Acid Soil, causes and effects on agricultural production

Acid soils, Causes , effects on plant growth and remediation strategiesm

1. Soil acidity – causes and problems, characteristics of
acid soil
and their reclamation

4. Definition:
“ A soil with a predominance of H
+
ions and probably of
Al
3+
and its various hydrated forms in proportion to
OH
-
ions, specially with pH less than 6.5 ”.
Acid soils are those having high degree of adsorbed
Aluminium and Hydrogen
Acid soils are usually common in humid regions.
In these soils, the concentration of H
+
ions exceeds that
of OH
-
ions

5. Occurrence
Rough estimates put the extent of acid soil
distribution on an area of 100 m ha out of the total
geographical area. (About one third of Indian soils are
acidic)
Of which :
12 % strong acidic (pH < 4.5)
48 % moderately acidic (pH 4.5 - 5.5)
40 % mildly acidic ( pH 5.5 – 6.5)

6. Sl.
No.
Name of the StateGeograp
hical
area
(GA)
Area ( m ha) under acid
soil
%
1Assam & N.E. States25.0 20.0 80
2West Bengal 8.8 3.5 40
3Bihar & Jharkhand17.4 5.2 33
4Orissa 15.6 12.5 80
5Madhya Pradesh &
Chhatisgarh
44.3 8.9 20
6Andhra Pradesh 27.7 5.5 20
Acid Soil Regions in India

7. 7Tamil Nadu 13.0 2.6 20
8Karnataka 19.2 9.6 50
9Kerala 3.9 3.5 90
10Maharashtra 30.8 3.1 10
11Uttar Pradesh29.4 2.9 10
12Himachal
Pradesh
5.6 5.0 90
13Jammu &
Kashmir
22.2 15.5 70

8. Occurrence :
a)Area basis:
Assam & NE states (20 m ha) > Jammu & Kashmir
(15.5 m ha) > Orissa (12.5 m ha) > Karnataka (9.6 m
ha) > Madhya Pradesh & Chattisgarh (8.9 m ha).
b) Per cent of geographical area :
HP (90 %) > Kerala (90 %) > Assam & NE states (80
%) > Orissa (80 %) > J&K (70 %) > Karnataka (50
%).

9. c) Soil Types:
Soil acidity is mainly seen in Oxisols (laterites), Alfisols (red
soils), Histosols (organic soils and Spodosols (forest soils).
Oxisols (laterite soil) : Chemical migration of silica and
accumulation of iron and Aluminium. Formed under tropical
and sub tropical climatic condition.

10. Laterization
(Later, meaning brick)
Laterization is the process that removes silica from the upper
layers and thereby leaving sesquioxides to concentrate in the
solum (Bo). The process of laterization occurs mostly in warm
and humid (tropical) climate with 2000 to 2500 mm rainfall,
continues high temperature(~ 25
o
C) throughout the year and at
high altitude (700 to 900 meter from the mean sea level). The
cemented sesquioxides rich horizon become very hard when dried
like a brick. Such soils are called as laterite or latosols (Oxisols).

11. Oxic (Bo) : A sub surface horizon enriched with Fe-
and Al-oxide with dominance of 1:1 type clay
minerals and from where silica has leached.

12. Alfisols (red soils): Soils are rich in ferric oxide and
located in humid (> 2500 mm and sub humid climate
< 1500 mm).
Alfisols (Pedalfer) (argillic or natric horizon,
medium-high bases).

Characters:
Clay enriched B horizon
Soils are rich in ferric oxide
Medium to high base status

13. Spodosols : Soils with sub surface illuvial
accumulation of organic matter and
compounds of aluminum and usually iron.
These soils are formed in acid, namely coarse –
textured material in humid and mostly cool or
temperate climates.

