See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/297737054

Soil Physical Properties

Chapter · November 2015

CITATIONS

4READS

44,012

3 authors, including:

Some of the authors of this publication are also working on these related projects:

Impact of organic and conventional farming practices on soil quality View project

Role of agricultural practices on soil physical conditions View project

V.K. Phogat

CCS Haryana Agricultural University

55 PUBLICATIONS   241 CITATIONS   

SEE PROFILE

Rita Dahiya

CCS Haryana Agricultural University

20 PUBLICATIONS   191 CITATIONS   

SEE PROFILE

All content following this page was uploaded by V.K. Phogat on 11 March 2016.

The user has requested enhancement of the downloaded file.

Chapter 6

Soil Physical PropertiesV.K. PHOGAT, V.S. TOMAR AND RITA DAHIYA

The Nation that destroys its soil destroys itself – Franklin D. Roosevelt,32nd President of the United States

6.1. IntroductionPhysical properties play an important role in determining soil’s suitability for agricultural,environmental and engineering uses. The supporting capability; movement, retentionand availability of water and nutrients to plants; ease in penetration of roots, and flow ofheat and air are directly associated with physical properties of the soil. Physical propertiesalso influence the chemical and biological properties. The most pertinent physicalproperties of soil relevant to its use as a medium for plant growth are discussed in thefollowing sections.

6.2. Soil TextureSolid phase of the mineral soil mainly consists of discrete mineral particles as the amountof amorphous material including organic matter is usually small. Mineral particles arenot exactly spherical but vary widely in their shape, therefore, these particles are usuallyclassified into three conveniently separable groups according to certain size range basedon their equivalent diameter (diameter of a sphere that has a velocity of fall in a liquidmedium equal to that of the specific particle). The groups of different size range ofmineral particles are known as soil separates, primary particles or textural fractions, namely:sand, silt and clay. Soil texture refers to the prominent size range of mineral particles,and is defined both qualitatively and quantitatively. Qualitatively, it refers to the feel ofsoil whether coarse and gritty or fine and smooth when rubbed between thumb andforefinger. Quantitatively, soil texture is the relative proportion of sand, silt and claycontent on weight basis. The term soil texture is often used interchangeably withmechanical composition of soil. It is more or less a static property affecting almost all othersoil properties. Land use capability and soil management practices largely depend on thetexture.

6.2.1. Classification of Soil ParticlesSoil particles of size less than 2 mm in diameter are included in the classification and areconsidered as soil material normally used in soil analysis. Some soils contain large sized

136 SOIL SCIENCE: AN INTRODUCTION

particles which may create problem in tillage operations but do not contributesubstantially to important soil properties. The particles greater than 2 mm are known asgravels (2-4 mm), pebbles (4-76 mm), cobbles (76-250 mm), stones (250-600 mm), and stilllarger (>600 mm) as boulders.

Several systems exist for the classification of soil particles but International Society ofSoil Science (ISSS), renamed as the International Union of Soil Sciences (IUSS), and theUnited States Department of Agriculture (USDA) are widely in use (Table 6.1). Someimportant properties of sand, silt and clay particles are described in Table 6.2. In addition,sand and silt particles consist of primary minerals such as quartz, feldspars and mica,while clay particles are mainly secondary minerals such as kaolinite, illite, vermiculite,montmorillonite, chlorite and hydrated oxides of iron and aluminium. Clay is a surfaceactive fraction with high degree of chemical and physical activities while relatively inertsand and silt fractions exhibit such activities to a lesser extent. The sand and silt may becalled the soil skeleton while the clay, by analogy, regarded as the flesh of the soil. All thethree fractions including the pore space in between form the matrix of soil.

Table 6.2. Important properties of sand, silt and clay particles

Particle Properties

Sand Visible to naked eye, generally spherical or cubical in shape, feel gritty, low water andnutrients holding capacity, loose when dry, very low plasticity and stickiness when wet.

Silt Not visible to naked eye, seen through an ordinary microscope, generally spherical or cubicalin shape, low to medium in capacity to hold water and nutrients, feel smooth, some plasticityand stickiness when wet.

Clay Visible only through an electron microscope, platy in shape, high water and nutrients holdingcapacity, hard when dry, high degree of plasticity and stickiness when wet, exhibit swellingand shrinkage behaviour.

Table 6.1. Systems of classification of soil particles according to their sizes

International Society of Soil Science (ISSS) United States Department of Agriculture (USDA)Particle Diameter (mm) Particle Diameter (mm)

Coarse sand 2.0-0.2 Very coarse sand 2.0-1.0Fine sand 0.2-0.02 Coarse sand 1.0-0.5Silt 0.02-0.002 Medium sand 0.5-0.25Clay <0.002 Fine sand 0.25-0.10

Very fine sand 0.10-0.05Silt 0.05-0.002Clay < 0.002

6.2.2. Mechanical AnalysisMechanical analysis or particle size analysis is a procedure of determining the sand, siltand clay contents in a soil sample. The primary soil particles, which are often aggregated,are separated and made discrete by removing the binding agents (organic matter, calciumcarbonate, soluble salts and oxides of Fe and Al) in soil-water suspension. The organicmatter is oxidized with H2O2 while CaCO3 and oxides of iron and aluminum are removedby treating the sample with dilute HCl and soluble salts by filtration with distilled water.Mechanical stirring is done to disperse the clay particles from each other and eliminate

SOIL PHYSICAL PROPERTIES 137

the air. A dispersing agent (Sodium hydroxide or sodium hexa-meta-phosphate or Calgonsolution) is added to prevent the clay particles to re-unite.

Once the soil particles are dispersed into ultimate particles, these are separated intodifferent sized groups by sieving through graded sieves up to approximately 0.05 mm insize. Sieves of different sized circular holes are used for particles larger than 0.5 mm. Forsmaller sized particles, wire mesh screens are used. For still smaller particles (<0.05 mm),method of sedimentation is used in which the relative settling velocity of particles ordensity of the suspension from which particles are settling is measured based on theprinciple of Stokes’ law.

6.2.2.1. Stokes’ Law (G.G. Stokes, 1851)According to Stokes’ law, the terminal velocity of a spherical particle settling under theinfluence of gravity in a fluid of a given density and viscosity is proportional to thesquare of its radius, and is given by:

…(6.1)

where v = terminal velocity of falling particles, cm s-1

ρs = density of solid particles, g cm-3

ρf = density of fluid, g cm-3

g = acceleration due to gravity, cm s-2

r = equivalent spherical radius of falling particles, cm

η = viscosity of suspending fluid, g cm-1 s-1 or poise

Derivation of Stokes’ LawA spherical particle falling in a vacuum encounters no resistance and its velocityaccelerates by gravity. The velocity of the falling particle increases as it falls. But whenthe particle falls in a fluid, it encounters a frictional resistance (Fr) in upward directionwhich is directly proportional to its radius (r) and velocity (v), and viscosity of the fluid(η). The resisting force due to friction, Fr, is given by:

Fr = 6πηvr ↑ …(6.2)

Initially, as the particle begins to fall, its velocity increases and eventually a point isreached at which the increasing upward frictional resistance force equals the constantdownward force due to gravity. At this point of time, the particle continues to fall,without acceleration, at a constant velocity known as the terminal velocity.

The downward force on a spherical particle due to gravity (Fg) is given by:

…(6.3)

where is the volume of the spherical particle.

When the terminal velocity is reached, the downward force equals to upward frictionalforce. Therefore, setting the two forces equal, i.e. Fr = Fg:

138 SOIL SCIENCE: AN INTRODUCTION

which is known as Stokes’ law …(6.4)

For a given fluid and falling particles, the ρs, ρf, η and g are constant at a specifiedtemperature, therefore, the sedimentation velocity is directly proportional to the squareof radius of the particles. The Stokes’ law is applicable to a solid sphere or soil particlefalling through a liquid or gas, or to a drop of liquid falling through a gaseous medium.

If d is the diameter of the particle and assuming that the terminal velocity is attainedalmost instantly, the time (t) needed for the particle to fall through a height (h) may becalculated as:

as and …(6.5)

Rearranging the equation (6.5):

…(6.6)

One method of measuring particle size distribution is to use a pipette to draw samplesof known volume from a given depth in the suspension at specific times aftersedimentation has begun. An alternative method is to use a hydrometer to measure thedensity of the suspension at a given depth as function of time.

Numerical: Using Stokes’ law, calculate the time required for sedimentation of silt(diameter = 0.02 mm) and clay (diameter 0.002 mm) size particles to a distance of 10 cm ina freshly prepared soil-water suspension at 20 oC.

Solution: Assuming particle density = 2.65 g cm-3, fluid density = 1.0 g cm-3, viscosity offluid at 20 oC = 0.01 poise = 0.01 g cm-1 s-1 and acceleration due to gravity = 981 cm s-2 andsubstituting these values in equation 6.4, we get the settling velocity v of silt size particles(r = 0.001 cm) as:

where h is 10 cm, therefore, time required for sedimentation of silt particles will be

sec or 4 min 38 sec.

Similarly, the time required for sedimentation of clay particles (r = 0.0001 cm) will come

out to be sec or 7 hr 43 min.

6.2.2.2. Assumptions and Limitations of Stokes’ lawi. Particles must be spherical, smooth and rigid: Clay particles are plate-shaped and fall

slower than spherical particles of the same mass.

SOIL PHYSICAL PROPERTIES 139

ii. Size of particles must be larger than the size of molecules of liquid so that Brownianmovement of molecules of the liquid may not affect the falling velocity of soil particles.The largest limit of particle exhibiting Brownian movement is approximately 0.0002mm. Therefore, in the gravitational field, particles from 0.10 to 0.0002 mm in size caneasily be determined with sedimentation method. Soil particles less than 0.00002 mmsize may be separated using a centrifuge.

iii. Fall must be unhindered: Many fast falling large particles may drag finer particlesdown along with them. Particles falling very near the wall of container (0.1 mmdistance) are also slowed down in their fall. The concentration less than 3% of soil inthe suspension may, however, reduce such hindrances.

iv. Particles must be of uniform density: Density of majority of mineral particles in mostsoils varies between 2.6 to 2.7 g cm-3 with an average value of 2.65 g cm-3 which can beused with reasonable accuracy.

v. The suspension must be still without any turbulence: Particles greater than 0.05 mmin diameter settle quickly and cause turbulence, therefore, are analyzed using otherprocedures. Any movement of the suspension alters the velocity of fall.

6.2.2.3. Methods of Mechanical AnalysisThe international pipette method and hydrometer method are widely used for mechanicalanalysis. The international pipette method is based on the principle of sedimentation, i.e.different sized particles fall at different velocities. If a sample of soil-water suspension istaken at a given depth at a particular time, it will contain all the particles which are stillin suspension at that depth. The international pipette method is regarded as a standardmethod for particle size analysis because of its accuracy but it is time-consuming,therefore, usually not employed where large number of samples are to be analyzed.

