Parent Material of Soil

Unconsolidated product of weathering from which soil develops is called parent material.

Generally there are two groups of parent materials, i.e. sedentary and transported. The transported materials are further subdivided according to transporting agency and place of deposition.

Sedentary or Residual

                                           The parent material formed in place and still at the original site.

Transported

                      The parent material carried and deposited by various transporting agencies.

By gravity- Colluvial

By water- Alluvial

By ice- Glacial

By wind- Eolian

The nature and properties of parent material together with climate are the most important factors affecting the kind and quality of soils. So to understand soils we must know the source of material, mechanisms of weathering and transporting agencies.

Sedentary or Residual Parent Material

                                                                 This material develops in place from the rocks, generally by long and intense weathering. In warm humid climate, it is well oxidized and leached of basic cations (Na, K and Ca). It has red and yellow colours due to oxidation of iron. In cooler and drier climates, weathering is less intense; leaching of basic cations does not take place. The nature of these materials depends upon the nature of rocks from which these materials have formed.

Transported Materials

Colluvial deposits

                                 A material transported by gravity that has been deposited at the base of foothills or mountains. These deposits are extremely variable in composition. Soils developed from these parent materials may be coarse and stony because physical rather than chemical weathering is dominant. This material has good drainage.

Alluvial deposits

                                                  The material which has been transported/deposited by river sand streams is termed as alluvium.

Alluvial soils are finely layered to great depths, which show marked changes horizontally. This is somewhat sandy near river bank to clayey away from river bank. These soils are generally level, fertile and highly productive, however if clayey there may be drainage problem.

Glacial deposits 

                                  This refers to all the material of glacial origin, whether deposited directly by associated water. It consists of heterogeneous mixture of debris, varying in size from coarse fragments to clay particles.

Eolian deposits

                                                These are material carried and deposited by wind. Such wind-blown materials are differentiated into loess, sand dunes, adobe and volcanic ash depending upon the type of original material that was subjected to such actions.

Loess

          It is generally silty in nature with some sand and clay.  Loess generally forms productive soils and these soils are quiet open and porous.

Adobe

          It is similar to loess but calcareous in nature.