LWM-501 (SOIL PHYSICS)
SOIL:
The term soil has various meanings, depending upon purpose and
general field in which it is being considered. To an Agriculturist ----“Soil is
substance existing on earth which provide habitat to plant where they grow up
flourish”. It is source of nutrients and water for plant growth.
Soil is a thin layer of material present on earth’s surface
in which plants have their roots. It is life supporting upper surface of earth,
which is basis of all agriculture. Soil contains minerals and gravels from
chemical and physical weathering of rocks, organic matter, micro-organism,
insects, nutrients, water and air. Study of soil is called as “PEDOLOGY”.
SOIL
PHYSICS: It is described as the branch of science, dealing with
physical properties of soil, as well as with the measurements. Predictions and
control of the physical process taking place in and through the soil.
SOIL FORMATION: Soil covers most of the land surface of earth in thin layer, ranging from a few centimeters to several meters deep. Soil is varying widely in composition and structure from place to place. They ae formed through weathering of rocks and minerals and breaking of organic matter. Weathering is the action of ice, wind, rain, sunlight (temperature) and biological processes on rocks. Which breaks them down into small particles. It can take 1000 years or more. Soil forms through a variety of soil formation processes, includes weathering rock “Parent Material” combined with dead and living organic matter and air. The parent material may be directly blow the soil, or great distance away if wind, or glaciers have transported the soil.
SOIL FORMING FACTORS:
1-
Time: How long the soil has
been forming.
2-
Parent material: eg Rocks, alluvium.
3-
Biotic factors: Plants, animals, microorganism
4-
Topography: Slope position, aspect shape and
amount
5-
Climate: Temperature, moisture and
seasonal distribution.
COMPOSITION OF SOIL: Soil is composed of rocks and mineral particles of many sizes mixed with water, air and living things, both plant and animal and their remains. In an ideal soil, air and water fill the pore spaces, and compose about 50% of the volume, organic matter about 1 to 5 %, of the soil volume, and mineral matter accounts for the remaining 45 to 49 %.
The relative proportions of these four soil components vary
with soil type and climatic conditions. The %age distribution of soil
composition materials is given bellow.
In brief soil contains!
A-
40% - 50% minerals
B-
0 % - 10 % biological
1-
Flora and fauna
2-
Live and dead (organic
matter)
3-
Macroscopic and microscopic
C-
50 % pore spaces
1-
Air
2-
Water
SOIL SEPARATES: Soil separates are mineral particles
ranging b/w specified size limits. The larger sizes are describes as coarse;
intermediate as medium, and the smaller as fine. The names and size limits of
major categories of separates recognized are:
Soil separate
|
Equivalent diameter
size (mm)
|
Gravel
|
> 2 mm
|
Sand
|
0.05-2 mm
|
Very course
|
1-2 mm
|
Coarse
|
0.5 – 1 mm
|
Medium
|
0.25 0.5 mm
|
Fine
|
0.1– 0.25 mm
|
Very fine
|
0.5 – 0.1 mm
|
Silt
|
0.002 – 0.05 mm
|
Clay
|
0.002 mm
|
SPECIFIC SURFACE AREA SOIL SEPARATES: The different soil separates have different surface areas. The surface area of soil can give insight understanding of several soil properties like fertility and water holding capacity. The smaller the particle size, the greater the total surface are in the given volume of soil. Therefore when comparing the three soil separates, clay has the greater surface area per gram, silt is second, and sand has the least. The greater the surface area of soil, the greater its capacity of holding water and nutrients is. A low total area is one of the primary reason why sandy soils tend to be drier and less fertile compared to finer textured soils. Think of a large boulder. It has a specific mass and a specific surface area. If you were to break up this boulder, it would it have the same mass, but the surface area would be greater. With each broken piece, more surface area is composed, and the sum of these individual surface area is greater than the intact boulder’s surface area. For example, if soil separate has spherical shape with radius “r” and particle density “ρ”, then Surface area (a) = 4πr2 and mass (m) = ρsV = ρs V = ρs (4πr3 /3) Specific surface area of aggregate (s) is therefore equal to = area/mass = a/m = 4πr2 / ρs(4πr3 /3) = 3/ ρs r.
Thus specific area is inversely proportional to the radius of soil separate.
Specific
surface area of soil particles
Particle
|
Efficiency
Diameter (cm)
|
Mass (g)
|
Area (cm2)
|
Specific surface area
(cm2/g)
|
Gravel
|
2×10-1
|
1.13×10-2
|
1.3×10-1
|
11.1
|
Sand
|
5×10-3
|
1.77×10-7
|
7.9×10-5
|
444.4
|
Silt
|
2×10-4
|
1.13×10-11
|
1.3×10-7
|
11.1×104
|
Clay
|
2×10-4
|
8.48×10-15
|
6.3×10-8
|
7.4×106
|
SOIL
TEXTURE: The proportion of the different size soil separates making
up a soil determines its soil texture. It is soil property used to describe the
relative proportions of the various soil separates in a soil. It has a large
influence on water holding capacity, water conducting ability and chemical soil
properties. It is determined by separating the amount of soil separates i.e.
sand, silt and clay in a soil and determining the % of each. If these
properties are known, a soil can be classified according to texture, using a
texture triangle. The three sides of textural triangle represent increasing or
decreasing percentages of sand, silt and clay particles. When the percent of
sand, silt and clay are added together they equal 100%.
