2. Soil
The word “Soil” is derived from Latin word ‘Solum’ means ‘Floor’ or ‘Ground’.
Definitions
“Soil is a natural body composed of inorganic and organic constituents, having a definite genesis
and a distinct nature of its own”
“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”.
“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”.
Pedological approach
The study of origin, classification and description of soils as they occur in their natural environment.
No practical use for crop production
Edaphological approach
The properties of soil in relation to plant growth, reasons for variation in soil productivity and
methods to improve soil productivity are studied in Edophology. More practical for crop production
3. Functions of soil
Medium for plant growth:
Soil supports the growth of higher plants, by providing a medium for plant roots and
supplying nutrient elements that are essential to the entire plant.
Soil is a source of:
◦ Physical support – anchoring to roots.
◦ Air –Ventilation.
◦ Water – Absorb rainwater and hold it where it can be used by plant roots.
◦ Temperature moderation – Insulates roots from extreme hot and cold.
◦ Protection from toxins –Vent toxic gases and decomposing or absorbing organic toxins.
◦ Nutrient elements – Supply plants with dissolved inorganic ions or mineral nutrients.
Regulate water supply:
Soil plays a vital role in cycling freshwater. Nearly every drop of water in our lakes,
rivers, estuaries and aquifers has either traveled through the soil or over its surface. Soil
filters and regulates the water supply by storing water after a precipitation event
4. Functions of soil
Habitat for Organisms: home to billions of organisms, belonging to thousands of species.
Soil combines a range of niches and habitats:
◦ Water-filled pores – Habitat for swim organisms like roundworms.
◦ Air-filled pores – Habitat for insects and mites. Areas enriched with organic matter.
◦ Highly acidic areas.
◦ Highly basic areas.
◦ Areas with varied temperature regimes.
Recycle wastes:
Soils have the ability to:
◦ Incorporate great quantities of organic waste, turning it into beneficial humus.
◦ Convert the mineral nutrients in the wastes into forms that can be utilized by plants and animals.
◦ Return the carbon into the atmosphere in the form of CO2.
Support for structures:
Soil is used for structures such as roads, causeways and as the foundation for buildings
and bridges. Soil is also used for the establishment of forestry and agriculture crops.
5. Soil phases
Soil is a complex system, made of solid, liquid and gaseous materials.
Soil is a three phase or polyphasic system comprising of (a) solid phase, (b) liquid phase,
and (c) gaseous phase in some proportions.
Normally the proportion is 50:25:25, but this may vary from soil to soil. In some
occasions, liquid or gaseous phase may be absent. For e.g., in water logged soil, air is not
present; similarly in desert dry sandy soils, water is not present.
6. Importance of Soil
Organic matter is a storehouse of nutrients in soil. It is responsible to get the
most desirable soil structure.
It promotes greater proportion of large pore sizes, improves water holding
capacity and aeration status of soil.
It is a main source of N, 5-6% of P, and 80% of S. It also supplies different
trace elements like boron, molybdenum etc.
It acts as a chelate, due to chelate formation between organic matter and
various metals; the availability of these metallic elements will be increased.
It contributes to cation exchange capacity in soils.
It reduces soil erosion; shades the soil and keeps the soil cooler.
7. Soil Components
Soil consists of four major components i.e., inorganic or
mineral materials, organic matter, water and air.
• (a) Soil mineral matter (SMM) - Size and composition of mineral matter in soils are
variable due to nature of parent rock from which it has been derived. The rock
fragments are disintegrated and broken portion of the massive rocks, from which
regolith through weathering, the soil has been formed.
8. Soil Components
(b) Soil organic matter (SOM) - Soil organic matter exists as partly decayed and partially
resynthesized plant and animal residues. These are continuously being broken down as
a result of microbial activity in soil. Due to constant change, it must be replenished to
maintain soil productivity.
The organic matter content in a soil is very small and varies from
only about 3–5% by weight in topsoil.
In addition to partly decayed plant and animal residues, soil organic matter contains
living and dead microbial cells, microbiologically synthesized compounds and
derivatives.
9. Soil Components
(c) Soil water - Soil water plays a very significant role in soilplant growth relationship.
Water is held within the soil pores with varying degree of forces depending upon the
amount of water present.
With the increasing amount of water in soil, the forces of retention of water by the soil
will be low and vice-versa.
