A soil develops from all kinds of rocks, deposits in lakes, rivers, and glacials, and wind-blown silt or sand which are all referred to as parent material. The interactions of the soil forming factors, namely, parent material, climate, topography, organisms, and time control the physical and chemical processes that change the parent materials into the soil.

The main purpose of this unit is to acquaint the student that soil is formed from different types of rocks by certain pedogenetic processes through some factors of soil formation.

The formation of soil from parent material is a slow but continuous process. The variability in the characteristics of soils such as texture, color, structure, porosity, and chemical and microbiological properties, usually result from the actions of soil-forming processes.

Importance of the Knowledge of Soil Origin and Development

In everyday language, Soil genesis is the study of the history of the soil. i.e. what has been going on in the soil that gave it the properties which it possesses today.

As we understand the history of our soils and their properties, we can make wise and better decisions concerning their use and management. Bearing this in mind, the reasons behind the quest for soil genesis include:

To help us point out some of the properties of our soils.

To help us point out some of the things we do not understand about the soils.
To be able to formulate law governing soils behavior
To help us provide an adequate method for management of our soils and make better interpretative usage.

Types of Rocks

The parent rock is the starting material of soils. Rock is defined as a consolidated mass composed of one or more minerals. A set of processes and factors act on the rock to produce soil.

Rocks are residual or in-place parent material that weathers into the soil without first being moved by wind or water. Rocks are divided into three main groups (igneous, sedimentary, and metamorphic) which are further subdivided by mineralogy, particle size, crystallinity, and specific mode of formation. The table below gives a summary of rock types.

Table: Major Rock Types, their Origin, and Properties

Rock type	Origin	Example	Properties
Igneous	Cooling of magma	Granite Basalt	Light colour coarse grained. Dark colour fine grained
Sedimentary	Deposition and	Shale Sandstone	Any colour, fined grained. Any colour,

	compaction	Limestone	coarse grained Light coloured, shells or CaCO₃ present
Metamorphic	Change in igneous or sedimentary	Slate Marble	Any colour, hardened shale. Any colour, changed limestone

1. Igneous rocks

These are formed from cooled magma (a hot mixture of elements). Magma that cools at the earth’s surface forms extrusive igneous rocks, while magma that cools below the surface forms intrusive igneous rocks.

It must be noted that the faster the rate of cooling the smaller the sizes of the mineral crystals. Cooling at the earth’s surface is usually very fast giving rise to extremely small crystals.

Generally, basic rocks give rise to basic soils. While acid parent rocks give rise to acid soils. The amount of silica determines the acidity of rock – the more basic the rock the less the amount of silica. Common members of igneous rocks are:

Granite: which consists mainly of quartz, silica (SiO2), feldspar, and mica (muscovite and biotite).

Gabro and Basalt: Dark colored. Have little or no quartz. These are basic rocks that form basic soils. Main components are feldspars, hornblende and augite minerals.

2. Sedimentary Rocks

These are sediments or precipitates laid down by the water or by wind, usually in the lakes, oceans, or lowland regions Examples include limestone, skeletons clay which consolidate into rocks over time, and under the pressure of overlaying materials.

Sedimentary rocks are further classified into five groups depending on hardiness, pressure, and composition.

i. Arenaceous: Mainly sandstone which originates from mostly sand-sized sediments.

ii. Argillaceous: Shale, clay from mostly clay-sized sediments, bituminous shale.

iii. Calcareous: Limestone which is rich in carbonates from the shells of lakes or ocean organisms.

3. Carbonaceous Rocks

Peat, usually vegetation remains which vary in color from brown to black. It is rich in nutrients, has a large cation-exchange capacity and high water-holding capacity, and example coal of different kinds.

4. Siliceous Rocks

Flit-compact form of silica.

5. Metamorphic Rocks

These are igneous or sedimentary rocks that have been subjected to heat and pressure and subsequently converted to new rocks. For example, marble forms carbonate, and calcite is re-crystallized.

Gneiss originated from granite, schist–foliated rock (like-leaf) easily split at foliation lines. The minerals here are quartz, feldspar, and mica (muscovite and biotite). Slate is formed from argillaceous sediments.