14. Spodosols (Gr. Spodos, meaning wood ash) (wood
ash or grey coloured E horizon soils, spodic
horizon, forests, low bases)
Characters:
Sandy soils (quartz rich)
Bleached,wood-ash-coloured eluvial E-
horizon(white coloured)

15. Dark Coloured illuvial B horizon enriched with free
sesquioxide and humus
Base saturation is very low (acidic in nature)
Form in humid, cool climate

16. Histosols (organic soils): Histosols (G.Histos, meaning tissue)
(Peats and Mucks)
The central concept of Histosols is that of soils formed on
organic soil materials. As a thumb rule, a soil without
permafrost is classified as a Histosol, if half or more of the upper
80 cm is organic
1.They must contain a min. of 12 % organic carbon if soil doesn’t
contain clay
2. 18 % organic carbon if soil contain 60 % clay.
These soils usually form under tropical climate with prolonged
water logging or in the cold climate due to non microbial
decomposition of organic matter. These soils posses high water
retention characters, higher CEC and lower BD. Some of the
nutrients like Cu and K are deficient.

17. Types of soil acidity
There are three types of acidity found in acid soils
namely;
i)Active acidity
ii)Potential acidity
iii)Non-exchangeable acidity
Active acidity and potential acidity are the two types of
acidity found in acid soil besides non-exchangeable
acidity.

18. Active acidity: It is the concentration of H
+
ions in soil
solution.
Potential acidity : It is the exchangeable or adsorbed H
+

and Al
3+
ions present on the soil colloidal complex.
Non-exchangeable acidity: The H
+
and Al
3+
fixed in the
inter-lattices of clay minerals.
- all these three make up for the total acidity.

19. Ions responsible for soil acidity:
Two types of cations are largely responsible for soil
acidity. They are:
•H
+
and
•Al
3+
with its various hydrated forms (hydrated
aluminum cations).

20. Why acid soils have Aluminum ?
Process of soil acidification :
M – clay + H
+
soil solution H – clay + M soil solution
M – basic cations such as Ca, Mg, K, Na etc.,

21. In H- clay, the adsorbed H+ ion no longer remains on
the crystal exchangeable positions. The H
+
ion attacks
the octahedral or tetrahedral sheets of clay crystals,
preferably attacks the octahedral layer and releases Al
3+

ions. Thus, the spontaneous decomposition of H-clay
leads to destruction of crystal structure and release of
Al
3+
.

22. How does Al
3+
contribute to soil acidity ?
Aluminum hydroxyl ions, Al (OH)
2+
, Al (OH)
2
+
are the
products of hydrolysis of Al
3+
ions,which in turn are
subject to hydrolysis, releasing H
+
ions during the
reaction.

23. How does Al
3+
contribute to soil acidity?
Aluminum hydroxyl ions, Al (OH)
2+
, Al (OH)
2
+
are the products of hydrolysis of
Al
3+
ions, which in turn are subject to hydrolysis, releasing H
+
ions during the
reaction.
Al
3+
+ H
2
O Al (OH)
2+
+ H
+
( strongly acidic soil, pH < 4.7)
Al (OH)
2+
+ H
2
O Al (OH)
2
+
+ H
+
( moderately acidic soil, pH 4.7-6.5)
Al (OH)
2
+
+ H
2O Al (OH)
3
+
+ H
+
( neutral to alkaline soil, pH 6.5 -8.0)
__________________________________________________________________
_
Al
3+
+ 3H
2O Al (OH)
3
+
+ 3H
+

24. Thus, each mole of Al
3+
or hydrated Al cation on
hydrolysis or ionization gives rise to three moles of H+
ions in the soil solution. These released H+ ions
increase the soil acidity by lowering the soil pH.
Iron (Fe) has similar reaction and products in soil
solution, but pH values differ.
In case of acid soils of Kerala and HP, Fe is mainly
responsible for soil acidity.

