Lal R., Shukla M.K. Principles of Soil Physics

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FIGURE 20.6 (a) Flood irrigation

without adequate drainage, (b)

Irrigation used for cotton-wheat

rotation in central Asia may lead to

secondary salinization if subsoil has

high salts, water is of poor quality, and

drainage is inadequate.

Soil salinity is used to designate a condition in which soluble salt concentration of soil

reaches a level that is harmful to crops. Moderate salinity can often go undetected

Principles of soil physics 594

because it causes no apparent injuries other than restricted growth. Leaves of plants

growing in salt infested areas may be smaller and darker blue to green in color than the

normal leaves. Salinity causes increased succulence, especially for a high concentration

of chloride ions in the soil solution. The appearance of plants in salt-affected soils and

moisture stress (drought) conditions is almost similar. The wilting of plants

FIGURE 20.7 (a) A saline soil with a

high water table, (b) Salt accumulation

in a poorly drained depressional land

in Haryana, India.

Freezing and thawing effects 595

is far less prevalent because the osmotic potential of the soil solution usually changes

gradually and plants adjust their internal salt content sufficiently to maintain turgor and

avoid wilting.

Symptoms of specific element toxicities, such as marginal or tip burn of leaves, occur

as a rule only in woody plants. Chloride and sodium ions and boron are the elements

most usually associated with toxic symptoms. Nonwoody species may often accumulate

as much or more of these elements in their leaves without showing apparent damage, as

do the woody species.

20.4.1 Osmotic Pressure

The water molecules are dipole and other ions in the solution are attracted to them by the

electric field to form clusters. The presence of solutes affects the thermodynamic

properties of water and lowers the potential energy. Consider two compartments, one

containing pure water and the other a solution, that are separated by a membrane

permeable to pure water and impermeable to solute. The pure water will continue to cross

over into the solution side, unless stopped by an opposing force. If the compartment on

the solution side is a flexible diaphragm type, then the pure water entry will expand it.

This will result in a rise in hydrostatic pressure that will eventually stop the flow of pure

water into the solution compartment. The hydrostatic pressure at equilibrium is known as

osmotic pressure (Π, erg cm

−3

) of a solution and for a dilute solution it is expressed as

follows

Π=C

(20.23)

where C

is concentration of solution (moles cm

−3

), R is universal gas constant (8.32×10

erg moldeg

−1

), and T is absolute temperature (K). The solute potential differences tend to

become uniform through the system owing to the process of diffusion and do not affect

the value of any other soil water potential components at equilibrium. However, when

membrane or diffusion barriers are present within the soil-water system and solute

system, the solute potentials need to be included in the analysis of potentials. In order to

further explain the osmotic pressure potentials and other components of soil water

potentials, let us consider a vessel filled (Fig. 20.8) with a solution of osmotic pressure

(or π) and a capillary tube filled with pure water. One end of the capillary has a perfect

semiinfinite membrane, that restricts flow of solute into a capillary but allows flow of

water into a capillary. The capillary risestops at equilibrium at height h.

Principles of soil physics 596

FIGURE 20.8 Schematic of a

capillary tube at equilibrium with a

solution of osmotic pressure (π).

At point A

The datum z=0 and P

atm

Evaluating the components of total soil water potential (Φ

)

Gravitational pressure potential: Φ

=0 because z=z

Air pressure potential: Φ

=0 because P=P

Matric potential: Φ

=0 because no soil is present

Hydrostatic pressure potential: Φ

=0 because no hydrostatic

pressure

Solute potential: Φπ=−π (by definition)

Therefore, Φ

=Φ

+Φ

−Φ

Or, Φ

=−π at point A

(20.24)

At point B

=0 because P=P

atm

=0 because pure water

=ρ

−P

=−2σ/R because the contact angle is zero, σ is

surface tension

Therefore, Φ

,=ρ

gh−2σ/R

(20.25)

Since the soil water system in the vessel and capillary is in equilibrium, Eqs. (20.24) and

(20.25) are equal

gh−2σ/R=−π

(20.26)

h=2σ/ρ

gR−π/ρ

(20.27)

Freezing and thawing effects 597

The equation shows that due to solute potential the rise of water in the capillary is smaller

than without it. If solute potential becomes larger than 2σ/R, there will be no capillary

rise.

