Soil science

1. SOIL ACIDITY
Concentrations of hydrogen and hydroxyl ions of a
solution determine acidity or alkalinity.
In aqueous system , an acid donates H+ , a base
that accepts H+ or donates OH-
Water molecules break up into hydrogen and
hydroxyl ions. ( Ionisation)
H2O H++OH
The H+ ion attaches to another water molecule to
give H3O+

2. Both H+ and OH- are produced by dissociation of a water
molecule. Water is both a weak acid and weak base.
The product of H+ and OH- concentration is the
dissociation constant of water or Kw
Kw=[H+ ][OH-]=[10-7][10-7]=10-14
Adding of an acid to water will increase [H+], but [OH-]
would decrease because Kw is a constant, viz. 10-14
Concentration is expressed by a logarithmic scale
pH ranges from 0 to 14, which is the negative logarithm
of hydrogen ion activity. Danish Chemist -Sorenson
pH = - logaH+ =log {1/aH+} or aH+ =10-pH
pOH = -logaOH- =log {1/aOH-} or aOH- =10-pOH
In pure water, pH + pOH = 14 and pH = pOH = 7

3. Source of Soil acidity
The important sources of soil acidity are
1. Exchangeable H+ & Al 3+
2. Al and Fe oxides
3. Soil organic matter
4. Clay matter
Two adsorbed cations-Hydrogen and Aluminium
are mainly responsible for the soil acidity.

4. Sources of Hydrogen and Hydroxyl Ions
 H+ held on surfaces of solid clay particles or organic matter
 Active acidity is due to the H+ in soil solution
 Potential acidity - H+ adsorbed on surface of solid particles
 Exchangeable H+ present in soil neutralise the negative
charge arising from isomorphous substitution of cations
 pH dependent charge arise from structural OH- groups at
soil clay minerals, dissociate into H+ ions
 At pH less than 5.0, Al3+ becomes soluble and present as
aluminium hydroxy cations
 Al3+ hydrolyze with liberation of H+
Al3+ + H20 = Al(OH)2+ + H+
Al(OH)2+ + H20 = Al(OH)2
+ +H+
Al(OH)2+ + H20 = Al(OH)3 + H+
 Fine-textured soils > Lighter soils

5. Moderately acid soils (pH 5.0 - 6.5)
Al3+ and H+ account for active hydrogen.
Al ions converted into aluminium hydroxy ions
Al3+ + OH- → Al(OH)2+
Al(OH)2++ OH-→ Al(OH)2
+
Some of the aluminium hydroxy ions are adsorbed and act as exchangeable
ions. They remain in equilibrium in soil solution.In the soil solution , they
hydrolyse and produce H ions and contribute soil acidty.
Neutral to alkaline soils.
Al(OH)2+ + H20 → Al(OH)2
+ H+
Al(OH)2
+ + H20 → Al(OH)3
+ H+
- dominance of Ca2+ and Mg2+ ions and other base-forming cations occupy
the exchange sites - aluminium hydroxy ions get converted to gibbsite
Al3+ and H+ are replaced on exchange complex by Ca2+ when soil is limed
and raising the concentration of OH- ion
2H+ -(clay) + 2H20 =Ca2+-(clay) + 20H-

6. At pH below 5.0, iron also behaves like Al, but acidity
generated by iron is much more less than Al.
In acid sulphate soil- H2SO4, Al2(SO4)3 and Fe2(SO4)3 cause
acidity.
At higher pH –Al and Fe hydroxide contribute acidity.
High Al- prevent the absorption of P and Al inside the cell
interfere with sugar phosphorylation.
Soil OM/ HUMUS-COOH and phenolic group-weak acid .

7. Classification of Acidity
(a)active acidity
- acidity developed due to concentration of hydrogen
(H+) and aluminium (Al3+) ions in soil solution
- H+ in soil solution due to active acidity is very small
- Needs meager amount of lime to neutralize acidity
- Plant root and microbes around the rhizosphere zone
are affected
(b) exchangeable acidity
- In strongly acidic soils, concentrations of exchangeable
H+ and Al3+ contributes exchangeable acidity.
- defined as acidity developed due to adsorbed H+ and
Al3+ ions on soil colloids.
- Ex. H+ and Al3+ concentration is meager in moderately
acid soils.

