by allophane and
The Colloidal Fraction: Seat of Soil Chemical and
Some of the many types:
• Layer silicate clays
• Iron and Aluminum
• Organic soil colloids:
Colloids are small particles in
soil that act like banks:
managing the exchange of
nutrient currency in the soil
Different soils, like checking accounts, have different
capacities to hold nutrient currency: cations and anions
Problem Soils in South and Southeast Asia (Adapted from
Ponnamperuma and Bandyopadhya 1980)
radii in nm*
Surface charge on soil colloids is developed in two
ways: isomorphic substitution (permanent charge) and
deprotonation of surface functional groups (pH dependent
charge). pH dependent charge occurs on the edges of layer
silicates, on variable charge minerals such as oxides of Fe and
Al, and organic matter. It is called pH dependent charge because
it increases in magnitude as the pH of the aqueous soil
environment increases. Most of the pH dependent charge
associated with agricultural soils is due to the deprotonation of
organic functional groups. As the pH of the soil environment
increases weak acid functional groups such as the carboxylic
acid donate a proton and generate negative charge:
COOH + OH- = COO- + H2O
One practical way to increase the CEC of agricultural soils is to
increase the organic matter content through tillage practices and
increase the pH by adding lime.
Isomorphic substitution is the replacement of one atom by another
of similar size in a crystal lattice without disrupting or changing
the crystal structure of the mineral. If you remember from the clay
and primary mineral tutorials, cations are coordinated to oxygen
or hydroxyl anions in mineral structures. The negative charge of
the anions is balanced by the positive charge of the cations that
are coordinated to it. Net negative charge is developed when a
cation of similar size and less positive charge substitutes for one
of higher positive charge. Isomorphic substitution can also take
place between cations of the same charge or a cation of higher
positive charge. In the case of isomorphic substitution between
cations of the same charge no charge is developed. In the case of
isomorphic substitution between a cation of higher positive charge
with one of lower positive charge a net positive charge is
developed. The important thing to remember is that isomorphic
substitution only occurs between cations of similar ionic radii. In
the tutorials below we will be strictly dealing with permanent
charge developed through isomorphic susbtsitution.
In clogged environments oxygen is present at lower
concentrations and is quickly
exhausted by the respiration of soil micro-organisms and plant
roots (Prade et al., 1990).
With the exhaustion of the oxygen, NO3 -, Mn4+, Fe3+, and
SO42- type elements can act as electron receivers for microbial
respiration and hence be found in reduced form in flooded rice
fields. Oxygen and nitrates are used in the early stages of the
flooding. Reduction of manganese (Mn4+) also occurs quickly,
since soil manganese content isusually low (Ponnamperuma,
1977). It is some days after flooding, when the redox potential
of the environment becomes lower than 180-150 mV, that the
reduction of the ferric iron Fe3+ starts (Patrick and Reddy,
1978). Under the reductive effect in this clogged environment
the ferric ion Fe3+ is reduced into ferrous ion (Fe2+), moreover
the bacteria present in the environment begin anaerobic
respiration that releases high quantities of ferrous ions (Fe2+);
in the solution
Potentiometric titration curve of H- and Albentonite (a smectitic clay).
Coleman and Harward (1953).
One of the most useful
ways to characterize soil
acidity is to use
and conductometric titration
one is investigating the
relationship between pH
and the amount of base
added (Fig. 9.10).
The first inflection point
or break in the titration
curve is ascribed to the
end of the neutralization
of exchangeable H+ and
the beginning of the
titration of exchangeable