20.4.2 Effects of Salinity on Water Movement

Darcy’s law states that the soil water flux is the product of hydraulic conductivity and the

driving force. The driving force consists of gravitational and pressure potentials for solute

free soils. For soils containing salts, the osmotic potential gradient is the additional

driving force for water movement through a semipermeable membrane by restricting the

flow of solutes and at the same time allowing the flow of water. For the situations where

solute flow is totally restricted the total hydraulic head is sum of all the three driving

forces (i.e., gravitational, pressure, and osmotic) and flux of water (q) for a soil of

hydraulic conductivity K can be given as follows

(20.28)

where Φ

,=Φ

+Φ

, ρ

is the density of solution, g is acceleration due to gravity, and x is

the distance along the direction of flow. When solutes are restricted to movement relative

to the water solvent, such a phenomenon is known as salt sieving. For field situations, a

total restriction of solute particles from flow seems unrealistic; therefore, an osmotic

efficiency factor (F

) is introduced in Eq. (20.28), which changes to

(20.29)

Experimental studies have demonstrated that F

is close to zero under saturated

conditions. However, for unsaturated conditions at high suction values, F

becomes

significant and is reported as 0.03 for suction of 0.25 to 1 bar (Letey, 1968). The solutes

have a profound influence on the saturated soil hydraulic conductivity (K

) because

aggregates tend to collapse by the dispersion of clay, which also blocks the interaggregate

pores, and high exchangeable sodium percentage and low salt concentrations cause

swelling and dispersion of clay—both of which ultimately reduce the K

of soil. The

negatively charged clay particles form a diffuse double layer by attracting cations. When

the solution concentration is less than 200–400 meql

−1

, this process of imbibition causes

swelling in soils, which reduces the osmotic pressure difference between the soil solution

(or more appropriately ambient solution, which is the soil solution away from soil

particles) and clay particle, and weakens interparticle bond (McNeal, 1974). This results

in dispersion of clay and reduction in K

of soil.

20.4.3 Leaching Requirement

In arid regions where irrigation with water containing salts is applied to crops, the twin

processes of evaporation and transpiration results in rise in salt concentration in the root

zone. On the other hand, if a shallow groundwater table exists in the area, then salt is

Principles of soil physics 598

brought in the root zone by the process of capillary rise. The excess salt present in the soil

is removed by leaching, which is a process in which the optimal quantity of water equal

to the leaching requirement is applied to the field and allowed to flow through and past

the root zone so that excess salts are removed (Richards, 1954). Leaching may result in a

slight increase in soil pH by lowering of salt concentration, but saline soils rarely become

strongly sodic upon leaching. Unless the water table is very deep and lateral movement of

water fast, the process of leaching can cause water table buildup. Therefore, an adequate

drainage system is a necessity for leaching. Leaching requirement (LR) is defined as the

fraction of irrigation water that must be leached out from the bottom of root zone to keep

soil salinity level within a specific limit (usually 4dSm

−1

). LR depends on the

evapotraspiration, salt tolerance of crops, and salt content of soil profile and irrigation

water. The LR can be obtained by first making a salt balance, which is the total salt input

and output for a given volume or depth of soil as follows:

)+M

−(M

+ρ

)=∆M

(20.30)

where V and c are the volume of water entering or leaving the soil root zone (per unit

surface area or equivalent depth) and concentration (EC), respectively, subscript r, i, and

g are for rainfall, irrigation, and groundwater, respectively. M

and M

are the mass of

salts from soil and soil amendment or fertilizers, M

and M

are mass of salt precipitated

and removed by crop, V

and c

is the volume of water drained from soil and

concentration, respectively, and ∆M is the total change in mass of salt. Disregarding the

changes in salt balance in soil profile by precipitation, agricultural inputs,

evapotraspiration, drainage, and groundwater or capillary rise, Eq. (20.22) is simplified as

follows:

(20.31)

Equation (20.31) is for the steady state conditions where water content and salinity of soil

profile is constant, and no precipitation or dissolution of salt is taking place.

