Major components of soil
Soils are the products of many forces and agents. The four major components of soil are air, water,
mineral matter, and organic matter. The relative proportions of these four components greatly
influence the behavior and productivity of soils. A typical mineral soil by volume basis, only about
half of the soil volume consists of solid materials (mineral and organic); the other half consists of
pore spaces filled with air and water. The solid materials, typically most is mineral matter derived
from the rocks of the earth crust. Only about 5% of the volume in this ideal soil consists of
organic matter. However, the influence of organic components on soil properties is much more
than its small proportions of volume. Since, it is far less dense than mineral matter, the organic
matter accounts for only about 2% of the weight of this soil.
The spaces between the particles of solid materials are just as important to the nature of soil solids.
It is in these pore spaces that air and water circulate, root grows, and microscopic creatures live.
Plant roots need both air and water. In the optimum condition for plants, 25% of the soil volume
consisting of water 25% consisting of air. If there is much more water than this, the soil will be
waterlogged. If much less water is present, plants will suffer from drought. Soils with much more
than 50% of their volume in solids are likely to be too compacted for plant growth. Compare to
surface soil layers, subsoils tends to contain less organic matter, less total pore space, and larger
proportion of small pores (micropores) tends to filled with water rather than air. The volume of air
and water in soil fluctuate. An optimum proportion of mineral part, water and air are very important
for plant growth. It is a three phase system, composed of solid phase, liquid phase and gaseous
phase in varying proportions.
Components of soil according to volume:
1. Mineral matter: sand, silt, clay etc. - 45%
2. Organic matter: protein, lignin, wax etc. -5%
3. Air (Soil air): N2, O2, CO2 etc. -25% (variable)
4. Water (Soil solution): NO3
2- etc. -25% (variable)
In a soil, the four components are mixed in complex patterns; however, the proportion of soil
volume occupied by each component can be represented in a simple pie chart as shown below:
It should be noted that only about half the soil volume consists of solid material (mineral and
organic); the other half consists of pore spaces filled with air and / or water. The four major
components of soil are briefly discussed below:
1. Mineral (inorganic) constituents of soil: Except in case of organic soils, mineral matter
consists of mineral particles. The larger soil particles, which include stones, gravel, and coarse
sands, are generally rock fragments of various kinds. That is, larger particles of soil are often
aggregates of several different minerals. Smaller particles tend to make of a single mineral. Soil
particles range in size from 2.0 millimeters (mm) to smaller than 0.002 mm in diameter. Sand
particle having 2.0-0.05 mm diameters are large enough to be seen by the naked eye and feel
gritty when rubbed between the fingers. Silt particles are somewhat smaller (0.05-0.002 mm
diameter). They cannot be seen without a microscope. The smallest class of mineral particles are
the clays having < 0.002 mm diameter, which adhere together to form a sticky mass when wet
and hard clod when dry. The smaller particles (< 0.001 mm) of clay (and similar sized organic
particle) have colloidal properties and can be seen only with the aid of an electron microscope.
The relative proportions of these particles (sand, silt, clay) in a soil is called soil texture such as
sandy loam, loam, silty clay clay loam etc. Sand, silt and clay particles can be thought of as
the building blocks from which soil is constructed. The arrangement of soil particles and
their aggregate into certain defined patterns is called structure. The primary soil particles
(sand, silt and clay) usually occur grouped together in the form of aggregates. Natural
aggregates are called peds, whereas clod is an artificially formed soil mass.
2. Organic matter(OM): Soil organic matter consists of a wide range of organic (carbonaceous)
substances, including living organisms (soil biomass), carbonaceous remains of organisms which
occupied the soil, and organic compounds produced by current and past metabolism in the soil.
The main source of soil organic matter is plant tissue. Animals are subsidiary source of soil
organic matter. It imparts brown or black colour of soil. Well decomposed organic matter is
known as humus. Over periods of time ranging from hours to centuries, organic matter is lost
from the soil as carbon dioxide produced by microbial respiration. Because of such loss, repeated
addition of new plant and/or animal residues is necessary to maintain soil organic matter.
Organic matter binds mineral particles into granular soil structure responsible for loose and
easily managed condition of soil and increase the water holding capacity of soil. Organic matter
is the major soil source of phosphorus and sulphur and the primary source of nitrogen. Organic
matter is also the main source of energy for soil organisms. Without it, biochemical activity
would come to a near standstil
3. Soil Air: Air is a changeable constituent of soil, which varies greatly from place to place in
composition, and concentration of different gaseous products. Soil air differs from atmospheric
air in several respects, which may be stated below:
The composition of soil air is quite dynamic. In local pockets, some gases are consumed by plant
roots and microbial reactions while others are released.
Soil air generally has higher soil moisture content than the atmosphere. The relative
humidity of soil air approaches 100% when the soil moisture is optimum.
Carbon dioxide in soil air is often several hundred times higher than the 0.035% commonly
found in the atmosphere. Oxygen decreases accordingly and, in extreme cases, may be only 5-
10%, or even less, as compared to about 20% for normal atmosphere. The term air capacity is
used to describe aeration status of soil. Air capacity of a soil refers to the volume of pore
space filled with air. Soil aeration refers to the exchange of O2 and CO2 between the soil
pore space and atmospheric air. The gaseous constituents of soil atmosphere plays a vital
role in modifying the soil physical, chemical and biological environment which intern
influence the growth of plants. CO2 in soil air helps in dissolving nutrients and making
them more available for plants. The composition of soil air is as follows:
Gas % by volume
Soil air Atmospheric air
O 20.00 21.00
N 78.60 78.03
CO2 0.25 0.03
Ar 0.90 0.98
4. Water: Like air, water is another changeable constituent of soil. Water is held in the soil pores
together with its soluble constituents, including nutrient elements like Ca, K, N, and P etc. Soil
water makes up the soil solution, which is the medium for supplying nutrients for growing
plants. Presence of H+
and OHin solution maintains the acidity and alkalinity of soil, which
mostly regulates the nutrient availability and uptake by plants and microbes. Soil water plays a
significant role in several natural processes that influence energy balance on the surface of the
earth, evaporation, infiltration and drainage of water, diffusion of gases, conduction of heat and
movement of salts and nutrients all are dependent upon the amount of water present in soil.
Major losses of water through the processes of transpiration by plants and evaporation from soil
surface the combined process is known as evopotranspiration.