Physical properties of soils

Dr. Muhammad IqbalDr. Muhammad Iqbal

Assistant ProfessorAssistant Professor

Institute of Soil & Environmental Institute of Soil & Environmental Sciences, UAFSciences, UAF

Soil Texture To learn and understand the

significance of the following concepts related to soil texture:

The definition of soil texture, soil separates and textural class. The relative size of particles defined as sand, silt, and clay.

How to determine a soil's textural class using the textural triangle when the soil texture is known.

How to estimate a soil's textural class by the Hydrometer or fell method.

Importance of soil texture

Soil Texture = %Sand, Silt & Clay in a soil.

Soil texture is the single most important Soil texture is the single most important

physical property of the soil. Knowing the physical property of the soil. Knowing the

soil texture alone will provide information soil texture alone will provide information

about: about: 1) water flow potential, 1) water flow potential,

2) water holding capacity,2) water holding capacity,

3) fertility potential, 3) fertility potential,

4) suitability for many urban 4) suitability for many urban

uses like bearing capacity uses like bearing capacity

Texture The Percent of sand, silt, The Percent of sand, silt,

clay in a soil sampleclay in a soil sample Critical for understanding Critical for understanding

soil behavior and soil behavior and managementmanagement

Soil texture is not subject Soil texture is not subject to change in the field but to change in the field but can be changed in can be changed in potting mixes.potting mixes.

Particle size distribution

Soil textureSoil texture Soil textureSoil texture is defined as is defined as

the relative proportion of the relative proportion of sand, silt and clay %. sand, silt and clay %.

The ranges of diameters The ranges of diameters of the three separates are of the three separates are (ISSS): course sand (2.0- (ISSS): course sand (2.0- 0.2 mm), fine sand (0.2- 0.2 mm), fine sand (0.2- 0.02 mm), silt (0.02-.002 0.02 mm), silt (0.02-.002 mm), and clay (<0.002 mm), and clay (<0.002 mm).mm).

Particle sizeParticle size

Diameter Ranges (USDA)

Soil FractionSoil Fraction Soil Separate Soil Separate Diameter Diameter (mm)(mm)

Sand Very coarse Very coarse sand sand

1 to 21 to 2

Coarse sandCoarse sand 0.5 to 10.5 to 1

Medium sandMedium sand 0.25 to 0.50.25 to 0.5

Fine sandFine sand 0.1 to 0.250.1 to 0.25

Very fine sandVery fine sand 0.05 to 0.10.05 to 0.1

Silt SiltSilt 0.002 to 0.05 0.002 to 0.05

Clay ClayClay < 0.002 < 0.002

Importance of soil Texture Soil texture is one of the most Soil texture is one of the most

important properties of a soil, and it important properties of a soil, and it greatly affects land use and greatly affects land use and management. management.

It affects the amount of water and It affects the amount of water and nutrients that a soil can hold and supply nutrients that a soil can hold and supply to plants. to plants.

Soil physical properties such as Soil physical properties such as structure, and movement of air and structure, and movement of air and water through the soil are affected by water through the soil are affected by texture. texture.

Importance of soil texture---cont

For example a fine textured clayey soil is For example a fine textured clayey soil is difficult to till, has poor aeration for good difficult to till, has poor aeration for good root growth, difficult to wet and difficult to root growth, difficult to wet and difficult to drain but it can retain more nutrients and drain but it can retain more nutrients and water.water.

Coarse sandy soil is easy to wet, easy to Coarse sandy soil is easy to wet, easy to drain and easy to till but loses more plant drain and easy to till but loses more plant nutrients and water by leaching.nutrients and water by leaching.

Sand

Feels gritty Feels gritty Considered non-Considered non-

cohesive – does cohesive – does not stick not stick together in a together in a mass unless it is mass unless it is very wet.very wet.

Sand

Sand has less nutrients Sand has less nutrients for plants than smaller for plants than smaller particlesparticles

Voids between sand Voids between sand particles promote free particles promote free drainage and entry of airdrainage and entry of air

Holds little water and Holds little water and prone to droughtprone to drought

Silt

< 0.05 mm to > 0.002 < 0.05 mm to > 0.002 mmmm

Not visible without Not visible without microscopemicroscope

Quartz often dominant Quartz often dominant mineral in silt since mineral in silt since other minerals have other minerals have weathered away.weathered away.

Silt

Does not feel grittyDoes not feel gritty Floury feel –smooth Floury feel –smooth

like silly puttylike silly putty

Silt Smaller particles – retains Smaller particles – retains

more water for plants and more water for plants and have slower drainage have slower drainage than sand.than sand.

Easily washed away by Easily washed away by flowing water – highly flowing water – highly erosive.erosive.

Holds more plant Holds more plant nutrients than sand.nutrients than sand.