Hydrometer (Bouyoucos 1927) method is based on the principle that there is acontinuous decrease in the density of soil suspension with time at the rate the particlesfall below the level of hydrometer. The density of the suspension progressively increasesdownward. Therefore, by measuring the density of soil suspension at required timeswith a calibrated hydrometer, the proportion of different sized particles can bedetermined. For quickly determining the silt+clay and clay fractions, it has beenrecommended to measure the density of the suspension at 4 minutes and 2 hours,respectively at 68 oF. The hydrometer method is considerably fast and reasonably accuratebut should not be used for soils having high CaCO3, organic matter content and salinityas the materials binding the soil particles are not removed in this method.

6.2.3. Textural ClassesThe overall textural designation of a soil as determined from the relative proportion of itssand, silt and clay contents is called the textural class. Textural class not only conveys thetextural composition of soils but also indicates their physical properties. Soils, based ontheir relative proportions of sand, silt and clay contents, are classified into twelve texturalclasses as shown in Textural triangle (Figure 6.1). There are three broad primary texturalgroups of soils: sandy, loamy and clayey to describe texture in relation to textural class ofthe soils (Table 6.3).

To illustrate the use of textural triangle, assume that a soil contains 40% sand, 45% siltand 15% clay by weight. First locate the point for 15% clay on the left side of the triangle.Draw a line from this point across the graph parallel to the base of the triangle. Then,locate the point for 40% sand on the base of the triangle and draw an inward line from

140 SOIL SCIENCE: AN INTRODUCTION

Figure 6.1. Textural triangle diagram according to ISSS system of classification of soil particles

Table 6.3. Textural groups to describe texture in relation to textural classes of the soils

Textural group Texture Textural class

Sandy Coarse SandLoamy sand

Loamy Moderately coarse Sandy loam

Medium LoamSilt loamSilt

Moderately fine Sandy clay loamClay loamSilty clay loam

Clayey Fine Sandy claySilty clayClay

this point parallel to the right side of the triangle. The two lines intersect at a pointcorresponding to 45% silt. The lines intersecting in the area demarcated as ‘loam’ indicatestextural class of the soil sample i.e. loam.

6.2.4. Textural Properties and Behaviour of Soils

6.2.4.1. Sandy SoilsSandy soils contain more than 70% of sand and less than 15% of clay and are sub-dividedinto two textural classes: sand and loamy sand. Sandy or coarse-textured soils are loose,

SOIL PHYSICAL PROPERTIES 141

absorb water rapidly and drain it quickly, well aerated and can be worked easily in bothmoist and dry conditions. These soils are also called as light textured soils due to lowerdraft power required to till these soils. In general, the sandy soils have lower water andnutrient holding capacity, lower organic matter content, no swelling and shrinkage, poorsealing properties for ponds and dams, higher leaching of nutrients and pollutants. Thefine sands are easily blown by wind while coarse sands resist erosion by water.

6.2.4.2. Loamy SoilsThe loamy soils are subdivided in seven textural classes: sandy loam, loam, silt loam, silt,sandy clay loam, clay loam and silty clay loam. To qualify for the designation as sandy or silt,a soil must have at least 40-50% of these separates. Therefore, a loam in which sand isdominant is classified as sandy loam. An ideal loam is a mixture of sand, silt and clay particlesthat exhibits the properties of these separates in equal proportions. It does not mean that thethree separates are present in equal amounts. From agricultural point of view, loam soilis most favourable as its capacity to retain water and nutrients is better than sandy whileits drainage, aeration and tillage properties are more favourable than clayey soils.However, under certain specific conditions for some specific plant species, sandy orclayey soil may be more suitable than a loam soil.

The medium textured soils dominating in silt content have medium to high water andnutrient holding capacity, moderate aeration, slow to medium drainage, medium to highorganic matter content, and moderate leaching of pollutants and nutrients. These soilsare easily blown by wind and susceptible to water erosion, easily compacted, having aslight swelling and shrinkage, and moderately difficult to till after rains.

6.2.4.3. Clayey SoilsA clayey soil must contain at least 35% of clay fraction. These soils are further sub-divided into three textural classes. If clay content is greater than 40%, the textural class iseither sandy clay, silty clay or clay depending upon the sand and silt contents. Sandy claycontains more sand than clay content. Similarly, silt content of silty clay is usually morethan the clay fraction. Clayey or fine textured soils tend to absorb and retain more water,become plastic and sticky when wet, hard and cohesive when dry, and difficult tocultivate, therefore, also called as heavy textured soils.

The clayey soils have high water and nutrient holding capacity, poor aeration, veryslow drainage unless cracked, high to medium organic matter content, medium to highswelling and shrinkage characteristics. These soils resist wind erosion and also resistwater erosion when aggregated. These soils have good sealing properties and retardleaching of nutrients and pollutants.

6.2.5. Specific Surface of SoilsSpecific surface is the surface area per unit mass or per unit volume of soil, expressed ascm2 g-1 or cm2 cm-3. Most of the chemical reactions and physical processes like adsorptionof water, swelling, shrinkage, plasticity, soil strength, cation exchange capacity, availabilityof nutrients, etc., depend on specific surface of soils. Specific surface increases as the sizeof soil particles decreases.

For same volume of soil, the specific surface increases in the ratio in which the size ofparticle decreases. For example, if the size of a cubical particle is 1 cm, then specificsurface area is 6 cm2 cm-3. If the cube is divided into 10 cubes, each of side 0.1 cm (1 mm),

142 SOIL SCIENCE: AN INTRODUCTION

then specific surface will become 60 cm2 cm-3. Thus, the specific surface area of sandparticle of 1 mm size and cubical in shape would be 60 cm2 cm-3, whereas clay particle of0.001 mm size would have specific surface of 60,000 cm2 cm-3 i.e. 1000 times more specificsurface than sand particle. As clay particles are plate shaped, therefore, their specificsurface would be even higher. Besides size and shape, type of clay minerals also affectspecific surface. Specific surface of soil separates, clay minerals and different texturedsoils is given in Table 6.4.

Table 6.4. Specific surface of soil separates, clay minerals and different textured soils

Soil separates / Diameter Specific surface Clay mineral Specific surfacesoil (mm) (cm2 g-1) (m2 g-1)

Coarse sand 2.0-0.2 45 Kaolinite 37-45Fine sand 0.2-0.02 446 Illite 120-170Silt 0.02-0.002 4458 Chlorite 130-180Clay 10-4 -10-6 1000 x 104 Montmorillonite 580-750Sandy loam 10x104 - 40x104 Vermiculite 780-900Loam 50x104 - 100x104

Clay 150x104 - 250x104

6.3. Soil StructureThe primary soil particles do not exist as such in natural conditions but are bondedtogether into larger units or aggregates usually termed as secondary particles. Theseaggregates formed under natural conditions are called peds whereas an irregular shapedcoherent mass of soil formed during tillage operations is called a clod. Soil structure isdefined as the arrangement of primary and secondary soil particles in a certain structural pattern.This arrangement results in formation of different sized soil pores, therefore, soil structuremay also be defined as the arrangement of various sized soil pores in a certain structuralpattern.

6.3.1. Importance of Soil StructureSoil structure influences almost all the plant growth factors viz. water supply, aeration,availability of plant nutrients, heat, root penetration, microbial activity, etc. Strongaggregation decreases detachability and transportability of soil particles by water or windand thus, reduces runoff and soil erosion. Soil structure is useful for classification of soils.It is affected by tillage, cultivation and application of fertilizers, manures, lime, gypsumand irrigation.

6.3.2. Classification of Soil StructureSoil structure is described and classified based on (i) the type, as determined by shape andarrangement; (ii) the class, as differentiated by size; and (iii) the grade, as determined bydistinctness and durability of peds.

6.3.2.1. Type of Soil StructureBased on shape and arrangement of peds, soil structure is classified as simple andcompound. In simple structure, natural cleavage planes are absent. Simple structure is oftwo types: single grained and massive. In single grained structure, particles are completely

SOIL PHYSICAL PROPERTIES 143

unattached to each other as in case of loose sand while in massive structure, particles arebonded in large cohesive, non-structured mass as in case of surface crust, plough pan andclay pan.

In compound structure, natural cleavage planes are visible with naked eye. The shapeof individual peds is described according to relative length of vertical and horizontalaxes, and by shape of their edges. Compound structure is of four types depending uponshapes and characteristics (Table 6.5).

Table 6.5. Types of compound soil structure and their characteristics

Type Shape and characteristics

Blocky All the three dimensions of peds are of about same size providing a shape ofblock having flat or rounded faces. These peds are further sub-divided intoangular blocky and sub-angular blocky. In the former, faces are flat, and edgesand corners are sharp while in the latter, faces and edges are mainly rounded.

The blocky structure is usually found in B-horizon and promotes good drainage,aeration and root penetration.

Prismatic The peds are elongated more in vertical than in horizontal direction giving acolumn like shape. Vertical cleavage planes are predominant. When the tops ofpeds are relatively angular and flat, it is called prismatic and when rounded, itis called columnar.

Prismatic structure commonly occurs in subsurface horizons in arid and semi-arid regions, and in poorly drained soils of humid region having swelling typeof clay.

Platy Horizontal axis is longer than vertical axis resulting in a plate like appearance.Horizontal cleavage planes are predominant. When peds are thick, they arecalled platy, and when thin, are called laminar.

Platy structure is often inherited from parent material and may also be formeddue to compaction of clayey soils by heavy machinery. Platy structure restrictsinfiltration, percolation and aeration in soils.

Spherical Peds are roughly spherical or granular and sub-divided into granular and crumb.Granular structure is less porous than the crumb due to low organic mattercontent.Spherical structure is formed by biotic activities in surface horizon andpromotes infiltration, percolation, aeration and root penetration in soils.

6.3.2.2. Classes of Soil StructureOn the basis of size of aggregates, each type of soil structure is further sub-divided intofive classes (Table 6.6). The size of aggregates is a criterion of quantitative classificationof soil structure. Coarse sand sized aggregates are more favourable for plant growth thanvery small or very large aggregates.

6.3.2.3. Grades of Soil StructureIt is qualitative means of classification of soil structure. Grades of soil structure areidentified on the basis of stability of aggregates. Stability of aggregates refers to theirresistance to disruption by impact of raindrops or under submerged condition. It isinfluenced by moisture content, amount and type of clay, nature of the adsorbed cations,and organic matter content of soil. High moisture content, kaolinite clay, divalent cations

144 SOIL SCIENCE: AN INTRODUCTION

Table 6.7. Grades of soil structure

Grade Characteristics

Weak Poorly formed, indistinct peds which are not stable

Moderate Moderately developed peds which are fairly stable and distinct

Strong Very well developed peds which are quite stable and distinct

Table 6.6. Classes of different types of soil structure along with their dimensions

Class Diameter (mm) Thickness (mm)Granular Blocky Columnar Platy*

Very fine < 1 < 5 < 10 < 1

Fine 1-2 5-10 10-20 1-2

Medium 2-5 10-20 20-50 2-5

Coarse 5-10 20-50 50-100 5-10

Very Coarse >10 >50 >100 >10

*Platy structure is designated as thin or thick instead of fine or coarse (Soil Survey Staff 1993)

and high organic matter content make the aggregates relatively soft. Three structuralgrades have been identified as weak, moderate and strong (Table 6.7).

