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A dynamic natural body composed of mineral and organic solids, gases, liquids and living organisms which can serve as a medium for plant growth.
                                                                (or)
Upper unconsolidated earth crust composed of minerals and organic solids which can support to the plant growth.

 Composition of Soil:

Soil is composed of 25 % air, 25 % water, ≈ 49 % minerals and ≈ 1 % organic matter

 Importance of Soil:

 What is Soil Science?

It is a scientific discipline that deals with the study of soil as a natural resource for plant growth.

 Branches of Soil Science: 

Physical properties of soil:

Soil texture, Soil Structure, Soil Density (Bulk density & Particle density), Soil Porosity, Soil Consistence, Soil Color, Soil Temperature etc.

1) Soil Texture:

It is defined as the relative proportion of sand, silt and clay in the soil (soil particles).

Size limits of soil separates, soil particles (Sand, Silt, Clay)

Soil texture is determined in the laboratory by Bouyoucus Hydrometer method and feel method in the field.

Use of Textural Triangle to name textural class of the soil.

Textural Classes: Sandy loam, Loam, Silt loam, Clay loam etc.

Importance of soil texture: Plant type, plant growth, nutrients availability, buildings, land use and management etc.

2 ) Soil Structure:

It is the arrangement and grouping of soil particles (Sand, Silt, Clay) in the soil.

Soil Formation: Soils are formed by the weathering of rocks and minerals.

Weathering: It is the process of disintegration and decomposition of rocks and minerals to more stable forms under atmospheric conditions. It transforms the original rock (parent rock) into parent material.

Rock: It is a natural aggregate of one or more minerals to form an appreciable part of the solid portion of the earth.

Mineral: It is a naturally occurring, inorganic, crystalline substance with a definite chemical composition and physical properties.

Examples: Quartz, Feldspars, Pyroxenes, Amphibole, Olivine, Mica, Apatite, Carbonates, Dolomite, Gypsum, Pyrite etc. (groups of minerals)

About 2000 minerals have been identified in the soil.

Types of minerals:

a) - Primary minerals: They are formed from the rocks by weathering.

b) - Secondary minerals: They are formed from the primary minerals by weathering.

Types of Weathering:

Processes of Weathering:

1- Physical Weathering: It is a process whereby rocks and minerals are cracked, crumbled, crushed and mellowed. The result is a material that varies in size from boulders to very fine particles with no appreciable change in chemical or mineralogical composition. The forces responsible for physical weathering are expansion, contraction, pressure, temperature and grinding etc.

2- Chemical Weathering: It involves processes that alter rocks and minerals that are stable at the given temperature and pressure at the surface of the earth. These processes are hydrolysis, oxidation, reduction, carbonation and hydration etc.

3- Biotic/ Biological Weathering: It can be defined as physical and chemical weathering by biological agents. The breakdown of the rocks and minerals is accelerated by plants, animals and microorganisms.

All these weathering processes work together to form soil.

Types of Rocks: 

1- Igneous Rocks: They are formed by the solidification of molten material called magma, which originates at considerable depth beneath the earth’s surface.

Examples: Granite, Basalt, Diorite, Andesite and Gabbro etc.

Magma: It is a natural, moldable, hot melt composed of rock forming materials and some volatiles that are held in solution by pressure.

2- Sedimentary Rocks: They are composed of particles derived from previously existing igneous rock by physical alteration.

Examples: Gypsum, Dolomite, Lime stone and Sand stone etc.

3- Metamorphic Rock: They are derived from pre-existing igneous or sedimentary rocks by chemical alteration.

Examples: Marble, Slate, Quartzite and Phyllite etc.

The relative abundance of these rocks in the earth crust is:

  Igneous- 64.7 %,                     Metamorphic- 27.4 %,                  Sedimentary- 7.9 %

Factors of soil formation:

S = F (Cl, O, R, P, T)         where

Cl (Climate), O (Organisms), R (Relief, Topography), P (Parent material), T (Time)

 Climate and organisms are considered as active factors, while parent material, topography and time are considered as passive factors.

 Parent material:

It refers to the consolidated or un-consolidated initial material from which soil develops.

                                             Rock - Parent material -  Soil

Types of soil parent materials:

 1- Colluvium, deposited by the action of gravity.

2- Alluvium, deposited by the action of water, Lacustrine (lake/ river), Glacial drifts (glaciers) 

3- Aeoline, deposited by the action of wind, Loess (silt size particles, in Potohar), Sand dunes (sand size particles, in Thal and Thar) .

Rate of soil formation:

A- Horizon = 1-20 years/ cm

A plus B- Horizon = 40-100 years/ cm

Complete weathering in the solum = 100-125 years/ cm

 Reference:

http://agrihunt.com/pak-agri-outlook/2727-basics-of-soil-science.html