8. (c) reserve / potential acidity
- Aluminium hydroxyl ions, H+ and Al3+ present in non
exchangeable form with organic matter and clays
- Non-exchangeable acidity contributes to titratable or total
acidity.
- Liming leads to form gibbsite from aluminium hydroxy
ions with increase in pH.
- acidity is much higher as compared to other acidity and
requires larger doses of lime for neutralization
-
- No attempt is made to neutralize reserve acidity, always
liming limited to neutralize active acidity and partially the
exchangeable acidity.

9. - The H ions in soil solution contribute active acidity which
is measured in terms of pH .
- The total acidity –caused by H ions held in different
chemical combinations, adsorbed on surface of solid
particle and organic colloids.
- When active acidity is neutralised progressively , the H
ions from the potential form are released to active form.
- Dynamic equilibrium exists between active, exch.and
reserve acidity.
- It occupies clay and organic surfaces.
- % Base saturation: Exch bases X 100
- CEC
- BSP-Less that 50-pH-5.5-acidity
- BSP-80% -pH- 6.5-Neutral

10. Buffering of Soils
Buffering is defined as the tolerance to changes in pH of a
solution
Rapid lowering of soil pH is prevented by buffering action as
that of organic acids are formed during organic matter
decomposition.
Soils having more of clay and organic matter exhibit higher
buffer capacity
Higher the exchange capacity of a soil, greater the buffer
capacity
Degree of buffering is highest between pH 4.5 and 6.0 and
drops off below pH 4.5 and above pH 6.0.
Buffering action is due to influence of weak acids and their
salts (Carbonates, bicarbonates and phosphates, Organic
acids act as buffering agents)

11. Soil Reaction Correlations
- cations Ca2+, Mg2+, K+, Na+ and Al3+ direct soil pH.
- At low pH, Fe, Mn and Al are highly soluble and attain toxic.
- Nitrification is slow below pH 5.5
- Under acid conditions, the NH4
+ fixation between lattices of
decreases with decreasing pH
- Phosphorus availability (H2PO-
4 and HP04
2- ions), is highest
in neutral or slightly acidic and declines soil becomes
strongly acidic or strongly alkaline
- cationic micronutrients availability (Fe, Mn, Cu and Zn)
increases with increase in soil acidity, whereas the
availability of molybdenum and boron, decreases.
- Bacteria and actinomycetes are active in mild acidic,
neutral and high pH soils.
- Fungi predominating in acid soils as compared to soils of
intermediate and higher pH.
- Rhizobia are less effective in acid soils.
- A pH range of 6.0 to 7.2 is suitable for most plants.

12. Acid Forming Factors
 Leaching due to Heavy Rainfall
 Acidic Parent Material
 Acid Forming Fertilizers and Soluble Salts
 Humus and Other Organic Acids
 Alumino silicate Minerals
 Carbondioxide (CO2)
 Hydrous Oxides
 Aluminium and Iron Polymers

13. Leaching due to Heavy Rainfall
Common in regions have rainfall
Leaches high amounts of exchangeable bases from soils
Leaves insoluble acidic compounds of Al and Fe in soil
Oxides and hydroxides react with water (H2O) and release
hydrogen (H+) ions in soil solution
H+ ions of the carbonic acid and other acids replaced the
basic cations of colloidal complex
C02 + H20 = H2CO3
H2CO3 + CaC03 = Ca(HC03)2 ↓
leachable

14. Acidic Parent Material
Acid parent materials -granite
Acid Forming Fertilizers and Soluble Salts
use of (NH4)2SO4 and NH4NO3
Humus and Other Organic Acids
Humus functional groups, carboxylic (-COOH), phenolic (-OH)
attracts hydrogen (H+) ions
Aluminosilicate Minerals
At low pH values Al3+ is present as hydrated aluminium ions
and undergoes hydrolysis
Carbondioxide (CO2)
Plant root and microbes activity: Hydrolysis of CO2
Hydrous Oxides
Oxides of Fe and Al undergoes hydrolysis and releases H+ ions

15. Aluminium and Iron Polymers
Al3+ ions displaced from clay minerals are hydrolyzed to
monomeric and polymeric hydroxyl aluminium complexes.
Hydrolysis of monomeric hexaquo aluminium or iron forms
with liberation of hydroniam ion (H30+) and lower soil pH
Al(H20)6
+ + H20 Al(OH) (H20)5
2+ + H30+
Al(OH)(H20)5
2+ + H20 Al(OH)2(H20)4
+ + H30+
Al(OH)2(H20)4
+ + H20 Al(OH)3(H20)3
0 + H30+
Al(OH)3(H20)3
0 + H20 Al(OH)4(H20)2
- + H30+

