• The term soil is derived from Latin word “Solum” means ‘Floor’ or ‘Ground’.
• Soil is the interphase between atmosphere and the mantle of rocks making up the earth’s
crust called “lithosphere”.
• Regolith: All loose material above bedrock (the unconsolidated material of weathered rock
and soil material).
• Soil Survey: Systematic examination, description and classification of soils.
• Pedology: Science dealing with genesis, survey, classification and laws of geographic
distribution of soils as a body in nature.
• Edaphology: It is the study of soils from stand point of higher plants.
Rocks:
• “Rocks” are defined as the aggregates of one (or) more minerals.
Eg: Limestone – aggregation of calcite mineral.
• Rocks have no definite chemical, morphological composition or definite symmetrical form
like minerals.
• Based on origin, rocks are classified as
1. Igneous rocks
2. Sedimentory rocks
3. Metamorphic rocks.
• I. Igneous rocks:
These are the most abundant and makeup 95% of all the earth crust.
Igneous rocks are “oldest rocks” and are also known as “crystalline (or) massive (or) fire
rocks.
These rocks mainly consist of primary minerals and more than half of the igneous rock
is containing “feldspar” minerals.
Igneous rocks based on their mode of formation are divided into two groups.
a. Extrusive (or) volcanic rocks: These are formed at the surface from the volcanic
magma. Eg: Basalt, diorite
b. Intrusive (or) plutonic rocks: These are formed by the cooling of the original magma
and occur below the earth crust. Eg: Granite.
Igneous rocks are also classified based on their chemical composition.
a. Acid rocks – contain more than 65% silica – Granite.
b. Neutral rocks – contain 50% - 65% silica – Diorite.
c. Basic rocks – contain less than 50% silica – Basalt.
II. Sedimentary rocks: These rocks are formed through the transportation and deposition of
weathered sedimens. The sedimetary rocks are mostly formed through the agency of
water, which are called clastic, aqueous or stratified rocks. Based on origin, sedimentary
rocks are classfied into 4 groups.
1. Residual sediments: These are formed from the products of weathering in situe that is at
same place. Eg: Laterite, bauxite.
2. Mechanical sediments: These are formed due to the deposition of pebbles, sand and silt. Eg:
Standstone, shale.
SC, ST, OBC & Minority Coaching Cell 1
, Agricultural College, Bapatla
3. Chemical sediments: These are formed due to evaporation of water, precipitation and
consequent accumulation of sediments.
Eg: Limestone.
4. Organic sediments: These are fomred due to partial decompostion of organic remains under
anaerobic conditions.
Eg: Peat
III. Metamorphic rocks: These are formed from the rocks by the action of heat and pressure
on pre-existing igneous and / or sedimentary rocks.
Minerals: A mineral is a naturally occuring, homogenous element or inorganic compound that has
a definite chemical composition and a characteristic geometric form. The minerals can be identified
by many of their physical properties like colour, lustre, streak, hardness etc.
• Lustre – general appearance of mineral in reflected light.
• Fracture – property of the mineral to break along an irregular surface i.e., surface produced
when the mineral breaks in direction other than cleavage plane.
• Streak – The colour of the powder of the mineral that is obtained by rubbing the mineral
against the unglazed porcelain plate.
Soil forming minerals mainly belong to the group of alumino – silicates. Minerals are classified
based on quantity (essential and accessory), origin (primary and secondary), specific gravity
( light and heavy) and chemical composition (native elements, oxides, sulphates, sulphides,
carbonates, halides, silicates).
Primary minerals –They are the original components of the rock, which have not been
altered chemically. They are formed from crystallization of magma (molten mass).
Eg: Quartz, feldspar etc.
Secondary minerals –are resulted from the decomposition and / or alteration of primary
minerals. These are formed due to subsequent changes in the rocks.
Eg: All clay minerals like kaolinite, illite except micas. Micas are primary minerals.
Minerals which form the chief constituent of rock and regraded as the characteristic
components of the rock are known as “Essential Minerals”. Eg: Quartz and feldspars.
Accessory minerals : The minerals which occur in small quantity in rocks They are not
concerned for naming or the nomenclature of the rocks. Such minerals are called as
“accessory minerals”. Eg: Apatite, pyrite, magnetite etc. These are not required for the
fomration of any rock.
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