Chemical properties of soil
Chemical properties are those properties that involved inn chemical reaction
Can’t be feel simply by touching soil.
Fertility of the soil is mainly due to it.
Chemical components of soil
Chemical properties are explained by following aspect
1.Minerals matter 2. organic matter
3.Soil Solution 4. Buffering Capacity
5.Soil air 6.Soil Water
7.Soil PH
1.Mineral/inorganic matter
The inorganic component of the soil is composed of many types of minerals which
influence the properties of the soil.
The differences among soils are mainly due to the differences in the type and relative
abundance of such minerals.
Minerals are the largest component in the soil, making up almost 45% of the total soil.
Properties of mineral matter
Plants need number of minerals for a healthy life and growth.
These minerals are absorbed from the soil via the roots of the plants and trees.
Soil minerals can also control adsorption, transformation, and other behaviour of
chemical constituents.
Minerals Formation
Minerals are basically formed by the break down of large rocks. There are 3 major processes
by which minerals form.
Crystallization from Magma
Magma is molten rock ,it’s from deep inside earth .When it cool element join
together to form minerals like Quartz
Precipitation
Certain type of prepicipitation happen when dissolve mineral in water form a soild
once water evaporate like limestone
Pressure & Temperature
Under extreme pressure and temperature, existing minerals become unstable and are
forced to rearrange its atoms to form a new mineral. (recrystallize).
Types of minerals
1) Primary minerals
2) Secondary minerals
Primary Minerals in soil:
They have not been chemically change since their deposit.
Similar to their parent materials.
Often are bigger in size and irregular in shape
Usually found in sand and silt.
Secondary Minerals in soil
They formed as the result of weathering of Primary Minerals.
Mainly found in fine silt or clay.
They have a large surface area that helps them retain moisture.
The size of these is much smaller, due to the weathering process.
Important Mineral Groups
Minerals are grouped by their chemical composition.The major mineral groups commonly
found in soil include:
1.Silicates, 2. Oxides,
3.Sulfates, 4. Sulfides,
5. Carbonates 6. Halides
2.Organic matter of Soi
Organic matter obtaint by plant and animal dead materials that subjected to
decomposition process.
In addition to providing nutrients and habitat to organisms living in the soil, organic
matter also binds soil particles into form soil aggregates
it improves the water holding capacity of soil.
Soil organic matter consists of two general groups of components
a) litter
Fresh or partially decomposed plant and animal residues generally occur in the sand and
silt fraction of the soil
b) Humus
The humus is a more complex product of decomposition and generally occurs in the clay
fraction of the soil
Decomposition of organic matter
Decomposition of organic matter is largely a biological process that occurs naturally.
Its speed is determined by three major factors:
soil organisms,
the physical environment and
the quality of the organic matter.
Products of decomposition
As a result of decomposition, a number of different organic products are released
1.Carbon dioxide
2. Energy,
3.Water,
4.Plant nutrients
HUMIFICATION
Successive decomposition of dead material results in the formation of complex organic
matter that is dark in colour, called Humus. This process is called Humification.
MINERALIZATION
Soil organisms use organic matter as food. As they break down the organic matter,
any excess nutrients (N, P and S) are released into the soil in forms that plants can use. This
release process is called Mineralization.
Compounds of humus
Humus component formed by humic substances, including humic acids and humins.
Chemically humus contains the following organic molecules:
Amino Acids, Protein, Aromatic Molecules, Amino sugars, Hexose Sugars, Sugar
Alcohols. It also contains fats, oils, waxes, lignin and some pigments.
Rate of decomposition of materials in soils
The organic matter is also classified on the basis of their rate of decomposition:
Rapidly decomposed : Sugars, starches, proteins etc.
Less rapidly decomposed : Hemicelluloses, celluloses etc.
Very slowly decomposed : Fats, waxes, resins, lignins ,etc
3.Soil air
air present in pore spaces.
Air is a mixture of few gases e,g,N, O2 Co2 e.t.c.
1. Soil aeration
The process by which air in the soil is replaced by air in the atmosphere.
Microbes add Co2 into soil.So need to remove
4.Soil Water
Water is held in the pore spaces due to the following forces:
Cohesion: The attractive forces between water molecules
Adhesion: the attraction between water molecules and soil particals.
Types of soil water
1) Hygroscopic water
2) Capillary water
3) Gravitational water
Hygroscopic water.
The water is retained in the form of thin films around the soil particals.
This water is unavailable to the plants due to the adhesion and cohesion
Capillary water
The water contained in the capillary spaces between the soil particals.
Available to plants.
Gravitational water
The movement of water downward towards the gravity.
The gravitational water is available to plants only when rain showers follow one
another in rapid succession
otherwise it percolates and thus remains unavailable
5.Soil solution
Minerals present in soil can be absorb only in solution form.
Exchange 2 types
Cation exchange and
anion exchange
Cation exchange
Most of the Soil negatively charge e.g
Humus and clay
Anion exchange
At anion exchange sites
Hold anions and exchange it.
Soil with positively
charge e.g organic colloids
6.Ph
• It is the measure of acidity and alkalinity.It effect on availability of minerals.
7.Buffering capacity of the soil
Resistance to change the ph of the soil
5)How buffering solution form
Buffer solution is formed by a mixture of weak acid and its salt