17. In a neutral soil, salt concentration affects the pH
value due to increase in the apparent strength of
acidic groups.
By increasing salt concentration in an alkaline soil,
the pH value is lowered due to reduction in the
hydrolysis of the exchangeable cations like sodium.
The bicarbonate concentration in the soil solution
due to free calcium carbonate and magnesium
carbonate influences the pH value.

18. Problems of Soil Acidity
1. Toxic effects
(a) Acid toxicity
(b) Toxicity of different nutrient elements
2. Nutrient availability
(a) Non-specific effects
(b) Specific effects
(i) Exchangeable bases
(ii) Nutrient imbalances
3.Microbial activity.

19. a. Acid Toxicity
Higher H+ concentration is toxic to plants under strong
acid conditions of soil.
b. Toxicity of Different Nutrient Elements
Iron and Manganese
Concentration of Fe2+ and Mn2+ in soil solution depends
upon the pH, organic matter and intensity of soil reduction
Due to increase in organic matter content and soil
microbes increases used the soil oxygen and results
reduction of soil.
Due to soil reduction, the nutrient elements like Mn4+ and
Fe 3+ reduce to Mn2+ (manganous manganese) and Fe 2+
(ferrous iron) respectively and very high toxicity of those
elements develops.

20. Toxicity of Aluminium (AI)
Aluminium toxicity is a problem in both upland and
lowland soils. plant growth affected by :
a. Restricts the root growth.
b. Affects plant physiological processes like division of
cells, formation of DNA and respiration etc.
c. Restricts the absorption and translocation of nutrient
elements from soil to plant like phosphorus, calcium,
iron, manganese etc.
d. Causes wilting of plants.
e. Inhibits the microbial activity in the soil.

21. Nutrient Availability
a. Non-specific effects.
It is associated with the inhibition effect of root growth
and thereby affects the nutrient availability.
b.Specific Effects
i. Exchangeable bases
Ion uptake and release process adversely affected due
to soil acidity.
- Deficiency of bases like Ca and Mg are found in acid
soils.
b. Nutrient imbalances

22. Microbial Activity
Bacteria and actinomycetes function better in soils having
moderate to high pH values.
Nitrogen fixation in acid soils is greatly affected by lowering
the activity of Azplobacter sp.
soil acidity also inhibits the symbiotic nitrogen fixation by
affecting the activity of Rhizobium sp.
Fungi can grow well under very acid soils and caused
various diseases (root rot of tobacoo, blights of potato etc).

23. AMELIORATION OF SOIL ACIDITY
Fertility status - affect plant growth and development
Yield gap – increased
Management practices
- lime and liming materials -by increasing the soil pH.

24. Principles of Liming Reactions
Lime reactions depends upon the nature and fineness of
liming materials.
Usually applied to soils in the form of ground limestone.
Limestones - calcitic (CaCO )& dolomite [CaMg(C0 ) ]
- limestones are sparingly soluble in pure water but
soluble in water containing carbon dioxide.
Reaction of limestone as :
CaC03 + H20 + C02 Ca(HC03)2
Ca(HC03)2 Ca2+ + 2HCO3
-
H++ HCO3
- H2CO3 H20 + C02
(soil solution) (from lime)
- H+ in soil solution react to form weakly dissociated
water
- Ca2+ from limestone undergo cation exchange reactions.
- Soil acidity is neutralized and per cent base saturation
of the colloidal material is increased.

25. Process of changing pH by lime

26. a. Moisture
- greater amount of moisture, more rapid rate of reaction.
- increased moisture allow for a greater volume of solution
and a lower concentration of reaction end products.
- Since the reaction is an equilibrium, accumulation of end
products would reduce reaction rate.
b. Temperature
- Liming materials react more rapid -low temperatures.
- (diffusion rates of end products away from reaction sites)
c. Amount of exchange acidity
- Amount of exchange acidity affects reaction rate.
- If a soil has a high lime requirement and sufficient
quantity of limestone is added to neutralize the acidity
present, the initial reaction will be quite rapid. However,
as the acidity becomes neutralized, the rate of reaction
decreases and finally, as neutrality is approached
becomes almost negligible.
Factors Affecting Liming/limestone Reactions

27. Naturally Occurring Liming Materials
defined as materials that are neutralize of soil solution
hydrogen (H) ions.
- oxides, hydroxides, carbonates and silicates of Ca or
Ca and Mg
- In addition to these, accompanying anion must reduce
the activity of H+ and Al3+ ions in soil solution.
- These are called "Agricultural liming materials".

