Learn About Soil Composition and Structure

SOIL CONSERVATON
TECHNOLOGY
CPT009

DEFINITION AND COMPOSITION OF SOIL
Introduction
 There are varieties of reason why soil is an important component in
agriculture. It is a natural resource of our country.
 Since Philippine is an agricultural country, our economy depends mostly to
agricultural products, it holds and purifies water, serves as medium for plant
growth and almost everything we need can be traced back from the soil i.e. food,
clothing, paper, timber, medicines, shade, and oxygen.
 This chapter will provide you an overview on what soil is and how it affects
agriculture as a whole.

What is Soil?
 a dynamic natural body formed from the weathering of rocks and minerals,
living because a handful of fertile soils contain millions of microorganisms,
composed of organic matter, mineral matter, soil air and soil water, and
possess distinct properties which in turn influence plant growth.

Concepts of soil:
■ Soil is a natural body derived from weathering of rocks and minerals
■ It is a porous mixture of organic and inorganic matter (allows exchange of gases in
the soil; it is in the pore spaces you can find water and air in the soil).
■ It is used as medium for plant growth (practical way to grow crop is through soils)
■ It has distinct composition and properties that influence the growth of the plant
(it has the essential nutrients which is needed for plant growth and development
■ It is a dynamic system (it is forever changing).

Approaches in the study of soil:
Different point of views was used in the study of soil. These are:
1.) Pedological approach- considered soil as a natural body with major emphasis
on properties related to genesis and classification as it’s occurs in the
environment. The pedological approach primarily studied soils on its capacity to
hold buildings and highways thus; the information gathered by the pedologist
would be an interest to highway engineers and building contractors. The study of
soil as they occur in nature with principal interest on characterization and
differentiation of their properties and with only minor emphasis on their practical
use. The soil is studied for their taxonomic classification.

Approaches in the study of soil:
Different point of views was used in the study of soil. These are:
2.) Edaphological approach - study of soil in relation to higher plants. It seeks to
answer the ff:
■ What are the soil properties that influence plant growth?
■ What is the variability of soil productivity?
■ How can we conserve and improve productivity?
 Further, edaphological approach studies on soils with emphasis on their
practical use, particularly the relationship of soil properties to plant growth.
Thus, more likely it is the study of soil fertility.

What is Soil Science?
 deals with the study of soil as natural resource on the surface of the earth
which includes soil formation, classification, and mapping, physical, chemical,
biological and fertility properties in relation to the management of the crop
production.

The fields of study in Soil Science
■ Soil Fertility - study of the inherent capacity of the soil to supply the nutrients
needed for growth and development of the plants. It refers to the study of the
soil with respect to the form, availability to plants of the nutrients necessary
to plant growth.
■ Soil Physics - it deals with the characteristics, properties, and reactions in the
soil related to the physical forces acting on the soils and can be described by
an equation.

■ Soil Chemistry and Mineralogy - study on the chemical composition of rocks
and minerals from which soils are derived. It deals with the characterization
of the chemical reactions in the soil. Soil chemistry also deals with study of
the interaction of the solid, liquid, and gas phase in the soil.
■ Soil Microbiology - study on the characterization of important groups of
microorganisms in the soil and their role in physical and biochemical changes.

■ Soil Conservation and Management/ Soil Technology - protection of the soil
against deterioration such as erosion and chemical deterioration. The totality
of the tillage operations, cropping practices, fertilization, liming, etc
conducted on soil for crop production and their effect to soil erosion.
■ Land Use - deals with the utilization or allocation of lands for general or broad
purposes such as for agriculture, settlement and military reservation.
■ Soil Genesis, Morphology, and Classification - it deals with the mode of
origin, structural characteristics, and systematic arrangement of soils.

• Soil is particulate and porous
Soil Solid
mineral matter and
organic matter
Physical Component of a Soil

Soil components:
The four major compositions of soil are
1.) mineral matter
2.) organic matter
3.) soil air and
4.) soil water.
 Its proportion in the soil greatly influences the behavior and productivity of
the soil.