25. Sl.No. Active acidity Potential acidity
1.The concentration or
activity of H+ ions in soil
solution
The exchangeable or adsorbed
H+ and Al ions on the
exchange / colloidal complex
2.Measured by using pH
meter
Measured by potentiometeric
titration
3.Expressed as pH Expressed in me/100 g soil
4.Plant growth is mostly
dependent on active
acidity
Plant growth is less dependent
on potential acidity
5.Magnitude of active
acidity is much less than
potential acidity
Magnitude of potential acidity
is much higher than active
acidity
Difference between active acidity and potential acidity

26. Active acidity and potential acidity are in equilibrium with each
other.
Exch. H
+
and Al
+3
on soil colloids H
+
and Al
3+
in soil solution
(potential acidity) (active acidity)

27. Factors responsible for Soil Acidity or Genesis or
Formation of Acid Soils :
1. Climate
2. Acid parent material
3. Carbon dioxide
4. Continuous application of acid forming / acid
containing fertilizers
5. H
+
ions released by plant roots
6. Acid rains
7. Crop removal of bases
8. Soils containing high sulfides (FeS
2
)

28. 1.Climate
Acid soils are mostly found in the areas of high rainfall.
In these regions, evaporation of water from the soil
surface is less than precipitation. Excess amount of
water moves downward in the soil and meets with
the ground water table. This downward moving
water carries a large amount of soluble salts from
the soil surface.

29. A reasonable amount of these salts is a must for
maintaining neutral reaction of surface soil.
In absence of these salts. Surface soil is, therefore,
rendered acidic in reaction.
Some compounds of iron and aluminium are resistant
to leaching and, therefore, left on the surface. These
compounds are acidic in nature and impart acidity to
the surface soil (eg. laterite soils).

30. 2. Acid parent material
Soil develops from some rocks (parent material) are
acidic in nature (e.g. Granite, Rhyolite (Igneous rocks).
Acid rock contains silica (SiO
2) and on reaction with
water it forms silicic acid. As a result, soil developed
from acid rocks becomes naturally acidic in nature.

31. Element Granite Basalt
0 48.7 46.3
Si 33.1 24.7
Al 7.6 10.0
Fe 2.4 6.5
Mn - 0.3
Mg 0.6 2.2
Ca 1.0 6.1
Na 2.1 3.0
K 4.4 0.8
H 0.1 0.1
P - 0.1
Total 100 100.1
Average chemical composition (%) of some typical rocks

32. 3. Carbon dioxide
The carbon dioxide evolved during organic matter
decomposition and root respiration dissolved in water
to from weak carbonic acid
CO
2 + H
2O H
2CO
3 (Carbonic acid)
H
2
CO
3
HCO
3
-
+ H
+

33. These acidified waters percolate through the soil and
cause soil acidity gradually. The percolating waters with
small amounts of H
+
ions continuously replace
solubilized basic cations such as Ca, Mg, K and Na.
The replaced cations are then leached from the root
zone.

34. 4. Continuous application of acid forming / acid
containing fertilizers
i)Ammonium containing fertilizers
Continuous application of acid forming fertilizers such
as ammonium sulphate, ammonium phosphate, DAP
etc. lead to soil acidity.
The ammonical fertilizers are oxidized by nitrobacteria
to from nitrate and hydrogen ions. For each mole of
NH
4
oxidized, 2 moles of H
+
are released resulting in
soil acidification.

35. Ca – clay + (NH
4
)
2
SO
4
NH
4
- clay + CaSO
4
(Leachable)

36. NH
4 + 2 O
2 NO
3 + 2 H
+
+ H
2O + energy

Nitrobacter

37. This relation applies to any source of ammonical
form of nitrogen including mineralization of NH
4

from organic matter. Some organic manures can
form acid because NH
4
is the first nitrogen product of
organic matter decomposition.

38. •Sulphur
Sulphur, an ingredient in some fungicides (CuSO
4
,
Bordeaux mixture) and fertilizers (CuSO
4
, FeSO
4
,
ZnSO
4
) oxidizes to form sulphuric acid which
dissociates into sulphate and hydrogen ions. Note that
sulphate itself is not acidic.

