Soil & soil forming materials

June 8, 2012
By Krishiworld

SOIL may be defined as a thin layer of earth’s crust which serves as a natural medium for growth of plants. It is the unconsolidated mineral matter that has been subjected to, and influenced by, genetic and environmental factors– parent material, climate, organisms and topography all acting over a period of time. Soil differs from the parent material in the morphological, physical , chemical and biological properties. Also, soils differ among themselves in some or all the properties, depending on the differences in the genetic and environmental factors.

Thus some soils are red, some are black; some are deep and some are shallow; some are coarse textured and some are fine-textured. They serve as a reservoir of nutrients and water for crops, provide mechanical anchorage and favourable tilth. The components of soil are mineral matter, organic matter, water and air, the proportions of which vary and which together form a system for plant growth; hence the need to study the soils in perspective.


Rocks are the chief sources for the parent materials over which soils are developed. There are three main kinds of rocks:
(i)igneous rocks,
(ii)sedimentary rocks, and
(iii)metamorphic rocks.

Igneous rocks. They are formed by the cooling, hardening and crystallizing of various kinds of lavas and differ widely in their chemical composition. They chiefly contain feldspars, maphic minerals and quartz. Rocks containing a high proportion of quartz (60-75%) are classified as acidic, whereas those containing less than 50% quartz are classified as basic. The common igneous rocks found in India are the granites(acidic) and basalts or the Deccan Trap (basic)

Sedimentary rocks. They are derived from igneous rocks and are formed by the consolidation of fragmentary rock materials and the products of their decomposition deposited by water. The common sedimentary rocks are conglomerate, sandstone, shale and limestone. Alluvial, glacial and aeolian deposits form the unconsolidated sedimentary rocks.

Metamorphic rocks. They are formed from the igneous or sedimentary rocks by the action of intense heat and high pressure or both resulting in considerable change in the texture and mineral composition. The common metamorphic rocks are gneis from granite, quartzite from quartz or sandstone, marble from limestone and slate from shale.

The rocks vary greatly in chemical composition. Table1 gives the average composition of four different kinds of rocks

Percentage Chemical Composition of Rocks

Oxides Igneous rocks Shales Sandstones Limestones
SiO2 59.07 58.90 78.64 5.20
Al2O3 15.22 15.63 4.77 0.81
Fe2O3 3.10 4.07 1.08 0.54
MgO 3.45 2.47 1.17 7.92
FeO 3.71 2.48 0.32 ..
CaO 5.10 3.15 5.51 42.74
Na2O 3.71 1.32 0.45 0.05
K2O 3.11 3.28 1.32 0.33
O2 .. 2.67 5.03 41.70
P2O5 0.30 0.17 .08 0.04
MnO 0.11 .. .. ..
TiO3 1.03 0.66 0.25 0.06
H2O 1.30 3.72 1.33 0.56
Miscellaneous 0.79 1.48 0.07 0.05


Weathering refers to the physical and chemical disintegration and decomposition of rocks which are not under equilibrium under temperature, pressure and moisture conditions on the earth’s surface. In the beginning, weathering precedes soil formation, more so in hard rocks. In other words, weathering creates the parent material over which soil formation takes place. Later, weathering, soil formation and development proceed simultaneously. The weathering may be physical or chemical.

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