SOIL CONSISTENCY

Soil consistency
• the physical condition of a soil at a given water
content

• used to designate the manifestation of the physical

forces of cohesion and adhesion acting within the
soil at various moisture content

• The factor long recognized as the major

determinants of sod consistency is the soil's degree
of wetness generally as the mass of water present
per unit mass of solids.
 Harsh consistency

• the consistency when the sod is dry

• When dry, a soil body is likely to be relatively hard

and brittle and exhibit a high degree of
cohesiveness for internal cementation and a high
resistance to tillage.
 Friable consistency
• this is the consistency of the soil when moist (not dry,
nearly saturated condition).

• When tilled, soil tends to crumble easily and form a

loose assemblage of relatively small soft clods. In this
state, the soil is at or near its “optimal wetness” for
tillage, as it can be tilled to best advantage with the
least investment in energy.

• It prescribes the physical conditions of the sod suitable

for plowing.
Plastic consistency
• the consistency that allows molding of a soil
to any shape without breaking

• When worked, instead of crumbling into

clods, it tends to be molded (puddle) into
lumps, which upon drying become extremely
hard.
 Sticky consistency

• the consistency when the soil is wet (almost

at saturation)

• If worked, soil becomes a muddy paste.

Viscous-consistency

• this is the consistency of the soil when

fluid (more than the saturation).
 SOIL MOISTURE

• Water serves in the nutrition of plants acting

as:
• the universal solvent
• a nutrient itself a carrier of nutrients
• stabilizes the temperature of the soil
Polarity
• Helps explain how water molecules relate to each
other.

• Each water molecules does not act completely

independent but rather is coupled with other
neighboring molecules.

• The hydrogen or positive end of one molecule attracts

the oxygen of another molecule and resulting in a
polymer-like grouping. The angle of association
(105°) has the hydrogen atoms encourage an open
tetrahedral lattice structure.
• The net association in liquid is more tightly packed
than in ice thereby accounting for higher density of
liquid water as compared to ice.

• Polarity also explains why water molecules are

attracted to electrostatically charged ions as Na, Ca,
and K and to charge clay surfaces.

• Polarity of water molecules also encourage the

dissolution of salts in water since the ionic
components have greater attraction for water
molecules than for each other.
Hydrogen bonding

• it is a phenomenon by which hydrogen atoms act as

connecting linkages between water molecules.

• It account,-, for the polymerization and lattice

structure of water and for the relatively high boiling
point, specific heat and viscousity of water.
Cohesion and adhesion

Cohesion (attraction between like molecules)

Adhesion (attraction between unlike molecules)

By adhesion, sod particles hold water molecules that

in turn hold water molecules by cohesion.
Together, these factors make it possible for the
soil solids to retain water and control its movement
and utilization.
Surface tension

• it is a phenomenon commonly evidenced at

liquid-air interfaces and results from a greater
attraction of water molecules for each other than
for the air above.

• The net effect is an inward force at the surface

which causes water to behave as if it's surface
were covered with a stretched elastic membrane.
Expressions of soil water content:
• relative to the mass of the dry soil (m)

This is given as:

Mw
m = _____
Ms

m – is the moisture content relative to the mass or weight of the soil

Mw - mass of water and Ms - mass of dry soil

 relative to the total volume of the soil (v)
Vw
v = ___
Vt

v - is the moisture content relative to the total

volume of soil
Vw – is the volume of water computed on the basis of
water removed until oven dryness
Vt – total volume of the soil
The relationship between the two expressions is
given as:
w
v = m x _____
b

where b – bulk density, w – density of

water which is approximately 1.0 g/cm3
 relative to the depth of the soil (hw)

hw =  v x hs

where: hw = depth or height of water

hs = depth of soil
 relative to the volume of pore spaces (% S)

Vw
%S = ___ x 100
Vp

where: %S – degree of saturation

Vw – volume of water
Vp – volume of pore spaces in the soil
 Energy status of soil water
• The retention and movement of water in soils, its
uptake and translocation in plants and its loss to the
atmosphere are all energy-related phenomena.

