This document discusses the origin and formation of soils. It states that soil develops from parent material through the processes of weathering and soil formation. The parent material can be igneous rocks like basalt and granite, sedimentary rocks like shale and limestone, or metamorphic rocks. Weathering breaks down rocks through mechanical and chemical processes like freezing, erosion, hydrolysis, and oxidation. Over time, weathered material accumulates and soil horizons like the A, B, and C horizons develop, forming a soil profile. The type of parent material and environmental factors like climate and organisms influence the properties of the resulting soil.

1. SOIL ORIGIN AND NATURE,
FORMATION OF SOILS

2. • 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

3. • 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

4. • 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

5. • 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

6. 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

7. METAMORPHIC ROCKS
• Are formed under heat and pressure
• Sandstone can change to quartzite, shale to slate and limestone to marble

8. 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.

9. 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

10. II. Chemical
a. Hydrolysis
b. Hydration
c. Carbonation and related acidic processes
d. Oxidation
e. Solution

11. 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

12. • 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

13. 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 2 Fe2O3 H2O
Hematite (red) Limonite (Yellow)

14. • Carbonation
Hydrogen ion, carbonic acid, HNO3, H2SO4
CaCO3 + H2CO3 Ca(HCO3)2
Calcite Carbonic Acid Soluble Bicarbonate
• Oxidation
Fe++ Fe+++
4FeO+O2 2Fe2O3
Ferrous Oxide Hematite

15. Solution
Dissolved CO2 and H+ ions
Fig. : How various kinds of parent’s material are formed transported & deposited

16. 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

17. FACTORS INFLUENCING SOIL FORMATION
• Climatic condition - particularly temperature and precipitation
• Living organism - especially native vegetation
• Nature of parent material – Texture & structure of soil
• Chemical & mineralogical composition of soil
• Topography of the area
• Time of soil formation

18. 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.

19. • 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

20. • 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