To describe a soil structure, the sequence grade, class and type is followed, forexample, strong coarse angular blocky, moderate thin platy and weak fine prismaticstructure.

6.3.3. Genesis of Soil StructureGenesis of soil structure refers to the causes and methods of formation of aggregates. Thefollowing two processes are involved in genesis of structure:

6.3.3.1. FlocculationIt is an electro-kinetic phenomenon in which positive and negative charges are involved.In a soil-water suspension, the clay particles with a high zeta-potential repel each other.With the addition of flocculating agent, zeta-potential is lowered, the particles comecloser and attract each other resulting in the formation of floccules of silt size. Flocculesare stable as long as the flocculating agent is present. Flocculation also takes place dueto dehydration, high soluble salt concentration and presence of divalent or trivalentcations.

6.3.3.2. Cementation of FlocculesIt refers to consolidation of clay floccules by cementing materials so that they may not getdispersed once these have been flocculated. As floccules are not larger than silt size,therefore, flocculation would be unfavorable for plants unless these are further aggregatedby inorganic (CaCO3, oxides of Fe and Al) and organic (organic matter) cementingmaterials. Flocculation of clay particles is, therefore, prerequisite for aggregation.Aggregation is always referred to flocculation plus where plus is the cementing agent.

SOIL PHYSICAL PROPERTIES 145

6.3.4. Factors Influencing Formation and Stability of Soil Structure

6.3.4.1. Physical FactorsThe physical factors include wetting and drying, and freezing and thawing. Alternatewetting and drying leads to formation of smaller aggregates, particularly in the finetextured soil. Drying causes shrinkage of the soil mass. When large dry clod is wetted,rapid entry of water causes unequal swelling in the clod and clod is fragmented alongcleavage planes. In the process of wetting, increasing pressure of the entrapped air causesdisruption of the large clod into smaller pieces. Similarly, freezing causes cracks in largeclods by expansion of water. These cracks take in more water upon thawing and getenlarged further on refreezing.

6.3.4.2. Chemical FactorsThe chemical factors include exchangeable cations and binding materials. Clay acts as acementing agent and also swells and shrinks upon wetting and drying. In certain soils,small aggregates are held together by covering of clay particles. Exchangeable ions suchas Ca2+, Mg2+ and K+ have a flocculation effect on clay while Na+ has a dispersing effect.The Na+ ions dominating on exchange complex of sodic soils cause particles to repel eachother and hinder the formation of aggregates. The divalent cations such as Ca2+ andMg2+cause the individual colloidal particles to come together and form floccules. Calciumalso helps in binding the organic colloids and clay particles. Hydrated sesquioxides formalmost completely irreversible colloids upon dehydration and help in forming stableaggregates as in lateritic soils (Oxisols and Ultisols) of the humid tropics. Sesquioxidesalso form complexes with humus. The CaCO3 precipitating around soil particles acts ascementing material for aggregation and imparts stability to soil aggregates. Soluble saltstend to enhance flocculation of clay even in the sodium-saturated clays.

Organic compounds play a key role in aggregation and stabilization of soil structure.Fats, waxes, lignin, proteins, resins etc., also help stabilizing the soil aggregates. Humushelps in aggregation by forming clay-humus complexes. Only Ca-humus is flocculateddue to its tendency to form complex with organic compounds through the coordinationlinkage. The Mg, K, H and Na humus is deflocculated and does not help in aggregation.Synthetic soil conditioners (long carbon-chain organic compounds) are also used for soilaggregation. These compounds attach themselves to the exchange sites of clay and bindthe clay particles together. Addition of these compounds in relatively small amounts canproduce a good structure even in sandy soils.

6.3.4.3. Biological FactorsThe plants and plant residues help the soil particles to bind together. Gelatinous organiccompounds excreted from roots binds the soil particles together. Root hairs penetrate soilclods and create points of weakness and ultimately break the clods into aggregates. Thesealso help the soil particles stick together in a granular form. Plant residues serve as foodfor microbes which play a prime role in aggregate formation and produce stickysubstances on decomposition. Microorganisms decompose plant and animal residues toform humus and this humus binds the particles together. Algae, fungi, actinomycetes andbacteria which constitute the living matter in soil, bind soil particles more effectivelythan the exchangeable ion. The small animals like rodents, earthworms, spiders, mites,nematodes, insects, centipedes, millipedes etc., facilitate the formation of soil aggregatesby way of burrowing, turning the soil or thoroughly mixing the organic residues with thesoil.

146 SOIL SCIENCE: AN INTRODUCTION

6.3.5. Evaluation of Soil StructureSoil structure can be evaluated both by direct and indirect methods.

6.3.5.1. Direct methodsIn direct methods, soil structure is characterized by observing the shape, size andarrangement of soil aggregates either microscopically or macroscopically. In microscopictechnique, thin soil sections are examined under various types of microscopes for shapeand size of the aggregates and voids. For macroscopic evaluation, a large soil clod isexcavated from the field and allowed to fall gently on smooth surface which breaks intopeds of different sizes and shapes. Shapes and sizes can also be observed directly fromthe cleavages of the peds in the soil profile.

6.3.5.2. Indirect methodsIndirect methods involve measurement of size distribution of stable aggregates, stabilityof aggregates or soil property which is a function of soil structure.

i. Size distribution of stable aggregates: Dry or wet sieving techniques are used fordetermination of different sized stable aggregates. In dry sieving technique, the proportionof stable aggregates against vibrating action, simulating the scouring effect of wind, isdetermined which provides an index for characterizing the susceptibility of soil towind erosion. In wet sieving technique, size distribution of water stable aggregates isdetermined.

ii. Measurement of stability of aggregates: It is evaluated by the degree to which soilaggregates resist dispersion. Several indices have been developed such as stabilityindex, structural coefficient and dispersion coefficient. Stability index is the differencebetween percent silt+clay as determined by mechanical analysis and that obtained bysuspension of soil sample in water. The greater the difference, the better is the soilstructure. Structural coefficient is given by (D-S/S), where D is the percentage of particlesless than 0.25 mm in diameter as determined by mechanical analysis and S is thepercentage of aggregates smaller than 0.25 mm in diameter as determined by wetsieving method. Higher the value of structural coefficient, the better is the soilstructure.

iii. Measurement of soil properties: Bulk density, infiltration rate, hydraulic conductivity,aeration, available soil water and degree of compaction of soil may be used forevaluation of soil structure.

6.3.6. Indices of Soil StructureAmount of different sized soil aggregates is represented by a single value for comparingthe structural status of different soils or the same soil under different managementpractices. Among various indices, Mean weight diameter of aggregates (MWD) is a commonlyused index of soil structure. The MWD gives an estimate of weighted percentage ofaverage sizes of all the aggregates. The proportion by weight (wi) of a given size fractionof aggregates to the total sample weight (W) is multiplied by the mean diameter (x– i) ofthat fraction. The sum of these products for all size fractions gives the MWD in mm.

SOIL PHYSICAL PROPERTIES 147

where n is the number of fractions; , where Wi is the weight of each size fraction

and W is total sample weight. The soils having MWD of aggregates greater than 0.25 mmare considered good for crop production. Change in MWD takes into account the stabilityof the aggregates both under dry and wet sieving conditions. The lower the differencebetween two MWDs, the better is the soil structure.

NumericalCalculate the mean weight diameter of water stable aggregates from the following dataobtained by wet sieving of 25 g of soil samples by Yoder’s wet sieving method foraggregate analysis.

Weight of aggregates 12.12 4.35 4.12 3.06 0.98 0.37retained in each size (g)

Solution: Total sample weight (W) = 25 g

Sieve size class Weight of aggregates Mean diameter of Fraction of total weight x– iwi

(mm) retained (g) each size fraction (mm) wi

Xi Wi x– i

4.0 12.12 4.0 0.485 1.9404.0-2.0 4.35 3.0 0.174 0.5222.0-1.0 4.12 1.5 0.165 0.2481.0-0.5 3.06 0.75 0.122 0.0920.5-0.25 0.98 0.375 0.039 0.0150.25-0.10 0.37 0.175 0.015 0.003Summation 25.00 2.820

= 2.82 mm

6.3.7. Management of Soil StructureThe general practices for improvement and maintenance of soil structure are:

i. Tillage operations be carried out within the range of optimum moisture condition toensure least destruction of soil aggregates.

ii. Zero or minimum tillage practices be adopted to maintain adequate aeration andreduce the loss of organic matter through oxidation and erosion.

iii. Soil surface be kept covered with crop residues to protect aggregates from the beatingaction of rain drops, check weed emergence and add organic matter.

iv. Incorporation of crop residues and manures into the soil for stabilizing aggregates.v. Introduction of legumes in crop rotation coupled with the application of phosphatic

fertilizers.vi. Green manuring and cover crops are good sources of organic matter.vii. Integrated use of organics and fertilizers for crop production.

148 SOIL SCIENCE: AN INTRODUCTION

Some soils have specific structural problems which need proper soil management practicesfor better soil structure.

i. Sandy soils: These soils have problem of low water and nutrient retention. The practicalmethod to improve structure of such soils is to add organic matter directly or throughgreen manuring. The addition of pond sediments, clay soil or even compaction ishelpful in increasing the proportion of small sized pores for enhancing water andnutrients retention capacity.

ii. Clayey soils: These soils have restricted drainage and aeration due to poor aggregation.Addition of organic matter, though helpful for improving the aggregation, but therequired amount is high. Therefore, crop rotation and use of phosphatic fertilizers areuseful. Ridge or raised bed cultivation can reduce the aeration stress problem to someextent.

iii. Puddled condition: In areas under prolonged waterlogging, the soil aggregates arebroken, leading to puddled condition. Proper drainage is possible solution of thisproblem.

iv. Dispersed condition: The problem of dispersion is found in sodic soils. Addition ofgypsum in combination with green manuring, manures or incorporation of cropresidues is successful in improving soil structure and amelioration of these soils.

v. Low stability of aggregates: Low stability of aggregates in soils of arid and semi-aridregions is due to low organic matter content. Stability of aggregates can be increasedby addition of organic matter. Mulching may also be useful as it provides favourableconditions for microbes to decompose organic materials.

6.3.8. Influence of Texture and Structure on Soil Porosity and Pore SizeDistribution

The porosity and pore size distribution in soil are affected by soil factors mainly textureand structure. The organic matter, bulk density, and management factors like tillage,cropping and irrigation, which affect soil structure, indirectly affect total porosity andpore size distribution.

