Soil science basics

Soil Science

Definition and Origin of Soil

The word “Soil” is derived from Latin word ‘Solum’ means ‘Floor’ or ‘Ground’.

Definition of Soil

1. “Soil is a natural body composed of inorganic and organic constituents, having a definite genesis and a distinct nature of its own”— Dokuchalev (1900).


2. “Soil is a natural occurring body that has been evolved owing to combined influence of climate and other organisms, acting on parent material, as conditioned by relief over a period of time”.


3. “Soil is the unconsolidated mineral matter on the immediate surface of the earth that serves as a natural medium for the growth of land plants”.

Soil Components:

Soil consists of four major components i.e., inorganic or mineral materials, organic matter,water and air. In a representative loam surface soil, the solid mineral particles comprise about 45 per cent of the soil volume and organic matter 5 per cent. At optimum moisture for plant growth,the pore space is divided roughly in half, 25 per cent, of volume being water space and 25 per cent air . The proportions of air and water are subjected to rapid and great fluctuations.


The four soil components occur in a thoroughly mixed condition in soil and this mixture encourages interactions with in and between the groups and permits marked variations in the environment for the growth of plants.

The proportion of different components in the diagram depicts the good soil condition for
plant growth. The air and water are extremely variable and their proportions determine in large
degree the soil’s suitability for the plant growth.

a) Mineral Matter: The Inorganic portion of soils is quite variable in size and composition.

It is composed of small rock fragments and minerals of various kinds.

Rock Fragment sizes

2.0 - 75.0 mm - Gravel or pebbles

75.0 - 250.0 mm - Cobbles (round), flags (flat)

> 250.0 mm - Stones or boulders

Soil Particles

0.2 - 2.0 mm - Course Sand (gritty)

0.02 - 0.2 mm - Fine sand (gritty)

0.002 - 0.02 mm - Silt (powdery)

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The proportion of different sized particles (texture) determines the nutrient supplying

power of the soil, considerably. Primary minerals (original) are prominent in sand and slit

fractions; where as the secondary minerals (formed) dominate in clay fraction and in some

cases the silt fraction. The inorganic fraction of soil is the original source of most of the

mineral elements that are essential for plant growth.

Primary minerals: 


Feldspar, Quartz, Mica, Limestone, Hornblende, augite, Olivine and serpentine.


Secondary minerals:


1:1 – One silica and one alumina layer: Kaolinite, Halloysite and Dickite.
2:1 – Two silica and one alumina layer: Montmorillonite, Vermiculite and Illite.
2:1:1 or 2:2 – The crystal unit is composed of one 2:1 unit: Chlorite.



Accessory minerals: Tourmaline, Topaz, Apatite, Rutile and Anatase.

Amorphous minerals: Allophane.

b)Organic Matter:

 It comprises an accumulation of partially disintegrated and decomposed
plant and animal residues and other organic compounds synthesized by soil microbes as
the decay occurs.


It is a transitory soil constituent as it is continuously broken down by soil organisms and
lasts from few hours to several hundred years. It requires maintenance by regular addition
to the soil of plant and/or animal residues. Organic matter content varies from 1.0 to 6.0 per
cent by weight in top soil and very less in sub soil. In respect of soil productivity organic
matter plays an indispensable role.


Decomposition of organic matter by humidification process gives humus.


In hilly and high altitudes areas, OM is above 1 per cent.

CN ratio of Organic matter is 10:1,

whereas it is an average of 14:1 for Indian soil.


Histosols are also called Organic soils.

c)Soil Water: 

Soil water is held in soil pores with varying degrees of tenacity depending on
the amount of water present and size of the pores. Soil water with its soluble constituents
(nutrients) makes up soil solution, which is the critical medium for supplying nutrients to
growing plants.

Soil water plays significant role in controlling energy balance of the soil
and regulates the gaseous exchange in the upper layer of the soil. The presence of water
in different amounts in soil governs its thermal, mechanical, physical, chemical and
biological properties.

d)Soil Air: 

The content and composition of soil air are determined largely by the water
content of the soil, since the air occupies those soil pores not filled with water. Soil air
always differs from atmosphere air in composition because of moisture content, root and
microbial activities.