Soil properties and concepts

1. SOILAND THEIR CONCEPT
The soil is the covering on the solid crust of the earth’s
landmass. The soil is made up of broken down rock material of
varying degree of fineness and changed in varying degrees
from the parent rocks by the action of different agency. Soil
was defined by Buckman and Brady as:
“A dynamic natural body on the surface of the earth in which
plant grow, compost of mineral, organic material and living
forms.”
Jeffy and Marbul defined soil as “ soil is natural body
developed by natural forces acting on natural materials. It is
usually differentiated into horizons from mineral and organic
constituents of variable depth which differ from the parent
material below in morphology, physical properties and
constituents, chemical properties and composition and
biological characteristic.”

2. SOIL ENVIRONMENT
Soil environment can be studied under physical, chemical and
biological environment.
The favorable physical environment for proper crop growth
is deep soil without hard layers or high water table so that crop
can exploit larger soil volume.
The soil has to contain higher available water holding
capacity, proper aeration and less soil strength or mechanical
resistance.
It should also have higher nutrient availability.

3. SOIL FERTILITY:
It is the capacity of the soil to produce crops of economic value
and to maintain health of soil without deterioration.
SOIL PRODUCTIVITY:
It is the capacity of the soil to produce crop with specific
systems of management and it is expressed in terms of yields.
All productive soils are fertile, but all fertile soil are need not be
productive.

4. PHYSICAL PROPERTIES OF SOIL:
Soil physical environment is control by soil characters like texture, structure,
aeration, water, mechanical resistance and depth of soil.
SOIL TEXTURE:
it refers to the relative proportion of sand, silt and clay. Rock fragments
larger then 2cm in diameter are stones, materials between 2 cms and 2 mm
diameter are graval and the soil mineral matter smaller than 2 mm in
diameter is the fine earth. Sand silt and clay together constitute fine earth.
•Sand: The particles of 0.2 to 2mm in diameter are classified as coarse sand
and of 0.02 to 0.2mm in diameter as fine sand. Sand particles are small pieces
of unweathered rock fragments.
Unless these particles are coated with clay or silt, they do not exhibit
properties such as plasticity, cohesion, stickiness, moisture and nutrient
retention etc.

5. •Silt: The size of silt particles are in the range of 0.02 to 0.002 mm in diameter.
Due to an adhering film of clay, they exhibit some plasticity, cohesion,
adhesion and adsorption.
These silt particles can holt more amount of water than sand but less
than clay. Both silt and sand particles are spherical and cubical shape.
•Clay: It is the most important constituent of soil that controls most of the soil
physical and chemical properties.
•These particles are of size less than 0.002mm in diameter.
•They have the highest surface area since surface area is inversely related to
size.
•They can adsorb and retain water and nutrients they exhibit properties like
flocculation, deflocculation and plasticity.
•It behaves like a weak acid which is neutralized by bases such as calcium and
magnesium ions, the serving as store house of serving nutrients.

6. Textural class Sand (%) Silt (%) Clay(%)
Sand 85-100 0-15 0-10
Loamy sand 70-90 0-30 0-15
Sandy loam 43-80 0-50 0-20
Loam 23-52 28-50 7-27
Silt loam 0-50 50-88 0-27
Silt 0-20 88-100 0-12
Sandy clay loam 45-80 0-28 20-55
Clay loam 20-45 15-53 27-40
Silty clay loam 0-20 40-73 27-40
Sandy clay 40-65 0-20 35-45
Silty clay 0-20 40-60 40-60
Clay 0-40 0-40 40-60
TEXTURAL CLASSES OF SOIL:
Example
If the soil contains more than 80% of silt fraction, the soil is designated as silty
soil and soil with more than 855 of sand is called sandy soil and so on.