It has already been mentioned that the coarse textured soils have lower total porositythan the fine textured soils, although the size of individual pores is larger in coarsetextured soils. The porosity in clayey soils is highly variable as soils exhibit swelling,shrinkage, aggregation, dispersion, compaction and cracking upon wetting and drying.The fine textured soils have larger proportion of micropores. Therefore, these soils retainhigher amount of water and are generally poorly drained. Sandy, loamy and clayey soilsmay have total porosity in the range of 30-45, 40-55 and 45-60%, respectively. Distributionof different sized pores is more important for plant growth than the total porosity. Insandy soils, most of the pores are relatively large and nearly of uniform size, hence, oncethese pores get emptied at a given suction, only a small amount of water is held in soilswhile in clayey soils, the pore size distribution is more uniform, therefore, water graduallydecreases with increase in suction (Figure 6.2). The amount of water retained at relativelylow suction (<1 bar) depends primarily upon the capillary effect and pore-size distribution,and hence is strongly affected by soil structure. Water retention in high suction range isdue to surface adsorption and influenced mainly by texture. Soil compaction decreasestotal porosity especially by reducing the volume of large inter-aggregate pores, therebyresulting in decreased water content at saturation and low suctions. The volume ofintermediate size pores is large but the intra-aggregate micro-pores remain unaffected bysoil compaction and for this reason the soil moisture retention for the compacted andaggregated soils may be nearly identical at high-suction range (Figure 6.3).

SOIL PHYSICAL PROPERTIES 149

Figure 6.2. Schematic representation of effect of texture on soil water retention

Figure 6.3. Schematic representation of effect of structure on soil water retention

6.4. Quantitative Relationships between Soil ConstituentsSoil texture and soil structure largely influence the weight and pore space of a soil. Withincrease in bulk density, the volume of pores decreases and vice versa. Major interest is,therefore, in studying the relationships between bulk density and pore space, as waterand air are stored in and move through the pores. Plant roots and other soil organismsalso require pore space for their growth and development.

6.4.1. Particle DensityThe particle density (ρs) of a soil is the oven-dried mass of soil (Ms) per unit volume ofsoil solids (Vs).

It is also called density of solid particles, mean particle density or true density. The ρs

depends on chemical and mineralogical composition of the soil. In most mineral soils, ρs

is in the range of 2.60 to 2.70 g cm-3. For all practical purposes average value of 2.65 Mgm-3 is used since this value is very close to the density of quartz - a dominant mineral insand and silt fractions of the soil. The presence of iron oxides and other heavy mineralsincreases but organic matter content decreases the ρs value as the soil organic matter(SOM) is light in weight as compared to mineral particles.

150 SOIL SCIENCE: AN INTRODUCTION

6.4.2. Bulk DensityBulk density (ρb) of a soil is the oven- dried mass (Ms) per unit volume (Vt) of soil as awhole including pore space.

The ρs is always greater than ρb as Vt is always greater than Vs. If the pores constitutehalf of the volume of soil then ρb is half of ρs, then ρb will range between 1.30 to 1.35 Mgm-3. The ρb of soil is influenced by texture, structure, moisture content, organic matterand management practices of soil. In coarse textured soils, ρb varies from 1.40 to 1.75 Mgm-3 while in fine textured soils, it normally ranges from 1.10 to 1.40 Mg m-3. The ρb

decreases with increase in organic matter content and fineness of soil texture. Highervalues of ρb indicate more compactness of the soil. The ρb generally increases with soildepth due to lower organic matter content and overburden of the upper soil layers. Inswelling soils, it decreases with increase in moisture content and vice versa. The bulkdensity is of greater importance than particle density in understanding the physicalbehaviour of soils and is used for computing the weight of a furrow slice of soil.

The reciprocal of bulk density is specific volume (Vb) i.e. the ratio of volume to drymass of soil, expressed as cm3 g-1 or m3 Mg-1. The Vb is an index of degree of compactionor looseness of the soil. A higher value of Vb indicates lower level of compaction. Thevalues of specific volume for agricultural soils may vary from 0.55 to 0.70 m3 Mg-1 forcoarse textured soils and from 0.70 to 0.90 m3 Mg-1 for fine textured soils.

6.4.3. Total PorosityThe total porosity (f) is the volume occupied by pores (Vf) per unit volume of soil (Vt). Itis an index of relative pore volume in soil and is generally expressed as percentage.

Its value varies between 30 to 60%. Porosity is lower in the coarse textured soils thanin the fine textured soils but the size of individual pores is larger in the coarse texturedsoils than in the fine textured ones. In clayey soils, the total porosity is highly variable asthe soil alternatively swells, shrinks, aggregates, disperses, compacts and cracks duringwetting and drying. Porosity is related to bulk density and particle density of the soil as:

Two types of pores (macro and micro) occur in soils without any clear demarcation.Usually, pores larger than 0.06 mm in diameter are considered as macropores (waterconducting) and those smaller are called as micropores (water retaining) or capillarypores. Macropores allow easy movement of water and air, whereas these movements arerestricted to some extent in the micropores. Pore space directly controls the amount ofwater and air in the soil and indirectly influences the plant growth. Distribution ofdifferent sized pores is more important for crop production than total porosity of the soilper se. The existence of approximately equal proportion of both macro- and micro-poresare ideal for optimum aeration, permeability, drainage and water retention; these alsooffer most favourable physical condition for optimum plant growth. Porosity of soil caneasily be altered.

SOIL PHYSICAL PROPERTIES 151

6.4.4. Void RatioThe void ratio (e) is the volume occupied by pores (Vf) per unit volume of soil solid (Vs).

Void ratio is preferred for engineering purposes. Void ratio is greater than porosity as Vt

is always greater than Vs and its value varies between 0.3 and 2.0. The relationship ofvoid ratio and porosity is:

6.4.5. Degree of SaturationDegree of saturation (s) is the ratio of pore volume occupied by water (Vw) to the totalvolume of pores (Vf) and usually expressed as percentage:

It ranges from zero in dry soil to 100% in completely saturated soil. However, completesaturation is rarely attained under field conditions since some air is always trapped evenin a very wet soil.

6.4.6. Air-Filled PorosityAir-filled porosity (fa) is the volume occupied by air (Va) per unit volume of soil (Vt) andexpressed as percentage.

It is an important criterion of soil aeration and is related to the degree of saturation ofsoil as:

fa = f (1-s)

6.4.7. Mass WetnessMass wetness (θg) is the mass of water (Mw) per unit mass of oven dried soil (Ms). It isoften termed as gravimetric water content and expressed as a fraction or percentage.

In mineral soils, the θg at saturation ranges between 25-60% depending upon bulkdensity. The saturated water content is usually taken as maximum water holding capacity ofthe soil. The saturated water content is higher in clayey than in sandy soils. In organicsoils (peat and muck), the saturated water content on the mass basis may exceed 100%.

152 SOIL SCIENCE: AN INTRODUCTION

6.4.8. Volume Wetness or Volumetric Water ContentVolume wetness (θv) is the ratio of total volume of water (Vw) occupied in the pore spaceto the total volume of soil (Vt), expressed as a fraction or percentage.

The θv can be computed from θg as: θv = θg x ρb

At saturation, θv is equal to the total porosity of soil. It ranges from 40 to 50% in sandy,approximately 50% in medium and can be 60% in clayey soils. In clayey soils (especiallywith expanding type of minerals), it may exceed the porosity of dry soil because of itsswelling upon wetting. Relationship of θv with s and fa can also be derived as:

and fa = f - θv

The expression of water content on volumetric basis is more useful and convenient asθv is directly involved in calculating water flux, volume of water added to soil by rains/irrigation and the volume of water extracted from the soil by the process of evaporationand transpiration by plants.

6.4.9. Depth of WaterThe depth of water (dw) in cm or mm may be calculated as:

dw = θv × dt, because θv = θg ×ρb, therefore, θv = θg ×ρb × dt

where dt is the depth of soil layer for which the depth of water is to be calculated

6.4.10. Weight of a Furrow SliceThe weight of a furrow slice per hectare is the oven-dried weight of soil of one hectarearea to a depth of 15 cm. This weight is used for calculating the amount of fertilizer andamendments to be applied. It is customary to consider that an average furrow slice of onehectare area of a medium textured soil having a bulk density of around 1.33 Mg m-3

weighs about 2000 Mg or 2x106 kg and is calculated as:

Weight = Bulk density (Mg m-3) × Furrow slice depth (m) × Area of one hectare (m2)

= 1.33 × 0.15 × 104 = 1995 or ≈ 2000 Mg or 2 × 106 kg

6.4.11. Numerical Problems with Solutions1. A cube of soil measures 10 cm x 10 cm x 10 cm (depth =10 cm and area = 100 cm2) and

has a total wet weight of 1460 g including 260 g of water. Assuming the particledensity of 2.65 Mg m-3, find out the water content on mass and volume basis, bulkdensity, porosity, water holding capacity, air-filled porosity, degree of saturation anddepth of soil water.Given: Depth of soil cube (dt) = 10 cm; Area of soil cube (A) = 10 cm x 10 cm = 100cm2, Volume of soil cube (Vt) = area x depth = 100 cm2 x 10 cm = 1000 cm3 , Wet massof soil (Mt) = 1460 g, Mass of water (Mw) = 260 g and particle density (ρs) = 2.65 g cm-3.

SOIL PHYSICAL PROPERTIES 153

Solution

i)

Ms = Mt – Mw =1460-260 = 1200 g

therefore,

ii) Bulk density

iii) Moisture content on volume basis (θv) = θg × ρb

therefore, θv = 21.7 × 1.2 = 26%

iv)

therefore,

v) Air-filled porosity (fa) = (f – θv)therefore, fa = (54.7 – 26.0) = 28.7%

vi)

therefore,

vii)

Alternatively,

2.. The bulk density and particle density of a soil is 1.5 and 2.5 g cm-3, respectively. If themoisture in the soil is 15%, find out the porosity, aeration porosity, degree of saturationand void ratio.Given: ρb = 1.5 g cm-3, ρs = 2. 5 g cm-3 and θv = 15%Solution

i)

ii) Air-filled porosity (fa) = (f – θv) = (40 - 15) = 25%

iii)

iv) . Void ratio is in fraction and not in

percentage.

154 SOIL SCIENCE: AN INTRODUCTION

3. The weight of 130 cm3 of a saturated soil is 250 g. If the bulk density of the soil is 1.6 gcm-3, find out its particle density, porosity and moisture percentage on mass andvolume basis.Given: Mt = Ms+Mw = 250 g, Vt = 130 cm3 and ρb = 1.6 g cm-3

Solution

i) Particle density . We need to find out the values of Ms and Vs. The

value of Ms can be obtained from the given values ρb and Vt as:

or . The value Vs may be

obtained as:Mw = Mt - Ms i.e. 250-208 = 42 g or Vw = 42 cm3 considering density of water asunity. The Vs is the difference of Vt and Vw i.e. 130-42 = 88 cm3

Therefore,

ii)

iii) Moisture content (mass basis)

iv) Moisture content (volume basis) θv = θg × ρb = 20.2 × 1.6 = 32.3%4. The bulk density of a 100 cm3 saturated soil weighting 200 g is 1.6 g cm-3. Find out the

moisture percentage and particle density.Given: Vt = 100 cm3, Mt = 200 g and ρb = 1.6 g cm-3

Solution

a.

