🧫 Soil Water Retention and Moisture Constants

Retention of Water by Soil

The soils hold water (moisture) due to their colloidal properties and aggregation

qualities. The water is held on the surface of the colloids and other particles and in

the pores. The forces responsible for retention of water in the soil after the drainage

has stopped are due to surface tension and surface attraction and are called surface

moisture tension. This refers to the energy concept in moisture retention

relationships. The force with which water is held is also termed as suction.

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The water retained in the soil by following ways

1. Cohesion and adhesion forces: These two basic forces are responsible for

water retention in the soil. One is the attraction of molecules for each other i.e.,

cohesion. The other is the attraction of water molecules for the solid surface of soil

i.e. adhesion. By adhesion, solids (soil) hold water molecules rigidly at their soil

water interfaces. These water molecules in turn hold by cohesion. Together, these

forces make it possible for the soil solids to retain water.

Adhesion

Cohesion

2. Surface tension: This phenomenon is commonly evidenced at water- air

interfaces. Water behaves as if its surface is covered with a stretched elastic

membrane. At the surface, the attraction of the air for the water molecules is much

less than that of water molecules for each other. Consequently, there is a net

downward force on the surface molecules, resulting in sort of a compressed film

(membrane) at the surface. This phenomenon is called surface tension.

3. Polarity or dipole character: The retention of water molecules on the surface

of clay micelle is based on the dipole character of the molecule of water. The water

molecules are held by electrostatic force that exists on the surface of colloidal

particles. By virtue of their dipole character and under the influence of electrostatic

forces, the molecules of water get oriented (arranged) on the surface of the clay

particles in a particular manner.

Each water molecule carries both negative and positive charges. The clay particle

is negatively charged. The positive end of water molecule gets attached to the

negatively charged surface of clay and leaving its negative end outward. The water

molecules attached to the clay surface in this way present a layer of negative

charges to which another layer of oriented water molecules is attached. The

number of successive molecular layers goes on increasing as long as the water

molecules oriented. As the molecular layer gets thicker, orientation becomes

weaker, and at a certain distance from the particle surface the water molecules

cease to orientate and capillary water (liquid water) begins to appear. Due to the

forces of adsorption (attraction) exerted by the surface of soil particles, water gets

attached on the soil surface. The force of gravity also acts simultaneously, which

tries to pull it downwards. The surface force is far greater than the force of gravity

so water may remain attached to the soil particle. The water remains attached to the

soil particle or move downward into the lower layers, depending on the

magnitudeof the resultant force.

Potentials

Soil water potential :

The retention and movement of water in soils, its uptake and translocation in

plants and its loss to the atmosphere are all energy related phenomenon. The more

strongly water is held in the soil the greater is the heat (energy) required. In other

words, if water is to be removed from a moist soil, work has to be done against

adsorptive forces. Conversely, when water is adsorbed by the soil, a negative

amount of work is done. The movement is from a zone where the free energy of

water is high (standing water table) to one where the free energy is low (a dry soil).

This is called soil water energy concept.

Free energy of soil solids for water is affected by:

i) Matric (solid) force i.e., the attraction of the soil solids for water (adsorption)

which markedly reduces the free energy (movement) of the adsorbed water

molecules.

ii) Osmotic force i.e ., the attraction of ions and other solutes for water to reduce

the free energy of soil solution.

Matric and Osmotic potentials are negative and reduce the free energy level of the

soil water. These negative potentials are referred as suction or tension.

iii) Force of gravity : This acts on soil water, the attraction is towards the earth's

center, which tends to pull the water down ward. This force is always positive. The

difference between the energy states of soil water and pure free water is known as

soil water potential. Total water potential (Pt) is the sum of the contributions of

gravitational potential (Pg), matric potential (Pm) and the Osmotic potential or

solute potential (Po).

Pt = Pg + Pm + Po

Potential represents the difference in free energy levels of pure water and of soil

water. The soil water is affected by the force of gravity, presence of soil solid

(matric) and of solutes.

Soil moisture constants

Earlier classification divided soil water into gravitational, capillary and

hygroscopic water. The hygroscopic and capillary waters are in equilibrium with

the soil under given condition. The hygroscopic coefficient and the maximum

capillary capacity are the two equilibrium points when the soil contains the

maximum amount of hygroscopic and capillary waters, respectively. The amount

of water that a soil contains at each of these equilibrium points is known as soil

moisture constant.

The soil moisture constant, therefore, represents definite soil moisture relationship

and retention of soil moisture in the field.