−V

(20.32)

where V

is volume of evapotranspiration. Transferring Eq. (20.24) into (20.23)

=(V

−V

(20.33)

(20.34)

or in terms of depth of irrigation water (d

), equivalent depth of evapotranspiration by

crop (d

), EC of drainage (s) and irrigation water (i), the equation can be written as

follows (Richards, 1954)

Freezing and thawing effects 599

(20.35)

Equation (20.35) suggests that by varying the amount of water for leaching the

concentration of salts in root zone can be reduced to the desired level.

20.5 SOIL WATER REPELLENCY

Water repellency is defined as a phenomenon of repulsion of water by soil particles. Soil

hydrophobicity, also called “water repellency or non-wetting,” reduces the affinity of soil

for water. Hydrophobicity can reduce the infiltration capacity of a soil to the extent that

the soil does not wet up even after weeks of being in contact with water. This can lead to

inhibited plant growth, increased overland flow and accelerated soil erosion, uneven

wetting patterns, and preferential flow generation. Hydrophobicity is known to vary

temporally, being generally most extreme after long dry periods and reduced or absent

after long wet spells (DeBano, 2003).

Water repellency is mostly associated with organic matter and its decomposition,

particularly where fungi growth is involved. Exudates and biomass produced by plant

roots and soil microbes can alter the surface characteristics of soil particles and lead to

the development of hydrophobic particle surfaces that may reduce water transport and

retention. This type of organic coating does not necessarily require covering the entire

soil particle; just a partial covering can render it water-repellent. The degree of soil

hydrophobicity is most severe at the soil surface and within the top 5 cm of soil profile,

but can be as deep as 15 cm or have patches of a hydrophobic layer within the soil profile

(DeBano, 2003).

Water repellency has been a concern for both land managers and researchers since the

early part of the twentieth century. It is a soil property with important repercussions for

plant growth, surface and subsurface hydrology, and soil erosion. It is generaly confined

to coarse-textured soils in regions with specific vegetation types and seasonally dries

climate and/or areas affected by fire. However, research conducted during the 1980s and

early 1990s showed that its occurrence is far more widespread. Water repellency can

occur at much lower levels or a localized scale in soil profile and can contribute to

preferential flow of water and nutrients. At low levels, repellency may not have a

deleterious impact on water retention and may even enhance microbial diversity through

the preferential alteration of soil pores by organisms. The hydrophobic substances

causing water repellency are also beneficial for conserving water by reducing the

capillary rise of water and the attendant evaporation, and leaching of nutrients. The water

repellency is often characterized in terms of wetting coefficients (C

=cos θ; where C

wetting coefficient and θ is contact angle) (Bahrani et al., 1970), surface roughness (Bond

and Hammond, 1970), and water surface tension and water-solid contact angles (refer to

Chapter 9; Watson et al., 1971).

Principles of soil physics 600

20.5.1 Wetting Pattern in Water-Repellent Soils

Dry soils are wetted when water is applied to them. A drop of water disappears and wets

soil because the force of attraction between soil particles and water results in loss of

cohesion in the latter, which lets it flow along the surfaces of particles. Once the

attractive forces between soil and water droplet are nonexistent, water remains as a

droplet and does not wet the soil. Before water starts infiltrating uniformly or percolating

inside the soil matrix, the presence of a continuous film of water over soil particle surface

is a prerequisite (Fig. 20.9).