Soil components:
1.) Mineral matter (45% of the total soil volume)
 coming from the weathering of rocks and minerals. It is extremely variable in
size:
■ Sand- 0.02-2 mm; feels gritty when rubbed between fingers; not sticky
■ Silt- 0.002-0.02 mm; feels smooth but not sticky when wet.
■ Clay- <0.002 mm; form sticky mass when wet and form hard clod when dry.

Soil components:
2.) Organic matter (5%)
 derived from the decomposition of plant and animal residues. Over a long
period of time, organic matter is lost in the soil to form CO2 by microbial
respiration.
 The repeated addition of animal and/or plant residues is necessary to
maintain the OM in the soil.

Soil components:
The general groups of OM
 Original tissue that is partially decomposed (peat).
– Sapric or totally or fully decomposed (muck)
– Hemic or ½ decomposed (peat)
– Fibric or less 1/3 decomposed (peat)
 Humus which is the most resistant product of decomposition is derived from those
synthesized by the microorganisms.

Soil components:
Important roles/ function of OM
a) Promote granulation of the mineral particles largely responsible for the loose,
easily managed conditions of productive soils.
b) Influence soil properties, and consequently on plant growth.
c) Increases the water holding capacity of the soils. Promote proportion of water
available for the plants.
d) Contain all essential nutrients and primary the source of nitrogen for most plants.
e) Source of food by the microorganisms in the soil.
f) Increases nutrient holding capacity.

Soil components:
3.) Soil water (20-30%)
 it is sometimes called soil solution because it has two components, the
solvent (water) and the solute (dissolved ions).
 The important function of soil water is primarily to replenish dilute nutrient
solution bringing it to the roots for ease of absorption.
 The soil water acts as:
1. universal solvent
2. nutrient carrier
3. nutrient itself
4. stabilizes soil temperature

Soil components:
4.) Soil air (20-30%)- the amount and composition of the soil air is determined by
the total water content in the soil. When the pore spaces are not occupied by
water, it is occupied by the air. Soil air is differed from atmospheric air in several
aspects:
a) The composition of the soil air varies from place to place.
b) Soil air generally has higher moisture content than atmospheric air.
c) The content of CO2 in the soil air is higher than in the atmospheric air.

Soil components:
 Only about half of the soil volume consists of solid material (organic and
mineral matter); the other half is the pore spaces which either occupies by air
or water.
 Of the solid material, 45% is the mineral matter which is derived from the
weathering of rocks and minerals and the other 5% is the organic matter,
coming from plant and animal residues.
 The mineral particle varies in size ranging from 2-<0.002 mm.
 Beyond 2 mm, it is no longer part of the fine earth fraction.
 Size <0.001 have colloidal properties and can be seen only by the aid of
microscope.

Elemental Composition of Earth Crusts
 The earth crust deep is about 16 km or 10 miles deep. It is the source of inorganic
material and is considered where soil parent material derived.
 The composition of the earth crust was estimated by Clark and Washington (1924)
as follows:
1) igneous rock - 95%
2) shale - 4%
3.) sandstone - 0.75%
4.) and limestone - 0.25%
 The most abundant minerals containing Al, O, and Si are alumino-silicates and its
oxides dominated by Fe, Al, and Si.

Table 1. The average composition of elements in the
earth’s crust (Clark and Washington, 1924)
Elements %(by weight) Oxides (%)
O 46.60
Si 27.72 SiO2 59.07
Al 8.13 Al2O3 15.22
Fe 5.00 Fe2O3 3.10
Ca 3.63 CaO 5.10
Na 2.83 Na2O 3.71
K 2.59 K2O 3.11
Mg 2.09 MgO 3.45
Ti 0.60 TiO 1.03
P 0.10 P2O5 0.30
Mn 0.09 MnO 0.11

Fig 1. Volume composition of the mineral soil
Components Optimal Soil Clayey Soil Sandy Soil
Mineral matter apprx 45% apprx 45% apprx 45%
Organic matter apprx 5% apprx 5% apprx 5%
Soil water apprx 25% > 25% < 25%
Soil air apprx 25% < 25% > 25%

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