39. 2S + 3 O
2
+ 2H
2
O 2 H
2
SO
4
Thiobacillus thioxidans
Oxidizing bacteria

40. Thiobacillus thioxidans is S – oxidizing bacteria.
H
2
SO
4
2 H
+
+ SO
4
=

41. H
+
ions released by plant roots
Some H
+
ions are also released by plant roots as H
+
ions
are exchanged (H pumping mechanism) for other plant
nutrient cations in soil solution. Also the root exudates
which are mainly organic acids also contribute for H
+

ions in soil solution.

42. 6. Acid rains
Acid rain falls when air borne sulphur oxides (mostly
sulphur dioxide, SO
2
) and nitrogen oxides (NO) are
converted to sulphuric acid (H
2
SO
4
) and nitric acid
(HNO
3
), respectively through oxidation and dissolution
in rain drops.
Sulphur and nitrogen oxides in atmosphere get further
oxidized and dissolved in rain drops / water

43. 2 SO
2
+ O
2
2 SO
3
(sulphur trioxide) + 2 H
2
O 2 H
2
SO
4
(Sulphuric acid)

44. 2 NO + O
2
2 NO
2
(nitrogen dioxide) + H
2
O HNO
3
+ HNO
2

(HNO
3
, nitric acid ) ; HNO
2
, nitrous acid)

45. Such rain water may have a pH as low as 2.0. Acid rain
is formed because of industrial pollutants. If the pH of
the rain water is less than 5.7, it is classified as acid
rain. It is a major pollutant of soil and lakes.

46. 7. Removal of bases by crops
Crop removal of bases like Ca, Mg, K, etc. makes the
soil base poor i.e., more acidic. i.e., influx (inflow) of
cations and efflux (outflow) of H
+
ions in the plant
system.

47. 8. Soils containing high sulphides (FeS
2
)

48. OCCURRENCE
The soils and sediments most prone to becoming acid sulphate
soils formed within the last 10,000 years, after the last major
sea level rise.
When the sea level rose and inundated the land, sulphate in the
seawater mixed with land sediments containing iron oxides and
organic matter.
Under these
 
anaerobic 
conditions, 
lithotrophic 
bacteria such as 
Desulfovibrio desulfuricans 
obtain oxygen for respiration
through the reduction of sulphate ions in sea or groundwater,
producing sulfides and hydrogen sulfides.

49. This in turn reacts with dissolved ferrous iron, forming
very fine grained and highly reactive
 crystals of iron
sulfides such as (pyrite).
Up to a point, warmer temperatures are more
favorable conditions for these bacteria, creating a
greater potential for formation of iron sulfides.
Tropical waterlogged environments, may contain
higher levels of pyrite than those formed in more
temperate climates.

50. Soils containing high sulphides (pyrites, FeS
2
) become
strongly acidic when drained and aerated for
cultivation are called acid sulphate soils or cat clays.
Acid sulphate soils develop mostly in indundated lands
covered by salt water such as sea water (Ece=35 dS/m).
When these waters are drained, FeS
2
oxidizes (biological
oxidation) partly to sulphuric acid by Thiobacillus
ferroxidans bacteria. Acid sulphate soils have sulphuric
horizon with pH less than 4.0.

51. Biological O xidation (accelerated by bacteria at pH below 4.0)
2 FeS
2
+ 2 H
2
O + 7 O
2
2 FeSO
4
+ 2 H
2
SO
4
Iron sulphide Ferrous sulphate Sulphuric acid
Thiobacillus ferroxidans

52. Harmful effects of soil acidity
(Or Problems of acid soils)
Effect of Soil Acidity on Plants
Strongly acid soils are not productive for most crops.
On Strongly acid soils, the majority of crop plants
produce yields less than their potential for one or more of
the following reasons.

53. 1)Some plants simply do not grow well at low pH
i.e., the plants are not adopted to low pH
2) Elements such as Al, Mn and Fe become so
soluble that they are toxic to plant growth. Thus,
toxicity of Al, Mn and Fe is noticed in acid soils
3) Phosphorus and Molybdenum become insoluble
and unavailable to plants. Thus the deficiency of
P (pH = 6.5-7.5) M (pH = 6.5-8.5 ) is observed in
acid soils.