The fundamental principle underlying the process of wetting shows that a reduction in

the surface tension of a solid (to be wetted) reduces its

FIGURE 20.9 Applied water droplets

make a film of water percolating in the

soil matrix due to suction gradients in

(a) wettable soil. It retains its shape as

a droplet in a (b) water-repellent soil.

TABLE 20.2 Water Repellency Classes

WDPT (seconds) Repellency class Water repellency

<5 0 Non-repellent

5–60 1 Slightly repellent

60–600 2 Strongly repellent

600–3600 3 Severely repellent

>3600 4 Extremely repellent

Source: Modified from Dekker and Ritsema, 2003.

Freezing and thawing effects 601

wettability, or a reduction in the surface tension of applied liquid increases the

wettability. The common method of classifying the water repellency is the empirical

water drop penetration time (WDPT). In this method, three drops of deionized water are

placed on a smoothened soil surface and the time over which drops are completely

absorbed is recorded. The time required for the drops to be absorbed depends on the

temperature of water and relative humidity of air. The increase in water temperature

reduces the surface tension and the time required for wetting. The increase in the relative

humidity of air increases the time for which the drops remain on the soil surface. The

water repellency classification given in Table 20.2 shows that soil is considered water-

repellent for WDPT >5 seconds, (Dekker and Jungerius, 1990; Dekker and Ritsema,

2003).

20.5.2 Effects of Water Repellency on Soil Processes

Water Infiltration

A water repellent soil does not get wet when water is applied under zero or negative

potential because contact angle is greater than 90°. Thus, a positive pressure must be

applied to force the entry of water into a soil. The value of the positive pressure depends

on the contact angle as well as pore dimension, and it increases with the contact angle

and decreases with the pore radius (Feng et al., 2001). The water content and the

attendant water pressure potential diagram (Fig. 20.10) with respect to time show that

non-water-repellent sand has a stable Richards-type imbibing front, slightly less saturated

than the total porosity of soil. The matric potential at the imbibing front for non-water-

repellent sand is negative. For a water-repellent soil, the matric potential behind the

imbibing front is slightly positive (Fig. 20.10b).

FIGURE 20.10 Schematic of matric

potential and moisture content for (a)

wettable and (b) water-repellent soil.

Principles of soil physics 602

The arrow points out to a negative

matric potential for wettable soil and

positive potential for repellent soil at

imbibing front. (Redrawn from Bauters

et al., 2003.)

In water-repellent soils, water movement is severely limited and the infiltration rates are

low. Most of the rainfall falling in a dry water-repellent soil may be lost as runoff.

However, as the dry water-repellent soil becomes wetter, the infiltration and water

movement gradually increases. The main difference between a hydrophilic and

hydrophobic soil is the shape of the wetting front. Infiltrating water in hydrophilic soils

forms an unconditionally stable horizontal Richards-type wetting front (refer to Chapter

13), whereas a hydrophobic soil forms an unstable front with fingers (Fig. 20.11).

Therefore, water distribution in the soil can have large variability with high water content

in the ectorganic layer (also known as “humus,” which protects the soil from erosion,

while enhancing aggregation) beneath which there can be a dry water-repellent layer,

which may be underlaid by a moist, less hydrophobic layer. The hydraulic conductivity

of water-repellent soil increases with depth of ponding (Carrillo et al., 2000).

Preferential Flow

The preferential transport of water and solutes can take place through soil matrix via

cracks formed in well-structured soils (such as clay or peat) due to shrink-swell

mechanism or biopores formed by soil fauna or the channels left behind by decayed

roots. In nonstructured sandy soils, the preferential

FIGURE 20.11 Irregular wetting in a

water-repellent soil.

flow can occur due to the formation of unstable wetting fronts, which can grow into

fingers because of the lateral diffusion (Ritsema et al., 1998).

Freezing and thawing effects 603