Download Soil origin and nature, formation of soils

SOIL ORIGIN AND NATURE,
FORMATION OF SOILS
•
Soil develops from parent material by the processes of soil formation
•
The process of formation soil from the hard rock (eg. granite) are divided into two
stages
– Rock weathering
– Soil formation
•
The mineral matter inherited form rocks are referred to as soil parent material
•
The principle parent materials of organic soils are formed due to decomposing of
plant materials
•
The most important properties of parent’s materials are texture and mineral
composition
•
Rocks - formed by the cooling of a molton mass called magma
•
The most common igneous rocks are basalt and granite
•
Basalt rock - most common lava rock which is the principle hard rock underlying
the ocean basins
•
Basalt rock due to weathering produces a large percentage of clay and less
percentage of stone
•
Granite rock - weathers to coarse grained rocks to produce much sandier soil
•
Granite usually produces a deeper soil than basalt
•
Igneous rocks – source - 10% of the earth soil area
•
The remaining 90% of the land area has a mantle of sedimentary or metamorphic
rocks
SEDIMENTARY ROCKS
•
Are formed by the cementation of iron of materials deposited by wind, water, ice
or gravity
•
Cementation is by iron, aluminum, silicon, CaCO3 etc)
•
Sedimentary rocks are shale, limestone, quartz stones etc
METAMORPHIC ROCKS
•
Are formed under heat and pressure
•
Sandstone can change to quartzite, shale to slate and limestone to marble
SOIL FORMATION
•
Weathering - responsible for the formation of the regolith and in turn the soil
•
Weathering - combination of destruction and synthesis
•
Simultaneously rock fragments and the minerals therein are attached by
weathering forces
•
These changes are accompanied by a continued decrease in particle size and by
release of soluble constituents.
PROCESS OF WEATHERING
• Mechanical disintegration
• Chemical decomposition
I. Mechanical
a. Temperature : differential expansions of minerals, frost betion and exfoliation.
b. Erosion and deposition – by water ice wind
c. Plant and animal influences
II. Chemical
a. Hydrolysis
b. Hydration
c. Carbonation and related acidic processes
d. Oxidation
e. Solution
MECHANICAL FORCES OF WEATHERING
Temperature
•
Sudden or wide variations of temperature influence the disintegration of rocks
•
Warming and cooling process are effective in disintegration of parent materials
• Freezing of water
It widens the cracks and dislodges rocks to fragments
• Water
Water has a tremendous cutting power in the valleys of rivers
• Ice
Ice is an erosive and transporting agency which disintegrates rocks
• Wind
Wind always had been important transporting agent for dust storms
• Plants
Mosses and lichens produces organic materials - help in disintegration and the growth
of roots in the rocks crevices and thus disintegration of rock
CHEMICAL PROCESS OF WEATHERING – DECOMPOSITION
• Hydrolysis
It is a decomposition reaction especially in case of feldspars & mica
• Hydration
It is the processes of attachment of H+ and OH- ions to the compounds
2 Fe2O3+3H2O
Hematite (red)
2 Fe2O3 H2O
Limonite (Yellow)
•
Carbonation
Hydrogen ion, carbonic acid, HNO3, H2SO4
CaCO3
Calcite
•
+
H2CO3
Carbonic Acid
Ca(HCO3)2
Soluble Bicarbonate
Oxidation
Fe++
Fe+++
4FeO+O2
Ferrous Oxide
2Fe2O3
Hematite
Solution
Dissolved CO2 and H+ ions
Fig. : How various kinds of parent’s material are formed transported & deposited
SOIL PARENT MATERIALS OR WEATHERED SOILS
– Residual parent material
– Alluvial debris
– Alluvial stream deposits
– Marine sediments
– Lacustrine
– Glacial
– Eolian (Sand size) silt size- Transported by wind
FACTORS INFLUENCING SOIL FORMATION
•
Climatic condition
-
particularly temperature and precipitation
•
Living organism
-
especially native vegetation
•
Nature of parent material
•
Chemical & mineralogical composition of soil
•
Topography of the area
•
Time of soil formation
–
Texture & structure of soil
SOIL PROFILE
• Horizontal distribution of soil layers are called soil horizons
•
The vertical sequence of soil horizons are termed as soil profile
•
Soil forming processes are grouped under four heads-O,A, Band C
Consolidated bed rock.
•
O Group: The organic horizons which lie above the mineral soil
•
A group: The mineral horizons which are at or near the surface characterized by
maximum leaching
•
B Group: The layer in which deposition from above or even below
•
It is the region of maximum accumulation of materials such as oxides of iron and
aluminum and silicate clays
•
These materials may have washed downward from the surface layers or they have
formed in the B horizon
•
In the arid region calcium carbonate and calcium sulphate may be accumulated
•
A & B horizon together is called the solum
•
C- Horizon: It is the unconsolidated material underlying the solution. Least
weathered accumulation of Ca , Mg carbonate, cementation, some times high bulk
density fragipans
•
R-Horizon: Consolidated bed rock. It may or may not be like the parent rock from
which the solution is formed