54. Example : Al
+++

+ H
2
PO
-
4
+ 2H
2
O 2H
+
+ Al (OH
2
) H
2
PO
-
4
(Insoluble)

55. 4) Bases such as Ca.Mg and K will be leached out and
deficiency of those nutrients is noticed
5) Nitrogen, Phosphorus and / or Sulphur deficiency is
observed because of very low decomposition
(oxidation) of organic matter due to reduced
microbial activity at low pH.
6) In addition to plant growth, the activities of the
following micro- organisms are greatly reduced in
acid soils

56. a)Nitrogen fixing bacteria (Nitrogenase Enzyme)
b) Nitrifying bacteria (bacteria that converts NH
+
4
to

NO
-
3
) (Nitrozomonas and Nitrobacter)
c) Various microorganisms that decompose organic
matter
Only fungus can survive in acid soil. Note that SO
4
=

itself is not harmful but harmful effects mentioned
above are noticed at higher concentration of H
+
ions
in soil

57. Reclamation of Acid Soils
Lime requirement (L.R) : Lime requirement refers to
the amount of liming material required to raise the pH
of an acid soil to a desired level under field conditions

58. Acidity in the soil is caused by excessive concentration
of hydrogen ions. Suppressing hydrogen ions by
applying lime is the best possible remedy of soil
acidity.
More than 90% of the lime used for this purpose in
our country is generally in the form of calcium
carbonate or lime stone (CaCO
3
). Besides, many other
liming materials are also used for the reclamation of
acid soils according to their local availability.

59. Acid soils can be reclaimed by amendments which
help in increasing pH and supply required cations.
Lime is the most commonly used, natural, cost effective
amendment added to acid soil for reclaiming them.
2 H – clay + CaCO
3 Ca- clay + H
2O + CO
2

60. Amount of lime required
Lime is added to mineral soils having pH < 6.5.
Amount of lime to be applied depends on :
1. pH of the soil : Lower the pH of the soil, higher will
be the quantity of the liming material required.
2. Buffer capacity of the soil : Higher the buffering
capacity of the soil, higher will be the quantity of the
liming material required.

61. 3.Fineness of the liming material : For quick results liming
material should be properly ground before application to the
field.
4. Application of coarse material may mislead us even if applied
in appropriate amount coarse particles take unusually long
time to dissolve in water and interact with hydrogen ions in
the soil and, therefore, quick response in terms of increased
Soil pH is not achieved.
Particles of the size which can pass through 80- 100 mesh
sieve (a sieve having 80-100 holes per square centimeter area)
are quite fine and show hundred per cent efficiency.

62. 5. Texture of soil : Fine textured soils require more
time than coarse textured soils for Reclamation.
6. Organic matter : Higher organic matter in soils more
will be buffering capacity hence higher the lime
requirement for reclamation.

63. Application of lime
Lime is less mobile in the soil. Therefore, it should be spread in
the field with maximum possible uniformity and worked well
into the soil.
This can be done during the preparation of field. Soil should be
sufficiently Moist at the time of liming or a light irrigation may
be given to the field after mixing the lime.
If slaked lime is used, seeds should be sown in the field at least
3-4 weeks after liming.
To maintain desirable soil reaction in the humid regions,
liming
at every 3-5 years interval is recommended.

64. Mobility of nutrients in
soil
Mobile Less mobile Immobile
Nitrate
(mass flow)
Ammonia
( mass flow)
Phosphorus
(diffusion)
Sulphur
(mass flow)
Potassium
(diffusion)
Zinc (diffusion)
Boron
(mass flow)
Calcium (root
contact)
Chloride
(mass flow)
Magnesium
(root cot act)
Manganese
(diffusion)
Copper
(diffusion)

65. Liming materials:
The common liming materials used to reclaim acid soils are;
1. Quick lime (CaO) / Burnt lime / Oxide of lime / Calcium
oxide.
2. Slaked lime ((Ca (OH)
2
) / Hydrated lime / Hydroxide of
lime
3. Calcic lime stone (CaCO
3
) / Calcite / Agricultural lime /
Carbonate of lime / Ground lime stone.

