What is Soil?
A dynamic natural body composed of mineral and organic solids, gases, liquids and living organisms which can serve as a medium for plant growth.
Composition of Soil
Soil is composed of 25 % air, 25 % water, ≈ 49 % minerals and ≈ 1 % organic matter
Importance of Soil
- Soil provides us food, fiber and shelter
- It is a medium for plant growth
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
- Soil Genesis: It is the branch of Soil Science that deals with weathering of rocks and minerals and factors and processes of soil formation.
- Soil Classification: It is the branch of Soil Science that deals with the systematic arrangement of soils into groups or categories on the basis of their characteristics.
- Soil Survey: It is the branch of Soil Science that deals with the systematic examination of soil in the field and laboratory, their description, classification, mapping, and interpretation according to their suitability for different management systems.
- Soil Physics: It is the branch of Soil Science that deals with the mechanical behavior of soil mass, the physical properties of soil, with emphasis on the state and transport of matter (especially water) and energy in the soil.
- Soil Chemistry: It is the branch of Soil Science that deals with the chemical constitution, chemical properties, and chemical processes taking place in the soil.
- Soil Biology: It is the branch of Soil Science that deals with soil inhabiting organisms, their function and activities.
- Soil Fertility: It is the branch of Soil Science that deals with the ability of a soil to supply the essential plant nutrients for plant growth.
- Soil Salinity: It is the branch of Soil Science that deals with the excess soluble salts present in the soil, their reclamation, and soil management for saline agriculture.
- Soil mineralogy: It is the branch of Soil Science that deals with the primary soil minerals and their contribution to the chemistry, physics, fertility, and biology of soils and their relationship to soil genesis.
- Soil Conservation: It is the branch of Soil Science that deals with the protection of soil from physical loss by erosion (by wind and water) or chemical deterioration.
Physical properties of soil
Soil texture, Soil Structure, Soil Density (Bulk density & Particle density), Soil Porosity, Soil Consistence, Soil Color, Soil Temperature etc.
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)
Very coarse sand
2.0- 1.0 mm (diameter)
1.0- 0.5 mm
2.0- 0.2 mm
0.5- 0.25 mm
0.25- 0.10 mm
0.2- 0.02 mm
Very fine sand
0.1- 0.05 mm
0.05- 0.002 mm
0.02- 0.002 mm
< 0.002 mm
< 0.002 mm
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.
It is the arrangement and grouping of soil particles (Sand, Silt, Clay) in the soil.
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
- Geochemical Weathering: It takes place below the soil solum.
- Pedochemical Weathering: It takes place in the soil solum.
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.
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