28. Application to an acid soil it dissociates into Ca2+ and SO4
2-
CaS04 Ca2+ + SO4
2-
Anion is sulphate and reacts with soil moisture produces mineral
acid (H2SO4)
SO4
2- + H20 H2SO4
Why Gypsum is not considered as a Liming Material?

29. Liming Materials types
a. Oxides of lime
b. Hydroxides of lime
c. Carbonates of lime
d. Slags

30. - Called as burned lime or quick lime.
- more caustic than limestones
Burned lime is produced by heating limestone
and dolomite
CaC03 + Heat CaO + C02 ↑
(Limestone)
CaMg(C03)2 + heat CaO + MgO + 2CO2↑
a. Oxides of lime

31. Produced by adding water to burned lime and is called
slaked lime.
CaO + H2O Ca(OH)2
(Burned lime) (Slaked lime)
- more caustic than burned lime (CaO)
- If it is kept open in moist air, calcium hydroxide occurs
Ca(OH)2 + C02 CaC03 + H20
- In case of Mg(OH)2
Mg(OH)2 + C02 MgC03 + H20
b. Hydroxides of lime

32. - Byproducts of certain industries and content of varies.
- Important minerals are
Calcite(CaC03) and dolomite [CaMg(C03)2].
c. Carbonates of lime.

33. 1. Blast furance slag
- It is a by-product of iron industry (Pig Iron).
- As a liming material, behaves as calcium silicate
- neutralizing value ranges from 75-90%.
2. Basic slag
- It is a by-product of basic open-hearth method of making
steel from pig iron or produced from high phosphorus
iron ores.
- The impurities in iron, including silica and phosphorus
are fluxed with lime
- neutralizing value ranges 60-70%.
3. Electric furnace slag
- Produced from electric furnace reduction of phosphate
rock during preparation of elemental phosphorus.
- Contains large amount of calcium silicate
4. Other liming materials
- Coral shell, chalk, wood ash, press mud, by-product
material of papper mills, sugar factories
d. Slags

34. Reacts with CO2 and water to form bicarbonate
CaO + H20 + 2C02 → Ca(HCO3)2
Ca(OH)2 + 2C02 → Ca(HCO3)2
CaC03 + H20 + C02 -> Ca(HC03)2
Reacts with soil colloid, replace H and Al ions from colloidal
phase to soil solution
2H+ (Clay) + Ca(OH)2 = Ca2+ (Clay) + 2H20
H (Clay) + Ca(HC03)2 = Ca2+ -(Clay) + 2H20 + 2CO2
Reactions of Lime with CO2 and Soil Colloids

35. 1. Neutralizing value (NV) or Calcium carbonate
Equivalent of liming materials
2. Purity of liming materials and
3. Degree of fineness of liming materials
Chemical Equivalence of Liming Materials
(Efficiency of Liming Materials)

36. - defined as acid neutralizing capacity of an agricultural
liming material expressed as a weight percentage of
CaCO3
- One molecule of CaO, MgO or Ca(OH)2 neutralizes the
same amount of acidity as does one molecule of CaCO3
- CaC03 equivalent of burnt lime (CaO) calculated by
ratio of molecular weights of CaC03 and CaO
CaC03 / CaO = 100 / 56 = 1.786
CaO equivalence of MgC03 is CaO/ MgC03 = 56/84 = 0.67
Mg equivalence of MgO is Mg/MgO = 24/40 = 0.60.
a. Neutralizing value (NV) or CaCO3 Equivalent (CCE)
Liming materials Neutralizing value of CCE (%)
Calcium oxide (CaO) 179
Calcium hydroxide Ca(OH)2] 136
Dolomite [CaMg(C03)2] 108.7
Calcite (CaC03) 100
Basic Slag (CaSi03) 86

37. - Finer materials increase the surface contact with soil
- Coarse, reaction will be slight
- Standard sieves size are used to measure fineness of
liming materials.
- Screen size indicates maximum diameter of particles
that can pass along with all the smaller particles.
- Fineness is measured as ability of a material to pass
through a sieve having 60 holes of equal size in one
linear inch.
b. Purity of liming materials
- more purer, higher effectiveness for amelioration of soil acidity
c. Degree of fineness of liming materials.