Textural
Triangle
SOIL
STRUCTURE:
Soil Structure refers
to the arrangement of soil separates into units called aggregates. An aggregate
possesses solid and pore space. Aggregates are separated by plane of weakness
and are dominated by clay particles. Silt and fine particles may also be a part
of aggregate. The arrangement of soil aggregates into different forms give a
soil its structure. The natural processes that aid in the formation of
aggregates are wetting and drying, freezing and thawing, microbial activity
that aids in the decay of organic matter, activity of roots and all animals,
tillage etc. the term structure-less soil refers to some structure that do not
have definite shapes such as single-grain soils (all particles act alone e.g.,
sand) and massive soils (all particles act such as a unit e.g., hardpan).
Soil
structure classes are differentiated on the basis of their aggregate size,
shape, arrangement etc. selected soil structure classes are:
1-
Platy: Soil structure that have
long horizontal axes. They are generally flat, but may be curved upward. They
are generally located at soil surface or parent material interface.
2- Blocky: Soil structures that have cube like shapes with equal height, length and depth axes. Types of blocky structures are:
A-
Angular blocky: Has a sharp, distinct
sides, corners and edges.
B-
Sub-angular blocky: has rounded sides, corners
and edges.
3-
Spheroids: Soil structures that have
round, spherical shapes. Types of spherical structures:
A-
Granular: hard can’t be crushed easily
B-
Crumb:
Soft, can be crumbled with hard pressure.
4-
Prismatic: Soil structures that have
long vertical axes. Types of prismatic structures;
A-
Prism: have sharp corners, sides.
Tops and bottoms.
B-
Columnar: have rounded, corners,
sides, tops, and bottoms.
Granular
and crumb structures are usually at the soil surface in the A horizon. The
subsoil predominately the B horizon, has sub-angular blocky, blocky, columnar
or prismatic structure. Platy structure can be found in the surface or subsoil
while single grain and structure-less structure are most often associated with
the C horizon.
Soil
structure is sometimes describes as ‘weakly structure”, “well structured”,
“good structure” or “bad structure” etc. The first two indicate the grade of
structure development. The last two refer to qualities important to crop
production. Structure, unlike texture, is modified by human practices.
DEFINITION’S
1-Pore
space:
pores result from the irregular shape of soil particles or from pushing and
aggregation forces. When pore space is very small, soil retain water and impede
drainage, resulting in poor aeration. Pore size and pore space connectivity in
form of micro joints is more important than total pore space.
2-Soil
moisture / water content (Ɵm): soil moisture or water content (Ɵm)
is a measure of the amount of water present in the soil. It is defined ratio of
mass of water (Mw) in soil to the mass of dry soil particles (Ms).
It is often referred as a gravimetric water moisture content.
Mathematically:
Ɵm = Mw / MS ×
100 (%)
The
natural water content for mist soils would be well below 100%, but organic
soils and some marine clay soil can have water contents greater than 100%.
3-Volumetric Water Content (ƟV):
Volumetric water or moisture content is measure of the volume of water resent
in the soil. It is defined ratio of volume of water (VW) in soil to
the total volume of sample (VT).
Mathematically:
ƟV =
VW / VT × 100 (%)
4-Void Ratio: (e)
It is defined as ratio of volume of voids (VV)
to the volume of solids (VS) .
It can be expressed as:
e = Vv / VS
5-Air Ratio (α)
It is defined as the ratio of volume of air to that
of the solids. It has relevance to plant growth and engineering applications.
α = Va / Vs
6-Prosity: (n)
Soil porosity is the ratio of
volume of voids (VV) to the total volume (VT) of soil
sample.
n = VV / VT × 100 (%)
7-Air filled porosity: (na)
It refers to the ratio of volume of air-filled pores
to that of total volume of sample.
na = Va / VT × 100 (%)
8-Degree of Saturation: (S)
The degree of saturation (S) is a measure of the
void volume that is filled by water, expressed as a percentage ranging from 0
to 100. It is defined as:
S
= VW / VV × 100 (%)
For a completely dry soil S=0%, and for a soil where
the voids are completely filled with water (saturated soil) S = 100%. Soils
below the water table are often saturated.
9-Unit weight: (ϒ)
Unit weight of a soil is simply the weight per unit
volume. However, because of the different phases present in the soil, several
forms of unit weight are used in soil physics.
1-
Bulk unit weight: (ϒb) The most
common one is the bulk unit weight (ϒb), which is also known as
total, wet or moist unit weight. It is the total weight divided by the total volume, and is
written as:
ϒb = WT / VT
2-
Dry unit weight: (ϒ) it is ratio of dry
weight of soil to total volume of soil.
ϒd= WS / VT
3-
Soil Density: (ρ) Density is the ratio
of mass and volume. It I commonly expressed in the units of g/cm3
and kg/m3. Density is defined an four ways as follows:
A-
Particle density (ρs): It is also
called the true density, and is the ratio of mass of solid (MS)
divided by the volume of solid (VS):
ρs = MS / VS
Particle density of inorganic soils ranges from 2.6 to
2.8 g/cm3. Note that density of organic matter is about half of that
of the inorganic mineral.
B-
Specific gravity: (G)
Specific gravity is defined as the ratio of the density of material to that of
water 40C and at Atmospheric Pressure. Since the density of water at
standard temperature is unity, the specific gravity is numerically equal to
density.
G = ρs / ρw
C-
Bulk density (ρb): It is also
called the apparent density, and is the ratio of mass of solid (MS)
and water (MW) to the total volume (VT). Its units are
also that of mass/volume as g/cm3 or kg/m3.
ρb = MT /
VT = MS + MW / VS + VW +
Va
Bulk density is
always less than particle density b/c for bulk density the volume of the
solids and the
voids are considered.
D-
Dry density expresses the
ratio of the mass of dried soil to its total volume.
Mathematically:
ρd = MS
/ vT = Ms / VS+VW+Va
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