The Ms is calculated from the given value of bulk density and total volume of soil

as:

So,

b.

5. The moisture percentage in a soil on wet-weight basis is 30%. Find out the bulkdensity if particle density is 2.5 g cm-3.Given: θg = 30% (wet weight basis), ρs = 2.5 g cm-3

Solution

SOIL PHYSICAL PROPERTIES 155

where Ms + Mw = Mt = 100 g and Mw = 30 g (or Vw = 30 cm3), therefore, Ms = 70 g

. For calculating Vt, we need to calculate Vs from values of ρs and

Vw

. Now Vt = Vs + Vw = 28+30 = 58 cm3

therefore,

6. A soil has an initial volumetric water content of 10% and its volumetric water contentat field capacity is 30%. How deep a 10 cm rain will wet the soil? How much water inneeded to wet the soil to 125 cm.Given: Initial θv = 10 % and θv at field capacity = 30%.SolutionWater content required to attain field capacity of soil = 30-10 = 20% i.e. 0.20 cm ofwater per cm of soil depth. Therefore, rainfall of 10 cm will wet the soil to a depth of

. The depth of water required to wet the soil to a depth of 125 cm

will be (0.2 × 125) = 25 cm7. A soil has volumetric field capacity of 30%. Its initial water content on weight basis

and its bulk density varied with depth and are given in the table below. How deep a 5cm rain will penetrate?

Depth (cm) Initial water content (%) Bulk density (g cm-3)

0-5 5 1.25-20 10 1.320-80 15 1.480-100 17 1.4

SolutionGiven that θv at field capacity is 30%, the depth of soil to be wetted by a rainfall of 5cm can be calculated as:

Depth Initial ρb Field Initial θv (%) Deficit field Depth of water(cm) θg (g cm-3) capacity θv (%) i.e. θv = θg × ρb capacity θv required to wet

(%) (%) the soil depth(cm)

0-5 5 1.2 30 6.0 30-6.0 = 24.0 0.24 × 5 = 1.205-20 10 1.3 30 13.0 30-13.0 = 17.0 0.17 × 15 = 2.5520-80 15 1.4 30 21.0 30-21.0 = 9.0 0.09 × 60 = 5.4080-100 17 1.4 30 23.8 30-23.8 = 6.2 0.062 × 20 = 1.24

The depth of water required to wet the soil up to 20 cm depth came out to be:(1.20+2.55) = 3.75 cm but rain water is only 5 cm, therefore, only 1.25 cm of rain wateris left to penetrate in the soil beyond 20 cm soil depth. As per calculation, an amount

156 SOIL SCIENCE: AN INTRODUCTION

of 0.09 cm of water will be required to wet 1 cm of soil in 20-80 cm soil layer.

Therefore, 1.25 cm of water will wet the soil to a depth of . Hence,

total depth of soil wetted by rain of 5 cm will be 34 cm i.e. 20 cm + 14 cm.

6.5. Dynamic Properties of SoilWhen certain forces act on a body, the forces do not produce any bodily motion, butproduce a relative displacement of particles of the body resulting in a change of shape orsize or both of the body. Physical behaviour of soils to an applied force is a dynamicproperty. These properties include soil consistency, crusting, compaction, strength andpermeability.

6.5.1. Soil ConsistencySoil consistency is the resistance of soil to deformation or rupture under applied pressure.Soil consistency is important for tillage operations and engineering purposes. The forcesof cohesion and adhesion acting within the soil are responsible for soil consistency. Theseforces undergo changes in soil with soil wetness. Therefore, soil consistency is expressedwith reference to soil moisture content. A soil may be hard when dry, friable when moistand plastic when wet. Apart from soil moisture, other factors affecting consistency are:

i. Type of clay: Soil consistency changes with the type of clay. For example, soil withmontmorillonite having larger specific surface area has higher consistency thankaolinite.

ii. Texture: With increase in fineness of the particles, soil consistency increases.iii. Organic matter: Organic matter has more cohesion than sand and silt but less than

clay.iv. Structure: A puddled soil has more consistency than a well-aggregated one because of

larger area of contact between the individual particles.v. Sesquioxides and calcium carbonate: Presence of these materials increases soil consistency.

6.5.1.1. Forms of Soil Consistencyi. Hard or Harsh consistency is observed in a dry soil. At low moisture content, the soil

becomes very hard and coherent due to cohesive forces between the dried particles.ii. Soft and friable consistency is observed in a moist soil. As moisture content increases,

water molecules are adsorbed on the surface of the soil particles and result indecreasing the coherence between the particles. The soil mass becomes friable.Friability characterizes the ease of breakdown of soil. The range of soil moisturecontent under friable condition is optimum for tillage operations.

iii. Plastic consistency is observed in wet soil. Soil can be moulded into any desirable shapewhich is retained even after the applied pressure is removed.

iv. Sticky consistency is observed in a very wet soil. In very wet condition, soil sticks tovarious objects. The moisture content at which soil ceases to stick to any other objectis called sticky point.

6.5.1.2. Soil PlasticityPlasticity is the property which enables a clay/soil to take up water to form a mass thatcan be deformed into any shape and to maintain the shape even after the deformation

SOIL PHYSICAL PROPERTIES 157

pressure is removed. Soils with less than 15% clay do not exhibit plasticity in any moisturerange. This amount of clay, however, depends on type of the clay and the organic mattercontent of the soil. The plate-like shape of clay particles and binding and lubricatingeffect of adsorbed water provide plasticity to the soil.

6.5.1.3. Indices of PlasticityPlasticity is exhibited over a range of moisture content referred to as plasticity limits(Atterberg limits). There are three indices of plasticity:

Lower plastic limit or plastic limit: The plastic limit is the lowest moisture content at whicha soil can be deformed without cracking. It is the upper limit of moisture content fortillage operation for most crops, except rice. Tillage operations in soil at moisture contentabove the plastic limit result in smearing and puddling of the soil.

Upper plastic limit or liquid limit: The liquid limit is themoisture content at which a soil ceases to be plastic. Itbecomes semi-fluid and tends to flow like a liquid underan applied pressure. This limit is used for classificationof soils for engineering purpose. The apparatus used fordetermining the liquid limit is liquid limit device i.e.Casagrande apparatus (Figure 6.4).

Plasticity index: The difference in the moisture contentbetween upper and lower plastic limit is that range ofmoisture content over which a soil remains plastic, and iscalled plasticity index. Soil with expanding type clayshas high liquid limit and plasticity index. Soils with highplasticity index are difficult to plough. The plasticityindex also gives an indication of compressibility. Thegreater the plasticity index, the greater is the soilcompressibility.

Shrinkage characteristics, liquid limit, plastic limit and different densities of soil arecalled the index properties of soil by engineers. Soil engineers usually determine theAtterberg limits for predicting suitability of soils for different construction purposes.Soils with a plasticity index of greater than 25 results in poor roadbeds or foundations.The expansiveness of a soil can be quantified as the coefficient of linear extensibility (COLE).A sample of soil is moistened to its plastic limit and moulded into the shape of a bar withlength, say LM. The bar of soil is then allowed to air dry which shrinks to length, say LD.The COLE is the per cent reduction in length of the soil bar upon shrinking:

6.5.2. Soil CrustingSoil crust is a thin compacted surface layer of higher bulk density than the soil

immediately beneath which is formed due to dispersion of soil aggregates as a result ofwetting and impact of rain drops, and its subsequent rapid drying. The thickness of crustmay vary from mm to few cm depending upon the amount and type of clay, and siltcontent of the soil. Soils having organic matter less than 1% are more prone to crusting.

Figure 6.4. Casagrande- A liquidlimit device

158 SOIL SCIENCE: AN INTRODUCTION

6.5.2.1. Mechanism of Crust FormationThe impact of high intensity raindrops or sprinkler irrigation disintegrates the surfacesoil aggregates. The disintegrated material disperses into suspension. Coarse particlesstart to settle faster but fine particles remain in suspension for a longer period. As thewater drains or evaporates, clay particles settling on the top of coarse particles form asurface crust on drying. Due to clogging of the macro-pores of the soil by dispersedparticles, the layer becomes dense, reducing the infiltration of water and exchanges ofgases through it. Upon drying, this layer becomes hard and its hardness/strength increaseswith decrease in its moisture content. Hardness of the soil crust also increases withincrease in silica, oxides and hydroxides of Al and Fe, CaCO3 and silt content of the soil.The crusting is more severe in the coarse and medium textured soils with weakeraggregates than in the fine textured soils.

6.5.2.2. Adverse Effects of CrustingAdverse effects of crusting are apparent mainly in germination and emergence of plantseedlings. However, such effects depend on crust strength and thickness, emergenceforce of seedlings and management practices. Re-sowing of poorly germinated cropswastes money on seed and labour, and escalate the cost of production. Soil crust reducesinfiltration rate into the soils and increases the runoff losses. The exchange of gasesbetween soil and the atmosphere becomes slow or even ceases.

Crust strength is measured by the modulus of rupture test or in terms of the resistanceto penetration of the penetrometer. The maximum crust strength of a sandy loam soil inarid and semi-arid regions can be about 3.5 kg cm-2 and thickness of about 3 mm. Theemergence forces of some of the crops are:

Crop Emergence force (kg cm-2)

Pearl millet 2.0

Cotton 3.0

Mungbean 3.2

Clusterbean 3.8

Maize 16.0

Thus, in sandy loam soils of arid and semi-arid regions, soil crust could be the limitingfactor for the emergence of pearl millet, cotton and mungbean but not for clusterbean andmaize.

6.5.2.3. Management of Soil CrustingSoil crusting can be minimized by vegetative mulching, application of FYM or straw atthe rate of 30-50 kg ha-1 on seed-line immediately after sowing of crop to protect the soilfrom raindrop impact. Ridge sowing of the crop can also be helpful for better emergenceof the seedlings. If the soil crust is formed, it can be immediately scrapped with a tinedhoe to ensure a better crop emergence. Addition of organic matter and use of certain soilconditioners can also reduce clay dispersion and crust formation. Light irrigation aftercrust formation can help in germination of the seeds sown.

