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, CaCO 3 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 Fe 2 O 3 +3H 2 O 2 Fe 2 O 3 H 2 O Hematite (red)Limonite (Yellow)

Carbonation Hydrogen ion, carbonic acid, HNO 3, H2SO 4 CaCO 3 + H 2 CO 3 Ca(HCO 3 ) 2 Calcite Carbonic Acid Soluble Bicarbonate Oxidation Fe ++ Fe +++ 4FeO+O 2 2Fe 2 O 3 Ferrous Oxide Hematite

Solution Dissolved CO 2 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 – Texture & structure of soil Chemical & mineralogical composition of soil Topography of the area Time of soil formation

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