4. Dolomitic lime stone (CaMg (CO
3
)
2
) / Dolomite

66. 5. Others : Industrial wastes like steel mill slag, blast
furnace slag, lime sludge from paper mills etc.
Calcite and dolomite react slowly with soil colloids
whereas burnt lime and hydrated lime react faster
and bring about changes in soil pH within few days.

67. Reactions of liming material in acid soils:
The changes which lime undergoes after it is added to acid soil are;
1.Reaction with CO
2
When liming materials like oxide, hydroxide or carbonates of Ca are applied to
acid soil, they get converted into bicarbonate form. This is because, CO
2
partial
pressure (0. 50 %) in soil is usually several hundred times greater than that in the
atmosphere air (0.035 %) which causes the formation of bicarbonates.
  Converted to bicarbonate form
1. with quick lime : CaO + H
2O + 2CO
2 Ca (HCO
3)
2
 
2. with slaked lime : Ca (OH)
2 + 2CO
2 Ca (HCO
3)
2
3. with carbonate lime:CaCO
3 + H
2O + CO
2 Ca (HCO
3)
2

68. 2. Reaction with soil colloids
All liming materials react with acid soils. The Ca and Mg replace H
+
ion on
the exchange complex. Both Ca (OH)
2
and CaCO
3
can react directly with acid
soils or indirectly by the formation of Ca (HCO
3
)
2
.
2 H-clay + Ca (OH)
2 Ca- clay + 2H
2O
2 H-clay + CaCO
3
Ca- clay + H
2
O + CO
2

 

2 H-clay + Ca (HCO
3
)
2
Ca- clay + 2H
2
O + 2 CO
2
On application of lime ;
CO
2 is evolved (in case of carbonate sources)
Adsorption of Ca, Mg and K increases the percentage of base saturation of
colloidal complex and hence increases the soil pH.

69. Estimation of lime requirement of acid soil based on exchangeable acidity
Problem: Calculate the amount of pure agricultural lime (CaCO3) required to
replace 2.5 me H
+
/ 100 g soil.
Step 1. To calculate amount of CaCO
3
required per 100 g soil
To replace 1 me of H
+
/ 100 g of soil = 1 me of CaCO
3
/ 100 g soil is required
= 1 mg equivalent of CaCO
3
/ 100 g soil
= molecular weight of CaCO
3
(mg) / 100 g soil
Valency of calcium
 
= (100 / 2 ) mg CaCO
3 / 100 g soil
= 50 mg CaCO
3
/ 100 g soil.

70. Step 2. To calculate amount (kg) of CaCO
3
required per ha of soil

71. Advantages of liming
Advantages of liming are three fold viz. physical,
chemical and biological. They are described below.

72. 1. Physical effects
Liming improves physical condition of heavy soils;
they become granular in structure and their water
holding capacity is improved. Liming also encourage
the decomposition of organic matter and consequently,
there is greater production of organic colloids.
The Ca-humus so produced is believed to be an
effective cementing agent in binding the soil particles.
Liming also prevents soil erosion because soils which
have received liming treatment support good plant
growth.

73. 2. Chemical effects
Among the principal chemical properties
influenced by liming is the reduction of H ions in the
colloidal complex. It increases the availability of
almost all the nutrients such as nitrogen,
phosphorus, potassium, calcium, magnesium, boron,
zinc, copper and molybdenum, and reduces the
toxicity caused by soluble iron, aluminium and
manganese.

74. 3. Biological effects
One of the outstanding biological effects of liming is
to encourage the microbial activity of the soil. By
raising the soil pH, it make the soil more congenial for
a number of micro organisms. Nitrifying and nitrogen
fixing bacteria, both symbiotic and non symbiotic are
stimulated by the addition of lime to an acid soil. Lime
also brings about a more rapid decomposition of
organic manure, both native and added, as a result of
improved microbial activity. This further increases the
availability of nitrogen phosphorus and sulphur.