SOIL PHYSICAL PROPERTIES 159

6.5.3. Soil CompactionSoil compaction is the process of increasing bulk density and reducing pore volume as aresult of the applied pressure. It leads to destruction of larger pores, re-arrangement ofsolid particles and compression of air within the pore spaces in the soil. The degree ofcompaction depends upon the nature of clay minerals, type of exchangeable cations,amount of energy applied, water content and extent of manipulation of the soil. Dry soilscannot be compacted to high densities due to incompressible nature of soil particles andhigh internal friction. An increase in moisture content decreases cohesion between theparticles and internal friction, and facilitates compaction. With increase in soil moisture,compaction increases to maximum and then decreases with further increase in soil water.The moisture content at which maximum compaction occurs is called proctor moisturecontent.

A compacted layer is commonly found just below the usually tilled layer of soil. Thislayer is termed as plough sole. The compacted layer often restricts root penetration, andreduces water and nutrient uptake by crops. For enhancing crop growth, the managementpractices which are capable of preventing formation or breaking of such hard layers(chiselling, sub-soiling, deep ploughing) should be followed. Compaction of coarse-textured soils is sometimes desirable for better seed germination and efficient utilizationof water and nutrients.

For engineering purposes, soil compaction imparts strength to soil for erecting stablestructures and reducing maintenance costs. Compaction occurring over time under heavyload causes uneven settlement and cracks in pavement or foundations. Different types ofequipments are used to compact the different textured soils. For example, kneadingtechniques use heavy sheep-foot rollers in clayey and a vibrating rollers or hammers insandy soils.

Soils having swelling type of silicate clay may be compacted by applying externalpressure but regain their original position upon removal of pressure. This makes themunsuitable for foundations. Standard proctor test is used to obtain optimum soil moistureat which maximum compaction of soil is achieved (Figure 6.5).

Figure 6.5. Soil moisture content and bulk density curve attaining optimum moisture for Proctor bulkdensity

6.5.4. Soil StrengthSoil strength is a measure of the capacity of a soil to resist applied pressure withoutdeformation. Soils which are unable to bear the applied pressure result in collapse of

160 SOIL SCIENCE: AN INTRODUCTION

engineering structures, for example, earthen dams under the pressure of impoundedwater, sliding of pavement or other structures on unstable sloppy lands. Strength ofcohesive soils declines under wet conditions and increases under dry and compactedconditions. There are different laboratory tests to estimate soil strength but the simplestis the direct unconfined compression test.

6.5.5. PermeabilityPermeability is the ease with which soil allows fluid to pass through it and helps indetermining the movement and retention of water, nutrients and air. Permeability is alsouseful for civil engineers. For example, constructing a building on highly permeable soilrequires water proofing before laying the foundations or raising the columns.

6.6. Soil ColourSoil colour provides valuable information regarding soil conditions and some propertiesof soils. For example, dark coloured soils absorb more solar radiation and warm up fasterthan the light coloured soils. Soil colour is also used for soil classification andinterpretation and description of soil profiles. The presence of excessive salts, soil erosionetc., can also be easily identified from the soil colour.

6.6.1. Determination of Soil ColourA standard system of accurate description of soilcolour involves Munsell Colour Chart (Figure 6.6).A small piece of soil is compared with thestandard colour chips in the soil colour book.Each colour chip is described by threecomponents: hue, value and chroma. The hue isthe dominant spectral colour which refers tousually redness or yellowness in soil. The valuerefers to the relative lightness or darkness of acolour (amount of reflected light), a value of zero(0) being black. The chroma represents the purityof the dominant colour (strength of the colour),a chroma of zero (0) being a neutral grey.

The Munsell colour notations are systematicnumerical and letter designations of each ofthese three parameters. For example, thenotation of 2.5YR5/6 means a hue of 2.5 YR,value 5 and chroma 6. The equivalent soil colourname for this Munsell notation is ‘red‘. Soilshave a wide range of colours of red, brown,yellow and even green. Some soils are nearlyblack while some are nearly white. Adjacent soilsmay even have different colours e.g., black andred soils exist side by side in Andhra Pradesh.Soil colour may vary with depth in soil profileand from place to place in the landscape. Thesoil horizons may have colours of same hue but of different chroma and value.

Figure 6.6. The Munsell color system showing

SOIL PHYSICAL PROPERTIES 161

6.6.2. Factors Influencing Soil ColourMost important factors influencing the soil colour are:

i. Mineralogy and chemical composition of soil: Presence of manganese oxides imparts blackwhile glauconite imparts green colour to the soil. Calcium carbonate which is foundin soils of semi-arid regions imparts white colour.

ii. Iron compounds: Red, yellow or brown colour is mostly related to the extent ofoxidation, hydration and diffusion of iron oxides in the soils. Yellow, red and browncolours are mostly due to the presence of goethite, hematite and magnetite,respectively.

iii. Organic matter: Organic matter present in the soil tends to impart dark brown to blackcolour to it.

iv. Soil moisture: Moist soils are darker in colour than the dry soils.v. Soil texture and structure: Coarse textured soils are usually light in colour while well

structured soils appear to be darker in colour.

6.7. Soil AirThe soil air is as important as nutrients, water and temperature for plant growth. Thecomposition of soil air is closely related to the atmospheric air. By volume, the atmosphereconsists of N2 (78.09%) and O2 (20.95%) with smaller quantities of CO2 (0.039%), inertgases such as argon (0.93%) and water vapour (variable). The concentration of O2 in soilair is slightly less (20.60%) and that of CO2 is several times higher (0.25 %) thanatmosphere as plant roots and soil microorganisms consume O2 and release CO2 duringtheir respiration. Under reducing conditions, O2 may completely be depleted and methane,hydrogen sulphide and ammonia are formed due to decomposition of organic matter.Soil air has a high relative humidity which is nearly 100% except at the surface duringsummer season. Soil air is not continuous due to discontinuity of soil pores. Thecomposition of soil air is, however, dynamic and varies largely with soil moisture content,degree of aeration, time of the year, temperature, soil depth, root growth, microbialactivities, etc., and from place to place.

6.7.1. Importance of Soil AirOxygen is required for the respiration of plant roots, microbes and the soil fauna. TheCO2 helps in increasing the availability of nutrients to plants. The N2 serves a substratefor the production of plant utilizable (available) nitrogen by symbiotic and non-symbioticbacteria. Water vapour prevents the desiccation of soil and helps in movement of waterwithin the soil. A constant supply of O2 is essential and its concentration should be atleast 10% for normal growth of the plants. Lack of O2 is more injurious to plants thanexcess of CO2 within the reasonable limits (<20%). An excess of O2 is also undesirablebecause it oxidises the organic matter rapidly and dries the soil quickly.

6.7.2. Factors Affecting Composition of Soil AirThe composition of soil air changes continuously due to consumption of O2 by roots andmicroorganisms, release of CO2 during respiration and decomposition of organic materials,and renewal by atmospheric air. Factors affecting the composition of soil air include soilphysical properties, crop grown, tillage practices, organic matter content, biologicalactivities, season etc.

162 SOIL SCIENCE: AN INTRODUCTION

6.7.2.1. Soil Physical PropertiesThe soil physical properties such as texture, structure, porosity, bulk density and moisturecontent largely control the composition of soil air. Sandy soils have low total porosity butthe size of pores is big whereas clayey soils have high total porosity but the size of poresis small. The soils with bigger pores promote rapid exchange of gases than soils withsmaller pores; therefore, the composition of soil air may not change in sandy soils. Thesoils of stable structure have large volume of macropores (> 0.06 mm equivalent diameter)and contain the amount of O2 equivalent to its content in atmosphere. On the other hand,puddled or flooded soils are higher in their CO2 content due to limited aeration. Thereduction in the volume of macropores upon compaction leads to increase in CO2 anddecrease in O2. The restriction in diffusion of gases with increase in soil moisture increasesthe concentration of CO2 in wet soils. In waterlogged soils, gases like H2S, CH4 and NH3

may accumulate and have adverse effect on plant roots. Since, surface soil diffuses directlywith the atmosphere, therefore, has relatively higher O2 as compared to subsoil.

6.7.2.2. RespirationThe rate of respiration by plant roots and microorganisms is the major cause of thevariation in the concentration of O2 and CO2 in soil air which is affected by soiltemperature, moisture and type of soil organisms.

6.7.2.3. Soil Organic Matter and Biological ActivitiesAddition of manures, crop residues, sewage sludge or other organic materials affects thecomposition of soil air to a large extent. During the process of microbial decomposition ofthese materials, the concentration of O2 decreases and that of CO2 increases.

6.7.2.4. CroppingThe growing plants tend to reduce O2 and increase CO2 concentration due to rootrespiration. The biological activities associated with crops also tend to increase theconcentration of CO2 in the soil.

6.7.2.5. TillageThe exchange of gases is faster in tilled soils. A shallow tillage encourages CO2 in the topsoil in comparison to a deep tillage. Puddling required for growing rice decreases themacropores and results in poor aeration for succeeding crop like wheat.

6.7.2.6. SeasonThe composition of soil air changes with season primarily due to changes in soiltemperature and moisture. In rainy season, soils have lower O2 and higher CO2

concentration as compared to summer when soils are dry and there is a greateropportunity of gaseous exchange. But in warm season due to higher microbial activitymore CO2 may be produced. The concentration of CO2 increases after rains because ofincreased decomposition of organic matter and slow gaseous exchange.

6.7.3. Soil AerationSoil aeration is the process of exchange of O2 and CO2 between soil air and atmosphere.Soil aeration replenishes O2 consumed and prevents accumulation of CO2 evolved during

SOIL PHYSICAL PROPERTIES 163

respiration of plant roots and micro-organisms. Aeration status of soil is usuallycharacterized by aeration porosity.

6.7.4. Processes of Gaseous ExchangeThe exchange of gases between soil and atmosphere (renewal of soil air) is a naturalprocess which involves two mechanisms namely, mass flow and diffusion.

6.7.4.1. Mass flowThe mass flow of air occurs due to total pressure gradient of gases which causesmovement of entire mass of air from a region of higher pressure to the region of lowerpressure. Mass flow of air may occur from atmosphere to soil or vice-versa and from onelocation to another in the soil. The difference in total pressure may arise due tometeorological factors such as temperature, pressure, wind, and air replacement due torain or irrigation.

i. Temperature: As per Charles’ law, air pressure is directly proportional to the airtemperature at a constant volume. So, whenever there is a temperature gradientbetween two points, pressure gradient also develops which causes gases to move.Temperature difference may arise within the soil or between soil and atmosphere.Therefore, movement of air may be within the soil and/or between soil andatmosphere.

ii. Pressure: As per Boyle’s law, volume of air is inversely proportional to the pressure ata constant temperature. With increase in pressure in the atmosphere, the volume ofsoil air decreases resulting in movement of air from atmosphere to soil. As pressurein the atmosphere decreases, the volume of soil air increases which causes themovement of soil air to atmosphere.

iii. Rainfall and irrigation: Rainfall and irrigation displace the soil air as such to the lowerdepths. When water is lost from the soil by evaporation, plant uptake or deepdrainage, the air moves from atmosphere to soil.

iv. Wind: Pressure and suction effects of high wind cause the exchange of gases betweenthe soil and the atmosphere but the effect is restricted to surface soil only.

