Parent Materials

Today we will know about the parent materials.

Parent material is the underlying geological material in which soil horizons form.
The nature of the parent material profoundly influences soil characteristics.
For example, a soil might inherit a sandy texture from a coarse-grained, quartz rich parent material such as granite or sandstone.

The chemical and mineralogical composition of parent material also influences both chemical weathering and the natural vegetation.
For example, the presence of limestone in parent material will slow the development of acidity that typically occurs in humid regions. Besides trees growing in limestone materials produce leaf litter that is relatively high in calcium.

Parent material also influences the quantity and type of clay minerals present in the soil profile.

Classification of Parent Materials:

Classes of parent material are based on placement and they are:

1. Residual: Residual parent material develops in place from weathering of underlying rock. Where the climate is warm and humid, residual parent materials are typically thoroughly leached and oxidized and show the red and yellow colors of various oxidized iron compound. The types of rocks that strongly influence the types of soils are: Limestone = clayey soil, Sandstone = coarse, acidic soil, Granite = coarse, acidic soil, Slate, Shale = clayey soil.
2. Transported: There are six types of transported materials. Here they are:

Colluvial Debris: Colluvial debris is made up of poorly sorted rock fragments detached from the heights above and carried downslope, mostly by gravity, assisted in some cases by frost action. These are usually rocky and stony. Physical weathering processes dominate relative to chemical weathering processes. These are well drained but unstable.

Alluvial Deposits: There are three general classes of alluvial deposits: floodplains, alluvial fans and delta deposits.

Floodplains: During flooding, water spreads and slows and fine sediment is deposited. The coarser materials being laid down near the river channel where the water is deeper and flowing with more turbulence and energy.
Horizontal and vertical stratification is seen. Terraces are old floodplains above the current floodplain.
This soils are very fertile and important for agriculture, forestry and wild life. It is a poor choice for urban development. To some degree, nutrient rich materials lost by upland soils are deposited in the river floodplain and delta.

Alluvial Fans: These are usually gravelly or stony in mountainous regions and can have finer materials as well. Stream leaver narrow upland channel which descends to broad valley below.
The soils derived from this debris often prove very productive although they may be quite coarse textured.

Delta debris: Much of the finer sediment carried by streams is not deposited in the floodplain. It is discharged into the lake, reservoir or ocean. Some of the suspended material settles near the mouth of the river forming a delta.
A delta often is a continuation of a floodplain. It is clayey in nature and it is likely to be poorly drained as well.
Delta marshes are among the most extensive and biologically important for wetland habitats

That's all from this post. We have learned 2 types of transported materials among the six.

Next we will learn about the other 4 materials.

Thank you.

Reference:

Brady, N.C. and Weil, R.R., The nature and properties of soil. 13th Edition.