• Different kinds of energy are involved including

potential, kinetic and electrical.
• However, to characterize the energy status of water,
the term “free energy” is used.
• Free energy is a sort of summation of all other forms
of energy.
• its level in a substance is a general measurement of
the tendency of that substance to attract soil water to
move or to otherwise change its status that is related
primarily to difference in energy levels from one soil
zone to another.

• The movement is from a zone where the free energy

of water is high to one where the free energy is low.
• Knowledge of the energy levels at various points in a
soil makes possible prediction of the direction of water
movement and gives some idea as to the forces to
which the water is subjected.
1) Matric force – refers to the attraction of the soil
solids (matrix) for water. This would markedly reduce
the free energy of adsorbed molecules and even
those held by cohesion.

2) Osmotic force – refers to the attraction of ions and

other solutes for water. This force also tend to reduce
the free energy of soil solution.

3) Gravitational force – refers to the force acting on

soil water, which tends to pull the water downward.
The free energy of soil water at some lower elevation.
It is this difference in free energy level, which causes
water to flow.
Maximum water holding capacity
• as water enters the surface of the soil due to the
heavy rain or application of irrigation water, air is
displaced from the dry soil and pores large or small
start to fill up.

• Continued addition of water to the surface results in

decreased downward movement of water. When all
pores are filled with water the soil is considered to be
its maximum water holding capacity. At this point,
suction is expected to be zero or near zero.
Field capacity
• field capacity may be defined as the maximum amount
of moisture the soil can hold after drainage in a given
period of time.

• The water have move away from the big pores

(macropores) and only the medium and small pores
(micropores) contain water. Water movement does not
stop, but such movement will be very slow to be
perceptible.

• Generally this moisture equivalent is attained at 1/3

bar.
 Wilting coefficient (WC)
or
Permanent wilting point (PWP)

• Wilting coefficient represents the moisture content of the

soil below which plants growing will wilt because the sod
can no longer supply at a sufficient rate to maintain turgor.

• For most soils, the suction, the suction of 15 bars, the

amount of available water (AW) may be readily be
estimated by subtracting the amount of water the soil
retains at 15 bars (WC) from the amount of water the soil
retains at 1/3 bar (FC). It is mathematically given as: AW=
FC-WC.
Hygroscopic coefficient (HC)
• HC is the Soil moisture constant representing the
condition wherein water is held so tightly and
appearing as a very thin film of water around a soil
particle and may move only through vapor transfer.

• The suction of water at this condition is about 31

bars.
Gravimetric Method

• this is the most commonly used method to measure

soil water content by mass.
• A sample of moist with known weight, usually taken
in cores from the field, is dried in an oven at a
temperature of 105-110°C for at least 24 hours or
until it has attained constant weight.
The moisture content can be calculated using the formula:

FW-ODW
m = ____________
ODW

where:

FW – is the fresh weight of the soil or the weight of the oil prior
to oven drying

ODW – weight of the soil after oven drying or the weight of the
moisture-free soil.
Resistance method
• this method takes advantage of the fact that the electrical
resistance of certain porous materials such as gypsum,
nylon and fiberglass is related to their water content.
When these blocks with suitably embedded electrodes are
planted in moist soil, they absorb soil moisture until
equilibrium is reached.

• The electrical resistance in the blocks is determined by

their moisture content and in turn by the tension or suction
of water in the nearby soil. The relationship between the
resistance reading the soil moisture percentage can be
determined by calibration.
 Neutron scattering

• it is a method of determining soil moisture in the

field. The neutron moisture is based on the
principle that hydrogen is relatively unique in its
ability to drastically reduce the speed of fast-
moving neutrons and to scatter them. The number
of slowed down neutrons is equal to the number of
water molecules with which they collide
Y- attenuation/scattering

• this is similar to that of the neutron

scattering, only that in here gamma rays
are used in place of neutrons.
 Heat Pulse method

• a heat pulse instrument equipped with

thermistor or transistor embedded in a ceramic
block is used. The temperature is then recorded
versus time and the rate of the decrease
temperature is a function of water content.
 METHODS FOR
DETERMINING SOIL WATER
POTENTIAL
Field tensiometer
• it measures the tension with which water is held
in soils. Its effectiveness is based on the
principle that water in the tensiometer is in
equilibrium through a porous cap with adjacent
soil water and the suction in the soil is the same
as the suction in the tensiometer.