75. Adverse effects or over liming
Lime may have adverse effects especially if the soil is
over limed. If lime is added to an acid soil until the pH
goes above 7, it produces certain deleterious effects on
the crop.
The harmful effect is more pronounced on soils that are
not well buffered such as sandy soils and soils poor in
organic manure.

76. The main effect of over liming is to reduce the
availability of some of the essential nutrients, both
major and minor, such as iron, manganese, boron,
copper, zinc, phosphorus, potassium, etc, and thus
bring about nutritional deficiency.

77. Excess lime also interferes with the absorption of
certain elements like potassium, phosphorus, boron,
etc. by plants thus hindering their utilization.

The very rapid decomposition of organic matter in
soils of arid and semi arid regions is also attributed to
the accumulation of excess lime in these soils

78. Element affected
Element
acting
B Ca Cu Fe K Mg Mn Mo N Na P S Zn
B / - - - - - - - - - A - -
Ca 1/ A / - A A A A - - A A - A
Cu - E / A - - A A - - A - A
Fe - - A / A - A - - - A - -
K A 2/ A - E / A E A A A - - -
Mg 1/ - A A A A / A E - A E E A
Mn - - - A - - / A - - - - -
Indicative summary of principal plant-nutrient
interactions

79. Element affected
Element
acting
B Ca Cu Fe K Mg Mn Mo N Na P S Zn
Mo - - A A - - A / - - - A -
N A E A - A E - E / - E - A
Na - A - - A A - - - / A - A
P - A A A A E A E A - / E A
S (as SO
4
2-
) - - - - - - - A 3 / - - E / -
Zn - - A A - A A - - - A A /
AA = Antagonises action, E= Antagonises action, E= Enhances action, - = Insufficient data or effect too variable to summarise simply= Enhances action, - = Insufficient data or effect too variable to summarise simply

80. Note : 1. Ca or Mg as carbonates may enhance action of other elements in Note : 1. Ca or Mg as carbonates may enhance action of other elements in
acidic soil by raising pH.acidic soil by raising pH.
2.2. With high B contents, high K levels can enhance B toxicity, With high B contents, high K levels can enhance B toxicity,
depending on conditions, and may reflect changes in the Ca: B ratiodepending on conditions, and may reflect changes in the Ca: B ratio
3.3. But note that the acidifying effect of gypsum, by decreasing CO But note that the acidifying effect of gypsum, by decreasing CO
33
2-2-

and OHand OH
--
, may increase Mo uptake. S as sulphur has acidifying effect , may increase Mo uptake. S as sulphur has acidifying effect
and can enhance uptake of other elements, sometimes to toxic levelsand can enhance uptake of other elements, sometimes to toxic levels
notably of A1 and Mn)notably of A1 and Mn)

81. VII. Crops Tolerant to Acidity
Though most plant species of agricultural importance
can grow satisfactorily in culture solutions having a wide
range of pH, their behavior in soil is very different.
Most crop plants grow well when the soil pH is
between 5.5 and 7.5, a fairly wide range even under field
condition. Serious trouble develops when the pH drops
below 5.0.
A large proportion of crop plants can tolerate slight to
moderate acidity, a few can tolerate even a fairly strong
acidity. Crops which can be successfully grown in acid
soils are listed in the following table.

82. Sl.
No.
Slightly
tolerant
(pH- 6.6- 6.1)
Medium
tolerant
(pH- 6.0- 5.6)
Very tolerant
(pH- 5.5- 5.1)
1. Sugarcane Maize Mustard
2. CauliflowerPotato Coffee
3. French beanWheat Rubber
4. Cabbage Soya bean Tea
5. WatermelonBarley
6. Lucerne Oats --
7.
--
Rice --
Crops Tolerant to Acidity