38. Material size Efficiency rating (%)
Material passing through a 60 mesh 100
Material passing through a 20 mesh but not a 60 mesh 60
Material passing through an 8 mesh but not a 20 mesh 20
- Liming reduced soil acidity and also supplies Ca and Mg
for plant uptake
- Frequency of liming varies with climate, soil and
cropping
- Soils having more amounts of clay and organic matter
needs more lime than sandy and highly weathered soil of
same pH level.
Type of clay is considered for to decide the dose of lime.
- smectitic clay requires more lime than kaolinite for an
equal rise in pH.
- More lime is needed for strong acid soil than weakly
acid soils

39. Effective calcium carbonate (ECC) rating of liming
materials is one product of its calcium carbonate
equivalent (CCE) and fineness factor.
Fineness factor is sum of product of percentage of
material in each of three size fractions multiplied
by appropriate effectiveness factor
Per cent ECC or NI = CCE x fineness factor
Per cent Effective CaCO3 (ECC) or(Neutralizing Index)

40. - growing of acid tolerant plant species and varieties
- Rice has good tolerance to acidity since flooding of rice
fields raises the pH to almost neutrality
- less response - minor millet and finger millet
- medium response - Bengal gram, lentil, groundnut,
maize, sorghum and field peas
- High response - Pigeonpea, soybean and cotton
- Liming improves base status, reduces P fixation and
danger of micronutrients toxicity
- stimulates microbiological activity and helps
mineralization of organic N and fixation of atm. N,
improves P availability.
Management of Acid Soils - agricultural practices

41. choice of amendment acid soils
- availability of lime source and its cost
Industrial wastes
- steel mill slag, blast furnace slag, lime sludge from
paper mills, cement kiln wastes, precipitated calcium
carbonate, etc.
- paper mills 65-85% CaCO3, 2% R203, (sesquioxides),
1% free CaO and 1.5% free alkali.
Indian slag contains 1-7% P2O5, 24-50% CaO and 2-10%
MgO
Industrial Wastes as Amendments for Acid Soils

42. Crops Optimum pH range
Cereals
Maize, sorghum, wheat, barley 6.0-7.5
Millets 5.0-6.5
Rice 4.0-6.0
Oats 5.0-7.7
Legumes
Field beans, soybean, pea, lentil etc. 5.5-7.0
Groundnut 5.3-6.6
Others
Sugarcane 6.0-7.5
Cotton 5.0-6.5
Potato 5.0-5.5
Tea 4.0-6.0
Relative tolerance of crops to soil acidity

43. Excessive lime affects growth of plants by
1. Deficiency of micronutrients will occur
2. P and K availability reduced
3. Due to high OH- ion, root development inhibited in
association with tip swelling by hydrations
4. incidence of diseases (scab in root crops)
increased
Effect of Overliming

44. Direct Benefits
1. Toxicity of Al and Mn effect reduced
2. Uptake of Ca2+ and Mg2+ in by plants from soil solution
improved
3. Removal of H+ toxicity damages root membranes and reduced
growth of beneficial micro organisms (bacteria)
Influence of Lime on Soil Properties
(in relation to Plant nutrition)

45. 1. Phosphorus availability
pH between 6.8 to 7.0 for maximum benefit of P
2. Micronutrient availability (Fc, Mn, Cu, Zn and, B etc.)
3. Nitrification
conversion of NH3 to NO3 requires ca with a pH of 5.5 to 6.5
4. Nitrogen fixation.
process of nitrogen fixation (symbiotic and non-symbiotic) is favoured
5. Soil physical condition.
fine soil textured improved by liming.
- increase in OM content and flocculation of ca saturated soils)
- decreases the bulk density of soils,
- increases infiltration and percolation rates of water.
- prevents soil erosion.
6. Diseases.
Club root disease of cole crops reduced with application of lime.
7. Efficiency of fertilizers.
- increases the efficiency of N and P fertilizers
Indirect Benefits