Soil, Soil profile, pedon, polypedon, soil forming factors, soil forming processes, USDA soil classification, soil orders, soils of India

1. PRIYA P. GURAV
Scientist
(Division of Soil Chemistry and Fertility)
ICAR – Indian Institute of Soil Science,
Bhopal
Soil its origin, classification
and distribution

2. What is soil?
• Soil can be defined as the organic
and inorganic materials on the
surface of the earth that provide the
medium for plant growth

3. SOIL ORIGIN and DEVELOPMENT
• SOILS are a SLOWLY RENEWABLE resource
• It takes 100’s to 1000’s of YEARS for SOILS to
DEVELOP…
…but they do GROW, CHANGE, and DEVELOP over
TIME

4. PEDON
• Soil individuals are
characterized in terms of an
imaginary three dimensional
unit called Pedon
• A Pedon (1 to 10 m2) is the
smallest sampling unit that
displays the full range of
properties that characterize a
particular soil
• The Pedon serves as the
fundamental unit of soil
classification

5. PEDON- POLYPEDON-LANDSCAPEMODEL
• A soil unit in a landscape
consists of a group of very
similar pedons (polypedon)
• A polypedon is a soil
individual
• Soil individuals that have in
common a suite of soil profile
properties and horizons that
fall within a particular range
are said to belong to the same
soil series.

6. COMPOSITIONOF SOIL

7. ROCKS: FORMATION NATUREANDCLASSIFICATION
• A rock may be defined as a hard mass of mineral matter
comprising of two or more rock-forming minerals.
 Petrography that deals with the description of rocks
 Petrogenesis that deals with the study of the origin of rocks.

8. FORMATIONOF ROCKS
 Cooling and Consolidation of Magma: Rocks, formed by cooling and
consolidation of molten magma within or on the surface of the earth, are termed as
primary rocks, e.g. igneous rocks.
 Transportation and cementation of fragmentary material: Disintegration and
decomposition lead to the breaking down of pre-existing rocks. The resulting
fragmentary material is either compacted in situ or transported in solution by the
natural agencies of wind, water or ice to low-lying areas, like oceans. Consolidation
of these materials after their deposition results in the formation of rocks, called
sedimentary or secondary rocks.
 Alteration or Pre-existing rocks: The primary and secondary rocks, when
subjected to earth’s movement and to high temperature and pressure, are partially
or wholly reconstituted or altered to form new rocks, called metamorphic rocks.

10. IGNEOUS ROCK
Created by the COOLING and SOLIDIFICATION of
MOLTEN ROCK
LAVA (extrusive) & MAGMA (intrusive)

11. SEDIMENTARY ROCK
Deposits of MUD and SAND (through wind and water erosion)
that are CEMENTED together by CHEMICALS or PRESSURE

12. METAMORPHICROCK
Formed when either IGNEOUS or SEDIMENTARY ROCK is
subjected to GREAT HEAT & PRESSURE

13. SOIL FORMATION
• SOIL is formed from the WEATHERING of
ROCK
PHYSICAL WEATHERING
• FRACTURING due to expansion and
contraction
• Frost Wedging – freezing and
thawing of water
• Heating and cooling

14. PHYSICAL WEATHERING
• EROSION or wearing down of rock
• Water
• Wind
SOIL FORMATION

15. CHEMICAL WEATHERING
• DISSOLUTION:Water DISSOLVES
minerals into SOLUTION
• HYDROLYSIS – Water reacts with
minerals in rock create NEW,
SOFTER COMPOUNDS
• OXIDATION – O2 reacts with
minerals in rock to form NEW
COMPOUNDS
SOIL FORMATION

16. BIOLOGICAL WEATHERING
• PLANTS ….can also PHYSICALLY and
CHEMICALLY WEATHER ROCK
• ROOT WEDGING – Plant roots
PHSICALLY force open cracks by ROOT
GROWTH
• LICHENS – exude acid which
CHEMICALLY WEATHERS rock
SOIL FORMATION

17. 17
Time
Soil
Parent material

18. Factors Influencing Soil Formation
Brady and Weil, 2004
 Parent material- (geological [~inorganic] and organic precursor)
 Climate- (temperature, precipitation)
 Biota- (vegetation, microorganisms, earthworms, humans)
 Topography- (slope, aspect, landscape position)
 Time- (period since formation began)

19. SOIL FORMINGPROCESSES
Additions: Materials added to the
soil, such as decomposing vegetation
and organisms, or new mineral materials
deposited by wind or water.
Losses: leaching, erosion, uptake by
plants
Transformations: The chemical
weathering of sand and formation of
clay minerals, transformation of coarse
OM into decay resistant organic
compounds (humus)
Translocations: Movement of soil
constituents (organic or mineral) within
the profile and/or between horizons.

