Chapter 4
Soil properties and soil mineralogy
General properties of soil
Soil is generally comprised of minerals, organic
matter, water, and air. The composition and proportion of
these components influence on soil properties.
Soil properties for civil
engineering concentration
can be divide in 2
important properties as
1. Physical properties
2. Chemical properties
Physical properties of soil
The physical properties of soils are
1. Texture 5. Consistency
2. Structure 6. Temperature
3. Density 7. Color
4. Porosity
1. Texture
The components of soil, sand, silt and clay, determine a soil's texture as illustrated
by USDA (United States Department of Agriculture) textural classification triangle.
2. Structure
The clumping of the soil textural components of sand, silt
and clay forms aggregates and the further association of those
aggregates into larger units forms soil structures called “Peds”.
Shape and arrangement of peds
2.1. Platy structure looks like stacks of dinner
plates overlaying one another. Peds are flattened one atop
the other 1-10 mm thick. Platy structure tends to impede
the downward movement of water (Lake sediment) and
plant roots through the soil.
2.2. Prismatic and columnar are long in the
vertical dimension, 10-100 mm wide. Prismatic peds have
flattops, columnar peds have rounded tops. Tend to form
in the high sodium soil where clay has accumulated.
2.3. Blocky (Angular and subangular) are
imperfect cubes, 5-50 mm, angular have sharp edges,
subangular have rounded edges. Tend to form in the clay
has accumulated and indicate poor water penetration.
2.4. Granular and crumb are spheroid peds of
polyhedrons, 1-10 mm, often found in the presence of
organic material. Crumb peds are more porous and are
considered ideal.
2.5. Massive soil has no visible
structure which is hard to break apart
and appears in very large clods.
2.6. Single grained soil consists of
individual particles that do not stick
together, common in very sandy soils.
Significance of soil structure
Soil structure also determines the amount and
arrangement of empty spaces in the soil, and thus has great
influence on how readily water moves through the soil. When the
soil is saturated, water moves down the cracks and pores in the soil.
Shape of the peds influences the path water must take to
get through the soil. Example in prismatic peds, the long direction
is vertical and the cracks between peds are mainly vertical, the
same direction that the water can move easily.
3. Density
- Bulk density is the ratio of mass of dry soil per all unit volume of soil.
- Particle density is the ratio of mass of dry soil per unit volume of solid.
- Particle density is the density of only the mineral particles that make up a
soil, it excludes pore space and organic material.
- When Water is the reference (1.0 g/cm3), averages approximately standard
value for mineral particle density in soils is 2.65 g/cm3
4. Porosity
Pores (voids) are spaces not occupied by soil solids (mineral or organic)
There are four categories of pores:
1. Very fine pores : < 2 pm
2. Fine pores : 2 - 20 pm
3. Medium pores : 20 - 200 pm
4. Coarse pores : 200 pm - 0.2 mm
5. Consistency
Consistency is the ability of soil to stick to itself and its ability to
resist deformation and rupture
Consistence — soil’s response to mechanical forces
- Resistance to rupture
+Soft/hard when dry or friable (crumbly) when wet
-Plasticity
+Tolerate considerable deformation W/out breaking
- Stickiness
+ which the soil is manipulated or even walked on
Consistency is generally measured at three moisture
conditions: dry, moist and wet; and in those conditions the
qualities depend upon the clay content.
1. Consistency of Dry Soil: loose, soft, slightly hard, hard,
very hard, extremely hard
2. Consistency of Moist Soil: loose, very friable, friable,
firm, very firm, extremely firm
3. Consistency of Wet Soil: nonsticky, slightly sticky, sticky,
very sticky; nonplastic, slightly plastic, plastic, very plastic
6. Temperature
Heat Capacity of the soil is the amount of heat (calories)
needed to raise 1 g of soil 1 degree Celsius. It is a function of
texture, organic matter and moisture content. Finer textured
soils (e.g., clays, clay loams) have higher heat capacity than
coarse textured soils (e.g., sand).
Heat Capacity is highly dependent upon water content:
Heat Capacity of water is (1.0 cal '1) about 5 times reater
S 2
than that of the soil itself (0.2 cal g'1).
Thermal conductivity of soil refers to the movement or
penetration of thermal energy into the soil profile.
Conductance is also strongly affected by texture
(increases with finer texture), organic matter (lowers
with increasing organic matter), and water content
(increases with increasing water content).
When soil water content is high enough
to bridge gaps between particles, further
increases in soil moisture have little effect upon
conductance.
Because water has higher heat capacity
and higher thermal conductivity than soil
minerals, Wet soils are harder to heat up initially,
but heat to deeper depths than dry soils.
Dry soils tend to get very hot at surface.
Moist soils are usually cooler than dry soils
because of their high specific heat, even though
conductance is also greater in moist soils.
Rain and irrigation water can also cool or
warm soil quickly, depending upon the
temperature of the incoming water compared to
the soil.
7. Color
Soil color is primarily influenced by soil mineralogy. Many
soil colors are due to various iron minerals. The development and
distribution of color in a soil profile result from chemical and
biological weathering.
As the primary minerals in soil parent material weather, the
elements combine into new and colorful compounds. Iron forms
secondary minerals of a yellow or red color, organic matter
decomposes into black and brown compounds, and manganese, sulfur
and nitrogen can form black mineral deposits
Soil color can be related to soil physical and chemical properties
Dark soils are often (but not always) enriched in OM.
colors can be associated with salts or carbonates.
Spots of different color (often rust colored) called
mottles can indicate periods of poor drainage.
Bluish, grayish, or greenish colors in subsoil indicated
prolonged periods of poor drainage.
Chemical properties, colloidal and mineralogy of soil
The main important on chemical properties of soils are soil
chemistry also determines its corrosivity, stability and ability to filter
water. It is the surface chemistry of clay and humus colloids that
involve soil's chemical properties.
A colloid is a small, insoluble and nondiffusible particle larger
than a molecule but small enough to remain suspended in a fluid
medium without settling. Most soils contain organic colloidal particles
called humus as well as the inorganic colloidal particles of clays.
The very high specific surface area of colloids and
their net negative charges, gives soil its great ability to hold
and release cations in What is referred to as cation exchange.
Cation-exchange capacity (CEC) is the amount of
exchangeable cations per unit weight of dry soil and is
expressed in terms of milliequivalents of hydrogen ion per
100 grams of soil.
Cation-exchange capacity (CEC)
Soil type CEC (cm0l(+)/kg) Compound CEC (cm0l(+)/kg)
Sand 1 — 5
fine sandy loam 5 - 10
loam & silt loam 5 - 15
clay loam 15 - 30
clay > 30
Humus 100 — 300
Vermiculite 80 - 150
Montmorillonite 60 - 100
Kaolinite 3 - 15
Oxide 0 - 3
Mineral Colloids (Soil clays)
Due to its high specific surface area and its unbalanced
negative charges, clay is the most active mineral component of
soil. It is a colloidal and most often a crystalline material. In soils,
clay is defined in a physical sense as any mineral particle less than
2 um (8>