• Maximum suction where tensiometer remained

useful is about 1 bar.
Resistance block

• the electrical resistance in the blocks is

determined by their moisture content and
in turn by tension or suction of water in the
nearby soil.
Suction method
• this makes use of tension plate and pressure
membrane apparatus. Tension plate apparatus is a
form of tensiometer used under laboratory
conditions.

• A core of soil is placed on a porous plate to which

suction is applied. The sample is weighted and the
relationship between suction and the soil content
is determined. The range of suitability of this
apparatus is from 0 to 1 bar only.
 Thermocouple psychrometer
• a thermocouple psychrometer consists of a double
junction of two dissimilar metals which when
subjected to different temperature would generate
voltage differences.

• One junction of which is equilibrated with the soil

atmosphere and the other is kept in an insulated
medium to provide temperature lag. The difference
in temperature is indicative of soil moisture
potential.
 Filter paper method

• in the technique, pF of the soil will be determined from

the known water characteristics of the Whatman no. 42
filter paper which had been soaked in 0.005% hgC12 and
dried to protect from fungal attack.
Biological Classification of Soil Water
1) Superfluous water – The water that occupies the
large pore space and would readily drain
downward due to gravity.

2) Available Water – it is the portion of water in a

sod that can be readily absorbed and assimilated
by plant roots.

3) Unavailable water – the water that is used by

the fine pore spaces with high energy of
retention.
Physical Classification of Soil Water
1) Free or gravitational water – water that moves into,
through or out of the soil due to the influence of gravity.
This corresponds to the superfluous water under the
biological classification.

2) Capillary water – water held in the capillary or fine pores

in the soil. This form of water corresponds to the
available water-under the biological classification.

3) Hygroscopic water – This also corresponds to the

unavailable water under the biological classification.
 SOIL AERATION

• Soil aeration involves the exposure of the soil air,

with gases moving both into and out of the soil.

• It determines the rate of gas exchange with the

atmosphere, the proportion of pore spaces filled
with air, the composition of the soil air, and the
resulting chemical oxidation or reduction potential
in the soil environment.
 Composition of Soil Air
• The composition of soil air is similar, but not identical
to the atmospheric air. It is similar since some soil
pores are open to the atmosphere through which
atmospheric gases may enter and move through. It 'is
not identical because movement of soil air is slow and
soil organisms would influence the composition of soil
air.

• Soil air is located in the mares of soil pores separated

by soil solids not occupied by water. The soil air has
higher moisture content than the atmosphere air.
Factors affecting soil air composition

• Microbial activity
• Vegetation
• Soil water content
• Drainage condition
• Soil layering
• Soil texture
 Gaseous exchange in soil

• Convection or mass flow – the entire mass of air

streaming from a zone of higher pressure to one of
lower pressure. The moving force is the gradient of
the total gas pressure.

• Diffusion – movement of individual gas whereby the

moving force is gradient or partial pressure.
 Soil Temperature

• Heat is a form of energy while temperature is a

measure of the heat energy that a substance
contains.
• Soil temperature is a function of energy
exchange processes (radiant, thermal and
latent energy), soil moisture levels,
vegetation, soil color, specific heat capacity of
soil and thermal conductivity of the soil.
• Radiation – all bodies can lose or gain energy by absorption
or emission of radiant energy. The major source of heat
energy that a soil receives is in the form of radiant energy.

• Convection – it involves the movement of heat energy by a

heat- carrying mass such as wind or water.

• Conduction – it is the flow of energy within a soil or other

body due to internal molecular motion.

• Latent heat transfer – latent heat transfer is the loss or gain

in heat energy resulting from changes of state.
• It plays an important role in soil formation, weathering, soil
OM balance, soil chemical reactions, microbial activity and
plant growth.
• It controls the intensity of biophysical, biochemical and
microbiological processes that takes place in the soil.

• The rate of mineralization of OM, the physical processes of

diffusion and viscous flow, the germination of seed, root
growth and its activity in terms of water and nutrient
absorption and respiration are strongly temperature-
dependent.