20. SPECIFICSOILFORMINGPROCESSES
• Calcification: it is the process of precipitation and accumulation of calcium carbonate in some part
of the profile. The accumulation of calcium carbonate may result in the development of calcic
horizon.
• Decalcification: it is the reverse of calcification that is the process of removal of calcium carbonate
ions from the soil by leaching.
• Podzolization: it is a process of soil formation resulting in the formation of podzols and podzolic
soils. Podzolization is the negative of calcification.
• Laterization: it is the process that removes silica, instead of sesquioxides, from upper layers and
thereby leaving sesquioxides to concentrate the solum.
• Gleization: it is the process of soil formation resulting in the development of a glei in the lower part
of the soil profile above the parent material due to poor drainage condition and where waterlogged
conditions prevail.
• Salinization: it is the process of accumulation of salts, such as sulphates, chlorides or calcium,
magnesium, sodium and potassium in soils in the form of salty horizon.
• Desalinization: it is the removal, by leaching of excess soluble salts from the soil profile by ponding
water and improving the drainage conditions by installing artificial network.
• Alkalization: the process involves the accumulation of sodium ions on the exchange complex of the
clay, resulting in the formation of Sodic soils.
• Dealkalization: the process refers to the removal of sodium ion from the exchange sites. The
process involves dispersion of clay.
• Pedoturbation: it is the process of mixing of soil. Mixing, to a certain extent, takes place in all soils.

21. WHYCLASSIFYSOILS
Whenever we call things by group
names, or give them labels that inform us
of their important properties, we are doing
classification.
The names or labels could provide
information about their appearance, make
up, functions or their other known
important characteristics

22. Classification
Family
Subgroup
Great group
Suborder
Order

23. Order
Suborder
Great group
Sub group
Family
Series
12
500+ in India
63
240+
1000+
8000
Kingdom
Phylum
Class
Order
Family
Genus
Species
Soil Taxonomy Hierarchy

25. The Twelve USDA Soil Orders
Entisols
Inceptisols
Mollisols
Alfisols
Spodosols
Histosols
Andisols
Oxisols
Ultisols
Gelisols
Vertisols
Aridisols

26. Gelisols
Soils with permafrost or
cryoturbation (mixing from
frost heaving)
Cold or frozen much of the year
Can be organic or inorganic soils
9% worldwide

27. Entisols
16% Worldwide
Young, weakly developed soil
Typically NO B horizon
(in some cases, can have a very
coarse-textured, poorly
formed Bw)
Typical profiles: C AC OC
OAC
Common on ridgetops, recent
deposits (rivers, beaches, ash…),
urban areas

28. Inceptisols
Relatively young soils – have B
horizons
More developed than Entisols but
less developed than any other
soil order
(soil doesn’t meet the criteria
of other orders)
10% Worldwide

29. Andisols
Developed from volcanic material
A horizons that are rich in Al-humic
materials
Secondary minerals are predominately
amorphous
Range in development from moderate
to well-developed
<1% Worldwide

30. Aridisols
Dry soils; arid to semi-arid
environments
Typically less plant growth, A
horizon with less OM
Can have many types of B
horizons, but Bk’s are
common
12% Worldwide

31. Mollisols
Have a ‘mollic’ A horizon (>50% Base
Sat., with good physical properties)
Usually under grasslands
VERY fertile soils
Profiles may be: A B_ C
A C
or even A E B_ C
7% Worldwide

32. Alfisols
Has a Bt horizon
Moderate Base Sat. (< 50%
and > 35%)
Lower fertility than Mollisols,
but still excellent soil
Common under deciduous
forests, grasslands
A, E, Bt1, Bt2, C
A, Bt1, Bt2, BC
10% Worldwide

33. Ultisols
Old, highly weathered soil, acidic
Low Base Sat. (< 35%)
Has a Bt horizon
Lower fertility than Alfisols
Common under forests
Develop in wetter temperate areas or
tropical areas with lots of leaching
O, A, Bt1, Bt2, Bt3, C
9% Worldwide

34. Oxisols
Most highly weathered, old soils
Laterization – has an oxic B horizon
Oxides prevalent, kaolinite common
A horizon low in organic matter
Common in tropical areas with high
rainfall, stable landscape
8% Worldwide

35. Spodosols
Has a Spodic horizon (Bs, Bhs)
Illuvial accumulation of Fe, Al, O.M.
Usually has an E horizon
Acidic, lower fertility
Found in cooler, temperate regions,
areas of high
leaching, coarse textured soil
O, E, Bhs, Bs, C
3% Worldwide

36. Vertisols
High content of shrink/swell clay in
the upper 1m of soil
Montmorillonite
Soil expands and contracts
Cracks open in dry season, material falls
in and cracks shut in wet season—
mixes soil
Subhumid to semi-arid areas
Deep A horizon
2% Worldwide

37. Histosols
Organic Soil (little mineral material)
O horizons
Found in waterlogged areas of any type
of climate: Wetlands
When drained, can be fertile but
difficult to manage due to poor physical
properties
Oa, Oe, Oi
1% Worldwide

38. DISTRIBUTIONOF SOIL