6.7.4.2. DiffusionDiffusion is the predominant process of soil aeration. In diffusion, individual gasconstituents move separately due to partial pressure gradient but the total pressure of airmay be the same. When partial pressure of CO2 in soil air increases due to root andmicrobial activities, the CO2 diffuses from soil to the atmosphere. Similarly, when O2 insoil air is consumed for respiration, its partial pressure is reduced and the O2 diffusesinto the soil. Diffusion increases with increase in temperature.

Fick’s law of diffusion: As per Fick’s law of diffusion, flux of a gas across a plane isproportional to the concentration gradient:

…(6.7)

where qi is the flux of ith gas constituent in x- direction, D is the diffusion coefficient, dc ischange in concentration in a small distance, dx, and dc/dx is the concentration gradient ofthe ith gas constituent. The flux (g cm-2 min-1) refers to the amount of gas diffusing in aunit time across a plane of unit area.

164 SOIL SCIENCE: AN INTRODUCTION

The equation for ideal gases is:

nRT PV = …(6.8)

where P is the partial pressure of gas, V is the volume of gas, n is the number of moleculesof gas, R is the gas constant (8.3 J K-1 mol-1) and T is the absolute temperature.

Therefore, where c = concentration

Substituting the value of c in equation 6.7:

…(6.9)

where D, R and T are constants, therefore, D’ is diffusion coefficient when gradient isexpressed in terms of partial pressure of individual gas constituents.

The coefficient of diffusion of gases depends on the texture, structure and moisturecontent of the soil. The rate of diffusion increases with temperature. Under dry conditions,the diffusion of gases is higher in fine than the coarse textured soil. Under moistconditions, the behaviour is reverse as the air-filled pores will be more in the coarsetextured soil.

6.7.5. Factors Affecting Soil AerationThe volume of macropores affects the total air and gaseous exchange in the soil. Thevolume of these pores largely depends on texture, structure, degree of compaction anddepth of soil. Due to large volume of macroporoes, coarse textured soils have adequateaeration than fine textured soils. Due to this, the concentration of CO2 is higher in the finetextured soils. Puddled soils have more CO2 content due to poor aeration. Well aggregatedsoils may have some patches of poor aeration due to the presence of compacted soillayer. The subsoils are usually deficient in O2 due to the reduced volume of macropores.

6.7.6. Characterization of Soil Aeration StatusSoil aeration status may be characterized by determining the concentration of O2 and CO2

in soil air using chemical or gas chromatographic methods. The air-filled porosity and airpermeability can also be used to determine the aeration status of soil. Other methodsinclude the determination of diffusion coefficient of gases and redox potential of the soil.However, the best method for characterization of aeration status is to measure the oxygendiffusion rate (ODR) in soil. The ODR is the rate at which O2 is replenished when it isused by plant roots or microorganisms. The ODR meter is used to measure ODR. Thecritical value of ODR of soils is 20×10-8 g cm-2 min-1 below which the growth of roots ofmost plants ceases. The optimum ODR range for most crops lies between 30×10-8 g cm-2

min-1 to 40×10-8 g cm-2 min-1. The ODR decreases with moisture and the depth of soil. Asoil condition where the ODR is at least 30 × 10-8 g cm-2 min-1 and O2 concentration of thesoil air is at least 10% in the root zone is considered as having adequate aeration.

6.7.7. Management of Soil AerationThe soil aeration can be optimized by managing soil structure, soil temperature, properdrainage, tillage, plant adaptation and regulation of plant roots.

SOIL PHYSICAL PROPERTIES 165

i. Soil structure: Soil structure may be improved by addition of farmyard manure, greenmanuring, crop residues, and growing of legume crops in order to increase the volumeof macropores. The fine pores retain water for a longer time and inhibit oxygendiffusion; therefore, it is necessary to avoid crusting and soil compaction.

ii. Drainage: Surface or subsurface drainage of fields is essential for the supply ofsufficient oxygen where soil becomes saturated due to continuous seepage from canals,perched or high water tables, heavy rains or irrigation. The drainage will decrease themoisture content and provide aerobic environment in the soil.

iii. Cultivation: A shallow cultivation of soil and inter-culture operations control weedsand help in exchange of gases, especially in the poorly drained heavy-textured soils.After rain, if crust is formed, it may hinder the gaseous exchange. A light cultivationwill break it and help in improving soil aeration. For the improvement of soilaeration in deeper soil layers, planting deep-rooted crops, or sub-soiling, and verticalmulching (the incorporation of organic residues into slits cut into the sub-soil) arevery useful.

iv. Temperature: An increase in soil temperature increases the oxygen diffusion rates,enhances the microbial activity and raises CO2 production in the soil. The net result ofthe increase in soil temperature on the partial pressure of O2 may, therefore, be eitherpositive or negative. In such situations, mulching plays an important role as mulchprotects the soil from the impact of the raindrops and therefore, helps in retaining thetilth. Thus mulching facilitates aeration but it also keeps the soil moist and restrictsthe soil aeration. The positive or negative effect of mulching on oxygen diffusion ratedepends on individual situation. However, it has been observed that mulching usuallydecreases O2 diffusion into the soil.

v. Plant adaptations: Plant roots, in general, are adapted to aerobic conditions. However,some of the plant species develop mechanisms such as increase in the air space ofroots (root porosity) or internal aeration through leaves and cortex cells, and growwell even in oxygen-deficient soils. The selection of crop species, therefore, isimportant for growing crops in waterlogged or poorly drained soils. For example, ricethrives well in submerged soil conditions. Soybean crop can tolerate temporarilywaterlogged conditions better than crops like maize, pigeon pea and other deep-rooted crops.

vi. Regulation of root respiration: Soil aeration may be managed by regulating respirationof roots and microbes by fertilization, cultural practices, plant population andincorporation of organic residues into the soil. Crops of lower O2 requirement orshallow-root systems may be grown in situations where only surface soil has somedegree of aeration.

6.8. Soil TemperatureSoil temperature is an important physical property that regulates evaporation, aerationand chemical reactions taking place in the soil. Soil temperature strongly influencesbiological processes such as seed germination, seedling emergence and growth, rootdevelopment and microbial activities. Temperature is a measure of the heat energy. Theunit of heat energy is calorie or joule and temperature is Kelvin (K) but it is often convenientto use degree Celsius (oC) or degree Fahrenheit (oF). Calorie is defined as the amount ofheat required to raise the temperature of one gram of pure water by 1 oC. The source ofheat in soil is primarily the solar radiation.

166 SOIL SCIENCE: AN INTRODUCTION

6.8.1. Thermal Properties

6.8.1.1. Heat Capacity and Specific HeatHeat capacity of a substance is the amount of heat required to raise the temperature of agiven mass of substance through 1oC and is expressed as cal oC-1. Specific heat is definedas the quantity of heat required to raise the temperature of one gram of the substancethrough 1 oC. In other words, it is heat capacity per unit mass of substance and expressedas cal g-1 oC-1 or J g-1 oC-1. The specific heat expressed on volume basis is called volumetricspecific heat i.e. product of specific heat and bulk density of the substance.

Specific heat of water is 1.00 cal g-1. Specific heat of most of the soil forming mineralsand humus is nearly 0.20 and 0.46 cal g-1, respectively. Practically, all substances havespecific heat lower than water. It takes five times more energy to raise the temperature ofwater by 1 oC than the soil; thus, the specific heat of soil is strongly related to watercontent.

6.8.1.2. Thermal Conductivity and Thermal DiffusivityThermal conductivity is the quantity of heat transmitted through a unit length of asubstance per unit cross-section per unit temperature gradient per unit time. It isexpressed in cal-cm s-1 cm-2 oC-1 or cal s-1 cm-1 oC-1. The flow of heat occurs in the directionof decreasing temperature gradient, i.e. from higher temperature to lower temperature.Thermal conductivity depends upon moisture content, texture, structure, mineralogicalcomposition, organic matter and degree of compaction of soil. It varies in the order ofsand>loam>clay>organic soil. Thermal conductivity increases with increase in wetness ofsoil up to a certain limit beyond which it decreases. Organic matter increases porosityand reduces the contact between soil particles. Therefore, thermal conductivity is low insoils with high organic matter content. Similarly, thermal conductivity increases withcompaction of the soil due to increased contact between soil particles. Thermalconductivity of quartz, water and air are in the ratio of 363:24:1.

Thermal diffusivity is the ratio of thermal conductivity to volumetric heat capacity. Itis a measure of the rate of change of temperature within soil due to net effect of thermalconductivity and volumetric heat capacity of soil under prevailing conditions. The unit ofthermal diffusivity is cm2 s-1.

6.8.2. Heat Balance in SoilThe heat balance in soil is the balance of gains and losses of heat energy. It is expressedas soil heat flux (amount of radiation received per unit area per unit time) which is thedifference between incoming heat fluxes and outgoing heat fluxes at the soil surface i.e.

JH = (Rs+Rnt) - (Hc + L*E + αRs)

where JH is the heat flux at soil surface representing vertical transport of heat into the soil

Rs - global solar radiation (sum of direct and scattered radiation)

Rnt - net long wave thermal radiation (radiation from sky - radiations from earth)

Hc - convective heat flux representing the transport of warm air from the soil surfaceto the atmosphere vertically above it

L*E - latent heat flux, which is the product of the latent heat of vaporization (L) andevaporation rate (E ), and denotes evaporation and subsequent transport of watervapour from soil surface into atmosphere

SOIL PHYSICAL PROPERTIES 167

α - albedo, which is fraction of incoming radiation to the reflected radiation back tothe atmosphere

6.8.3. Diurnal and Annual Variations of Soil TemperatureDepending upon the position of the sun from the earth, the exchange of solar radiation atsoil surface varies causing variations in both diurnal and annual soil temperature.

6.8.3.1. Diurnal VariationThe temperature of surface soil variesaccording to incoming radiation duringthe day, increasing to a peak valueduring noon and falling thereafter.During the night also it falls but muchslowly than during the day. Soiltemperature below the soil surface tendsto follow the changes in surface but thediurnal variation is reduced gradually(Figure 6.7). The variation in minimumand maximum temperature alsodecreases considerably with depth andat a certain depth below the surfacetemperature variation is practicallyabsent. In general, diurnal changes in soil temperature are lower in moist than dry,compacted than loose, and deeper than shallower soil layers.

6.8.3.2. Annual VariationAnnual variations in the soiltemperature (mean maximum andminimum) at different soil depths(Figure 6.8) influence the crop growth.These changes may occur even beyond1.0 m depth. Soil temperature variationduring the year is a major factor thatdetermines the length of growing seasonand suitability for different crops. Innorthern and central India, themaximum soil temperature is critical forcrop growth during summer andminimum soil temperature duringwinter.