7. SOIL STRUCTURE:
The primary soil particles viz. sand, silt and clay are usually grouped
together to form aggregates into define patterns is called soil structure.
Natural aggregates are called fragments while the word clod is used
for an artificially formed soil mars.
Soil structure is an important soil property which influences the soil
environment through its effect on the amount and size of pore space,
water holding capacity, availability of plant nutrients and growth of
microorganisms

8. SOIL WATER:
water is held around soil particles in thin layers due to adhesion and
cohesion.
It is held with greater force (10000 bars) near the particles surface.
As the thickness of water layer increases, the force with which it is held is
reduced.
When the soil particles are holding water with less than 0.3 to 0.1 bar force,
it is lost into deeper layers due to gravitational pull.
The soil water held between -0.3 to -15 bars, is considered as available water.
Soil water moves from higher potential to lower potential and this
movement of water is an important factor in water availability.

9. FUNCTIONS OF SOIL WATER:
It has major influence on modifying the soil environment. The soil strength is
reduced due to its presence.
It moderates the soil temperature from extremes. Nutrient availability and
mobility increases with water. Biological activities including mineralization
also increases.
SOIL TEMPERATURE:
it is one of the most important soil properties that affect crop growth.
The major source of heat is sun and heat generated by the chemical and
biological activity of the soil is negligible.
The amount of solar energy received on a unit area of the soil decides the soil
temperature.

10. SOIL AIR:
The volume ratio of the three constituent phases of soil, namely, solid, water
and air are continually changing as the soil undergoes wetting or drying,
swelling or shrinking, tillage or compaction and aggregation or dispersion.
Water and air compete for the same pore space, and their volume fractions are
so related that an increase of one decreases the other.
Field air capacity is the fractional volume of air in a soil at field capacity.
The air capacity depends on soil texture.
In sandy soils, it is of the order of 25% or more, in loamy soils it is generally
between 15 and 20 % and in clay soils it is likely to fall below 10% of the total
soil volume because clay tend to retain more water.

11. Other physical properties of soil of some importance are:
•Soil profile
•Soil depth
•Water table depth
•Soil strength
•Soil colour

12. CHEMICAL PROPERTIES OF SOIL
•The soil chemical environment is dynamic and reactions that maintain
dilute solution of nutrient element are indispensable for continual plant
growth.
•The nutrient transformation and its availability in soils depend on pH, clay
mineral, cation and anion exchange capacity.
SOIL pH
•It is defined as the negative logarithm of hydrogen ion activity. The pH of
the soil indicates its acidity or alkalinity.
•It influences nutrient availability, soil physical condition and plant growth.
•pH influences rate of nutrient release through its influence on
decomposition, cation exchange capacity and solubility of materials.
•Soil reaction preference differs with crop.
•Rice and tea prefer acidic soil reaction and most of the other crops prefer
neutral pH.

13. CATION EXCHANGE CAPACITY (CEC):
It is very important property of the soil from plant nutrition point of
view. Colloids or micelles have larg surface area per unit weight.
Most clay micelles have a crystalline structure while humus micelles
are amorphous. These micelles contain negative charges on their
surfaces.
The negative charge on clay micelles are due to isomorphic
substitution, ionization of hydroxyl groups and exposed carboxyl and
hydroxyl groups.

14. ANION EXCHANGE CAPACITY:
It is less in magnitude compare to CEC.
The sites for anion for exchange to and from soil solution and also from
soil constituents are present at the amine groups in humus and at the
edge of silicate clay mineral when the bonding between clay mineral
and cation terminates.
In addition, release of OH- from Fe and Al cause positive charges on
clay minerals. Similar to exchange of cations from clay mineral and soil
solution, anions are also exchange.

15. TRANSFORMATION:
The nutrients present in the soil are subjected to physical chemical and
biological changes or transformations.
Several substances are produce during transformations which are not directly
connected to plant nutrition.
Some of these may be toxic and others harmless. These transformation may
either release or fix nutrients.
MODIFICATION OF CHEMICAL ENVIRONMENT:
Chemical environment of the spoil is modified by the addition of organic
matter , fertilizers and by irrigation/ tillage.