The three classes of water (gravitational, capillary and hygroscopic) are however

very broad and do not represent accurately the soil - water relationships that exists

under field conditions.

Though the maximum capillary capacity represents the maximum amount of

capillary water that a soil holds, the whole of capillary water is not available for

the use of the plants. A part of it, at its lower limit approaching the hygroscopic

coefficient is not utilized by the plants. Similarly a part of the capillary water at its

upper limit is also not available for the use of plants. Hence two more soil

constants, viz., field capacity and wilting coefficient have been introduced to

express the soil-plant-water relationships as it is found to exist under field

conditions.

1. Field capacity : Assume that water is applied to the surface of a soil. With

the downward movement of water all macro and micro pores are filled up. The soil

is said to be saturated with respect to water and is at maximum water

holding capacity or maximum retentive capacity. It is the amount of water held in

the soil when all pores are filled.Sometimes, after application of water in the soil

all the gravitational water is drained away, and then the wet soil is almost

uniformly moist. The amount of water held by the soil at this stage is known as the

field capacity or normal moisture capacity of that soil. It is the capacity of the soil

to retain water against the downward pull of the force of gravity. At this stage

only micropores or capillary pores are filled with water and plants absorb

water for their use. At field capacity water is held with a force of 1/3

atmosphere. Water at field capacity is readily available to plants and

microorganism.

2. Wilting coefficient : As the moisture content falls, a point is reached when the

water is so firmly held by the soil particles that plant roots are unable to draw it.

The plant begins to wilt. At this stage even if the plant is kept in a saturated

atmosphere it does not regain its turgidity and wilts unless water is applied to the

soil. The stage at which this occurs is termed the Wilting point and the percentage

amount of water held by the soil at this stage is known as the Wilting Coefficient.

It represents the point at which the soil is unable to supply water to the plant.

Water at wilting coefficient is held with a force of 15 atmosphere.

3. Hygroscopic coefficient : The hygroscopic coefficient is the maximum amount

of hygroscopic water absorbed by 100 g of dry soil under standard conditions of

humidity (50% relative humidity) and temperature (15°C). This tension is equal to

a force of 31 atmospheres. Water at this tension is not available to plant but may be

available to certain bacteria.

4. Available water capacity : The amount of water required to apply to a soil at the

wilting point to reach the field capacity is called the "available" water. The water

supplying power of soils is related to the amount of available water a soil can hold.

The available water is the difference in the amount of water at field capacity (- 0.3

bar) and the amount of water at the permanent wilting point (- 15 bars).

5. Maximum water holding capacity : It is also known as maximum retentive

capacity. It is the amount of moisture in a soil when its pore spaces both micro and

macro capillary are completely filled with water. It is a rough measure of total pore

space of soil. Soil moisture tension is very low between 1/100 [th] to 1/1000 [th]

atmosphere or pF 1 to 0.

of an

6. Sticky point moisture : It represents the moisture content of soil at which it no

longer sticks to a foreign object. The sticky point represents the maximum

moisture content at which a soil remains friable. Sticky point moisture values vary

nearly approximate to the moisture equivalent of soils. Summary of the soil

moisture constants, type of water and force with which it held is given in following

table.

Soil water capacity

Moisture equivalent : It is defined as the percentage of water held by one

centimeter thick moist layer of soil subjected to a centrifugal force of 1000 times

of gravity for half an hour.

Soil moisture constants and range of tension and pF

S.No.	Moisture class	Tension (atm)	pF
1	Chemically combined	Very high	---
2	Water vapour	Held at saturation point in the soil air	---
3	Hygroscopic	31 to 10,000	4.50 to 7.00
4	Hygroscopic coefficient	31	4.50
5	Wilting point	15	4.20
6	Capillary	1/3 to 31	2.54 to 4.50
	Moisture equivalent	1/3 to 1	2.70 to 3.00
	Field capacity	1/3	2.54
	Sticky point	1/3 (more or less)	2.54
	Gravitational	Zero or less than 1/3	<2.54
	Maximum water holding capacity	Almost zero	---

Relationship between soil moisture and tension

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Summary Cheat Sheet

Quick Recall Points

• Field capacity and permanent wilting point define plant-available water.
• Gravitational water drains quickly; capillary water is most useful to plants.
• Texture and structure control retention behavior.

Exam Traps

• Total water content is not equal to available water.
• Hygroscopic water is held too tightly for most crop uptake.

References

2 sources • [1] [2]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

FAO Soils Portal

Website