6.8.4. Factors Affecting Soil TemperatureSoil temperature is controlled largely by the factors which affect incoming and outgoingheat on and within the soil. The major source of heat in soil is radiation from the sun.Other sources which are of minor importance include the heat from interior of the earth,radioactive substances, and chemical and biological processes occurring within the earth.The factors affecting soil temperature are both environmental (external) and soil (internal).

Figure 6.7. Schematic diurnal variations intemperature at different soil depths

Figure 6.8. Schematic annual variations intemperature at different soil depths

168 SOIL SCIENCE: AN INTRODUCTION

6.8.4.1. Environmental FactorsSolar radiation reaching on the soil surface depends largely on the angle of incidence ofradiation at earth surface. Highest radiation is received at lower latitudes i.e. at theequatorial regions. The solar elevation is lower in winter than in summer season; moresolar radiation reaches the earth surface in summer causing seasonal fluctuations in soiltemperature. During the day, solar elevation is highest in the noon than in dawn anddusk, causing diurnal changes in soil temperature.

The presence of water vapour, clouds, dust, smoke, fog, etc., in atmosphere alsoaffects the soil temperature. The processes such as evaporation, condensation, thawingand freezing, cause increase or decrease in soil temperature depending upon the releaseor absorption of heat during the process. Rain usually has a cooling action and vegetationtends to decrease the soil temperature through reflection of incident radiation, insulationeffect, transpiration, etc.

6.8.4.2. Soil FactorsThe properties of soil which affect the albedo are mainly responsible for variation in soiltemperature. Albedo, the fraction of the incoming radiation reflected from the earth’ssurface, depends on colour, moisture content and surface condition of the soil, and angleof incidence of solar radiation at soil surface. The albedo is higher at dawn and dusk thanat other times of the day, and in winter than summer. Moist soils are relatively darker,have lower albedo than the dried soils. Similarly, smooth surface has higher albedo thanthe rough soil surface. During day, heat flows in downward direction from warmer soilsurface to cooler sub-surface and vice versa during night.

A soil having high specific heat exhibits less fluctuation in soil temperature. Therefore,clayey soils with high moisture content remain cool while sandy soils which hold lesswater warm up quickly. Soil texture, structure, compaction and moisture affect soiltemperature by influencing the thermal conductivity of soil. Dark soil absorbs more andreflects less radiation than the light coloured soils; consequentially the former soils warmup more quickly. In addition, biological activities associated with decomposition oforganic materials evolve heat and raise the soil temperature.

6.8.5. Heat Flow in SoilFlow of heat in any material may occur by conduction (through contact), convection(through air currents) and radiation (without anymedium). The heat flow in soil is mainly throughconduction while convection and radiation are oflesser importance. The heat flow throughconduction is described by Fourier’s law.According to this law, the heat flux in a block ofsoil (Figure 6.9) is directly proportional to the ratioof temperature difference (T1-T2) between the hotand cool ends of the block and its thickness (x1-x2). The proportionality constant is thermalconductivity, k. The Fourier’s law in thedifferential form may be written as:

Figure 6.9. Heat flux in a block of soil

SOIL PHYSICAL PROPERTIES 169

where Jq is heat flux, (J m-2 s-1) or quantity of heat Q, transferred across a unit cross-

sectional area, A (m2) of soil in a unit time, t (s), and is the temperature gradient over

distance x (oC m-1). The negative sign indicates that flux and gradients are in oppositedirection. A major factor influencing the heat conduction in soil is its moisture content. Adry and loose soil is a poor conductor of heat than a wet and compacted soil.

6.8.6. Measurement of Soil TemperatureMeasurement of soil temperature is based on changes in thermometric properties of themeasuring system in equilibrium with the soil. These methods are contact and non-contact types. In contact type thermometers, expansion of a solid (bimetallic stripthermometer), liquid (mercury thermometer), gas (constant pressure or volumethermometer) or changes in electrical properties of material (thermistors and thermocouplethermometer) with change in temperature is measured. The non-contact type thermometermeasures temperature from thermal radiation emitted by the object such as opticalpyrometers, total intensity radiometers and infrared thermometers. Mercurythermometers, thermistors and thermocouple thermometers are widely used to measuresoil temperature.

6.8.7. Soil Temperature and Plant GrowthSoil temperature influences the plant growth directly by affecting seed germination androot growth, and indirectly by affecting soil moisture, aeration, structure, microbialactivities and availability of plant nutrients.

6.8.7.1. Direct EffectsMost of the crops require optimum temperature in the range of 10-35 oC to germinatewithin a reasonable time as shoot meristem, the site of temperature perception of manycrops, lies below the soil surface for an appreciable period. The plant growth is initiatedwhen the minimum temperature is reached, and rate of growth increases up to theoptimum temperature and declines thereafter. The minimum and maximum temperatures,however, differ for different crops and stage of the crop growth. At very low or hightemperatures, root growth is stunted which reduces the absorption of water and nutrients.Some plants are adapted to low temperatures while others adapt better under hightemperatures. The optimum temperature for root growth of most of the crop plants liesbetween 20-25 oC and is often lower than that for shoot growth. The optimum temperaturefor activities of most of the micro-organisms is between 25-35 oC. The relation oftemperature to plant life is shown in Table 6.8.

Table 6.8. Approximate soil temperature ranges (oC) for some crops

Soil temperature range Maize Rice Sorghum Wheat

Optimum range - plants flourish and produce best 25-35 25-30 2-30 15-27Growth range - plants can grow 10-39 15-38 15-35 5-35Survival limit* (minimum and maximum temperature) - plants survive 0-43 12-42 7-37 0-40

* depends on stage of growth and duration of exposure to such extreme temperatures.

170 SOIL SCIENCE: AN INTRODUCTION

6.8.7.2. Indirect Effectsi. Soil moisture: Soil moisture decreases with increase in soil temperature due to increased

rate of evaporation. Temperature increases vapour pressure, and difference in vapourpressure causes movement of water in vapour phase in the soil. This type of movementis very important, particularly, in sandy soils where water percolates below root zoneand moves upward in vapour phase. These vapours get condensed in the root-zone atnight and become available to plants.

ii. Aeration: Increase in soil temperature increases diffusion which is important forexchange of gases between soil and atmosphere.

iii. Soil structure: The temperature affects soil structure through its effect on moisturechanges, and freezing and thawing processes. Soil temperature is also important instabilizing the soil aggregates.

iv. Microbial activity and availability of nutrients: Soil temperature has a profound effect onmicrobial activities which transform nutrients through decomposition of organicmatter and make them available to plants.

6.8.8. Management of Soil TemperatureTemperature of surface soil may be managed by suitable cultural practices such asmulching, irrigation, drainage and tillage.

Mulch with plastic cover or crop residues may increase or decrease the temperatureof surface soil, depending upon the type of mulch and the environmental conditions.Crop residue mulch reduces the solar radiations and lowers the maximum soiltemperature which has significant effect on root growth and crop yields in the aridregion. Polyethylene mulch raises the maximum soil temperature which is desirableduring winter season, as polyethylene is transparent only to the incoming solar radiations.Mulch also conserves soil moisture by altering infiltration and evaporation of water.

Irrigation causes rapid and substantial reduction in maximum temperature in summersdue to lower temperature and higher heat capacity of irrigation water and greaterevaporative cooling in irrigated soils. In winter season, irrigation increases minimum soiltemperature due to relatively higher temperature and heat capacity of irrigation waterthan the soil. As poorly drained soils have higher heat capacity, therefore, removal of amajor portion of soil water by drainage raises the soil temperature which is quite importantin cold humid region.

Tillage makes the soil loose, increases porosity and decreases thermal conductivity,and consequently, the soil temperature. A tilled surface soil shows higher temperaturethan the underlying untilled or compacted soil.

6.9. ConclusionsPhysical properties have significant influence on the behaviour of soil for agriculturaland engineering uses. Soil texture and structure determine the total porosity and the sizedistribution of pores which influence water, heat and air relationships in the soil. Soiltexture is a static property but structure may be manipulated through managementpractices. It is essential to carry out the tillage operations at optimum soil moisture toavoid deterioration in soil structure. Management of physical, chemical and biologicalfactors can help in maintaining proper soil physical conditions for plant growth. Soilaeration and soil temperature affect the quality of soils for plants and other organisms.Soil water has a major influence on both soil aeration and temperature. It competes with

SOIL PHYSICAL PROPERTIES 171

soil air and moderates soil temperature. Soil consistency, plasticity, compaction, strengthetc., help in determining the stability of soil against loading forces from traffic, tillage orbuilding foundations. Looking at the current stress on soil as a natural resource for foodsecurity and safety, due emphasis is needed for maintaining soil physical fertility byadding organic materials, introduction of legumes in rotation, adoption of conservationtillage, etc.

Study Questions1. Calculate the total porosity of a soil whose bulk density is 1.30 Mg m-3 and particle

density of 2.65 Mg m-3.(Ans. 50.9 %)

2. Calculate the weight of 1 hectare 15 cm deep soil whose bulk density is 1.4 Mg m-3.

(Ans. 2100 Mg)3. Calculate the depth of irrigation for one hectare area of cotton whose effective root-

zone is 90 cm, from the following soil data taken before irrigation.

Soil depth ρb θg Field capacity Wilting point ρs

(cm) (Mg m-3) (%) (%) (%) (Mg m-3)

0-30 1.4 8.0 20.0 6.0 2.6530-60 1.5 9.0 21.0 7.0 2.6560-90 1.6 10.0 22.0 8.0 2.65

(Ans. 16.20 cm)4. Describe the factors affecting soil structure.5. Explain different kinds of soil structure and give significance of soil structure in

relation to plant growth?6. What is the applicability of Stokes’ law in mechanical analysis of soil? What are its

assumptions and limitations?7. What do you mean by soil separates? How does clay content in soil affect soil physical

properties?8. Describe the importance of soil air in plant growth.9. Suggest methods to manage soil temperature suitable for plant growth.

Suggested Further ReadingsBiswas, T.D. and Mukherjee, S.K. (1994) Text Book of Soil Science, Second Edition, Tata McGraw-

Hill Publishing Company Limited, New Delhi.

Brady, N.C. and Weil, R.R. (2004) The Nature and Properties of Soils, Thirteenth Edition, PearsonEducation, Inc. New Delhi.

Das, D.K. and Agrawal, R.P. (2009) Physical Properties of Soils. In: Fundamental of Soil Science, theIndian Society of Soil Science, pp 449-460.

Foth, H.D. (1978) Fundamentals of Soil Science, Sixth Edition, John Wiley & Sons, Inc. New York.

Hillel, D. (1971) Soil and Water - Physical Principles and Process, Academic Press, Inc. New York.

Kohnke, H. (1968) Soil Physics, Tata McGraw-Hill Publishing Company Ltd. New Delhi.

Lal, R. and Shukla, M.K. (2004) Principles of Soil Physics, Marcel Dekker Inc., New York.

View publication statsView publication stats