The movement and retention of water in the soil is primarily influenced by the
characteristics of the soil viz., texture, nature of inorganic and organic colloids, type
and amount of exchangeable cations, size and total amount of pore spaces etc.
10. Soil Components
(d) Soil air - Pore spaces in soil consist of that portion of the soil volume not occupied by
soil solids, either mineral or organic.
Under field condition, pore spaces are occupied by air and water. The relative amounts
of air and water in the pore space fluctuate continuously.
During rainy season, water replaces air from the soil pore spaces, but as soon as water
leaves by downward movement, surface evaporation, and transpiration etc.
Air gradually replaces the water, as it is lost form the pore spaces. Soil air contains
various gases like CO2, very small amounts of O2 and N etc. Generally, soil air contains
much more CO2 and small amount of O2 than that of atmospheric air due to microbial
respiration when large amounts of CO2 releases into the air and O2 is taken up by soil
microorganisms.
11. Branches of Soil Science
1. Pedology: The Science dealing with the genesis, survey and classification and the
laws of geographical distribution of soils as a body in nature. Pedology is the study of
soil as a natural body and does not focus primarily on the soil’s immediate practical use.
2. Edaphology: The science that deals with the influence of soils on living things,
particularly plants, including man’s use of land for plant growth. Edaphology is the study
of soil from the stand point of higher plants.
3. Soil Fertility: The quality of the soil that enables it to provide essential chemical
elements in quantities and proportions for the growth of specified plants.
4. Soil Chemistry: Deals with the chemical constituents, the chemical properties and the
chemical reactions of soil in relation to crop needs.
5. Soil Physics: Study of various physical processes that are taking place in and through
the soils.
12. Branches of Soil Science
6. Soil Microbiology: Deals with microscopic population of the soil, its role in various
transformations and its importance in plant nutrition and crop production.
7. Soil Conservation: Deals with the protection of soil against physical loss by erosion
and against chemical deterioration.
8. Soil Genesis: The study of the mode of origin of soils, with special reference to the
processes responsible for the development of Solum or true soil from the
unconsolidated parent material.
9. Soil Survey: The systematic examination, description, classification and mapping of
soils in an area.
13.
14. Sandy soil has good drainage, but can be bereft of nutrients. Because the soil
particles are large, the medium is very porous, allowing chemical constituents
to pass through before plants can absorb them. Sandy soils can dry out quickly
for the same reason and prone to erosion, particularly by wind erosion.
Loamy soils are typically good for agricultural because they have the right
balance between nutrient-retention and drainage. Moreover, loam is usually
described as having humus (or organic carbon). The humus in loams provide
the structure for nutrient retention as well as providing nutrients as they
degrade into simpler compounds.
Clay soils are diverse since they are defined by their chemical constituent. Due
to their water and nutrient retention properties, they’re technically fertile, but
their microscopic structure are tight enough that water has difficulty
flowing/draining. Hence, clays are known to be poor soils as plant roots may
not be strong enough to penetrate the clay medium, which is dense/strong
enough to resist human tools as well.
16. Essential Plant Nutrients
Plant contains more than 90 elements, out of which, 17 elements are known to be
essential which are classified as macr-onutrients and micro-nutrients, based on their
relative abundance in plants.
19. Nutrient Deficiency Symptoms of Plants
1. Complete crop failure at the seedling stage.
2. Severe stunting of plants.
3. Specific leaf symptoms appearing at varying times during the season.
4. Internal abnormalities such as clogged conductive tissues.
5. Delayed or abnormal maturity.
6. Obvious yield differences, with or without leaf symptoms.
7. Poor quality of crops, including differences in protein, oil, or starch
content, and storage quality.
8. Yield differences detected only by careful experimental work.
20. Inorganic Fertilizers
Fertilizers: The inorganic materials which are added to the soil to supply certain
elements essential to the growth of plants.
Classification of Fertilizers
Straight fertilizers: Fertilizers which contain only one primary or major nutrient, e.g.
Urea.
Binary fertilizers: Fertilizers which contain two major nutrients e.g. Potassium nitrate.
Ternary fertilizers: Fertilizers which contain three major nutrients e.g. Ammonium
potassium phosphate.
Complete fertilizers: Those fertilizers having all the three major nutrients viz. N, P and K.
Complex fertilizers: Such fertilizers contain more than one primary or major nutrient
element e.g. DAP, Ammonium phosphate.