Silt is Silt is responsible for responsible for silting over silting over gravel beds in gravel beds in rivers that are rivers that are needed by fish needed by fish for spawning.for spawning.

www.pedrocreek.org/ fishcommittee.html

Clay

< 0.002 mm < 0.002 mm Flat plates or tiny Flat plates or tiny

flakes flakes Small clay particles Small clay particles

are colloidsare colloids If suspended in water If suspended in water

will not settlewill not settle

Clay Wet clay is very sticky Wet clay is very sticky

and is plastic or it can and is plastic or it can be molded readily into be molded readily into a shape or rod.a shape or rod.

Easily formed into long Easily formed into long ribbons ribbons

Clay

Pores spaces are very Pores spaces are very small and convoluted small and convoluted Movement of water and air Movement of water and air

very slowvery slow Water holding capacity Water holding capacity

Tremendous capacity to Tremendous capacity to adsorb water- not all adsorb water- not all available for plants.available for plants.

Chemical adsorption is Chemical adsorption is largelarge

Textural Triangle

Sandy Soils

Coarse Coarse texturetexture SandsSands Loamy Loamy

sandssands

Loamy Soils

Moderately Moderately coarse coarse texturetexture Sandy loamSandy loam Fine sandy Fine sandy

loamloam

Loamy Soils- Coarse

Medium textureMedium texture Very fineVery fine

sandy loam sandy loam LoamLoam Silt loamSilt loam SiltSilt

Loamy Soils - Fine

Moderately Moderately fine texturefine texture Sandy Sandy

clay loamclay loam Clay loamClay loam Silty Silty

clay loamclay loam

Clayey Soils

Fine textureFine texture Silty claySilty clay ClayClay Sandy claySandy clay

Changing Soil Texture

Soil texture can be Soil texture can be changed only by changed only by mixing with another mixing with another soil with a different soil with a different textural class in small textural class in small quantitiesquantities

Changing Soil Texture

Adding sand to a clay soil Adding sand to a clay soil creates a cement like creates a cement like substancesubstance

Adding peat or compost to a Adding peat or compost to a mineral soil is not considered mineral soil is not considered changing the texture – since it changing the texture – since it only adds organic matter not only adds organic matter not sand, silt or clay.sand, silt or clay.

So why add peat or compost?So why add peat or compost?

Changes in soil texture

Over long periods Over long periods (1000(1000’’s yrs) s yrs) pedologic processes pedologic processes alter soil horizon alter soil horizon textures.textures.

As soils get older As soils get older sand weathers to silt sand weathers to silt and silt weathers to and silt weathers to clay….therefore old clay….therefore old soils have more clay.soils have more clay.

Soil Texture

Methods for Textural DeterminationMethods for Textural Determination

Hydrometer methodHydrometer method Feel MethodFeel Method

Soil texture can also be determined by feeling Soil texture can also be determined by feeling the soil. the soil.

This procedure takes practice but eventually This procedure takes practice but eventually a person can become very proficient and will a person can become very proficient and will be able to estimate the % clay within 3% of be able to estimate the % clay within 3% of the actual value.the actual value.

Determining Soil Texture - Feel Method

Wet soil in handWet soil in hand Make ribbonMake ribbon Length of ribbon Length of ribbon

indicates clay indicates clay contentcontent

Grit or lack of grit Grit or lack of grit indicates sand or indicates sand or siltsilt

Smoothness Smoothness indicates siltindicates silt

Determining Soil Texture Hydrometer Method

The velocity of settling The velocity of settling (V) is proportional to the (V) is proportional to the square of particle square of particle diameters (d)diameters (d)

Bigger particles settle Bigger particles settle more quicklymore quickly

Density of the water (due Density of the water (due to suspended silt and to suspended silt and clay) holds up clay) holds up hydrometer hydrometer

Greenhouse Effect:- Gaseous efflux of CO2, CH4, NOx - C sequestration aggregation

Particulate matter in air:- Wind erosion- Blowing salt

Fresh water resources and quality:- Suspended and dissolved loads- Biological and chemical O2 demand- Pathogens

Acid Rain:- Water quality- Vegetation cover- Biodiversity

Soil Physics and

Environment Quality

Applications of soil physics to environment quality

Environmental Soil Physics

Soil physical properties and processes

Air quality

Water quality

Soil buffers and filters pollutants out of environment

Soil quality

Quality of Life

Specific Surface Area

Properties related to SSA are

CEC, retention and movement of chemicals, swell-shrink

capacity, plasticity, cohesion and strength

SSA is expressed as:

Surface area per unit mass (am)

Surface area per unit volume (av)

Surface area per unit bulk volume (ab)

SSA is expressed as:

Surface area per unit mass (am)

Surface area per unit volume (av)

Surface area per unit bulk volume (ab)

3

2

3

2

2

m

m

V

Aa

m

m

V

Aa

g

m

M

Aa

t

sb

s

sv

s

sm

As – total surface area

Ms – mass of soil

Vs – volume of soil solids

Vt – total volume

SSA can be determined by:

For powdery substances such as clay

Adsorption isotherms

Using inert substances such as N2, water vapor

ethylene glycol

Amount adsorbed

Solution concentration

Methods of measuring SSA

By Ethylene Glycol

- Dry soil sample is saturated with ethylene glycol in a vacuum desiccator

- Excess polar liquid is removed under vacuum

- Surface area is calculated from weight of ethylene glycol retained

BET Method: Brunauer, Emmett, Teller (1938)

Assumptions:

1. Nonpolar gas molecules are adsorbed in multilayer on a solid surface

2. Amount of adsorbed gas in monolayer in contact with the surface can be determined by constructing an adsorption isotherm and analyzing it mathematically

Main assumption for BET equation

1. The molecules adsorbed on the first layer (directly on surface) are more energetically adsorbed than molecules on subsequent layers

2. Heat of adsorption of all layers after the first is equal to the latent heat of condensation of gas

ommo p

p

cx

c

cxppx

p 11

)(

Linear form of BET equation

x = weight of gas adsorbed at equilibrium pressurep = equilibrium gas pressurepo = saturation vapor pressure at temperature Txm = weight of gas in a complete monolayerc = exp(E1-L)/RTµE1 = heat of adsorption in the first layerL = latent heat of condensationR = gas constant/mole (1,336 calories/mole)T = absolute temperature

mm

t ANM

xS

Total surface area of soil sample

St= Total surface area of the sample

xm= experimentally determined weight of gas in an adsorbed monolayerM = molecular weight of the adsorbate (28.01 for N2)N = Avogadro’s Number (6.02 x 1023) (calculated value of the number of atoms, molecules, etc. in a gram mole of any chemical substance)Am = cross sectional area of gas molecule in the monolayer (16.2 x 10-20 m2 for N2)

The specific surface area, am, is obtained by dividing the total surface area by the sample weight.

Remember adsorption experiment must be conducted at or below the temperature of condensation of gas in order for significant adsorption to occur

.

Soil degradation

Land degradation indicates temporary or Land degradation indicates temporary or permanent long-term decline in ecosystem permanent long-term decline in ecosystem function andfunction and

Productive capacity. It may refer to the Productive capacity. It may refer to the destruction or deterioration in health of destruction or deterioration in health of terrestrial ecosystems, thus affecting the terrestrial ecosystems, thus affecting the associated biodiversity, natural ecological associated biodiversity, natural ecological processes and ecosystem resilience. processes and ecosystem resilience.

Soil deg-------- cont

It also considers It also considers the reduction or loss of the reduction or loss of biological/economic productivity and biological/economic productivity and complexity of croplands, pasture, complexity of croplands, pasture, woodland, forest, etc. Across the world, woodland, forest, etc. Across the world, over 20% of cultivated areas, 30% of over 20% of cultivated areas, 30% of forests and 10% of grasslands are suffering forests and 10% of grasslands are suffering from degradation, affecting about 1.5 from degradation, affecting about 1.5 billion people.billion people.

Causes of land degradation include:

Clearance of vegetative coverClearance of vegetative cover Soil erosion by wind or water Soil erosion by wind or water Natural conditions e.g. soil type, topography (e.g. steep Natural conditions e.g. soil type, topography (e.g. steep

gradient),weather/climatic conditions e.g. High intensity gradient),weather/climatic conditions e.g. High intensity rainfall, natural hazardsrainfall, natural hazards

Invasive speciesInvasive species PollutionPollution Drought i.e. precipitation is significantly lower than Drought i.e. precipitation is significantly lower than

average recorded levels for a prolonged period. average recorded levels for a prolonged period. Unsustainable agricultural practices Habitat alteration e.g. Unsustainable agricultural practices Habitat alteration e.g. urban expansionurban expansion

Effects of soil degradation

Decline in the chemical, physical and/or Decline in the chemical, physical and/or biological properties of soil e.g. lower biological properties of soil e.g. lower organic content and organic content and nutriments level, nutriments level, salinisation, pH changes in soil salinisation, pH changes in soil (acidification or alkalinization)(acidification or alkalinization)

Reduced availability of potable waterReduced availability of potable water Lessened volumes of surface waterLessened volumes of surface water Depletion of aquifers due to lack of re-chargeDepletion of aquifers due to lack of re-charge

Impacts on livestock and agriculture e.g. loss of animals due to dehydration, reduced yields

water and food insecurity, famine Biodiversity loss General reduction of the ability for the community

to depend on the natural environment for livelihood Decline in economic productivity and national

development Conflict over access to resources Mass migration

Role of agricultural practices

Intensification and increased mechanization of agriculture Intensification and increased mechanization of agriculture have led to the abandoning of many sustainable traditional have led to the abandoning of many sustainable traditional agricultural practices in favor of increased yields and agricultural practices in favor of increased yields and faster production. These include:faster production. These include:

Overgrazing of pasture land over cultivation of cropland Overgrazing of pasture land over cultivation of cropland and mono-cropping, waterlogging and salinization of and mono-cropping, waterlogging and salinization of irrigated land, over-extraction of wells, rivers and dams, irrigated land, over-extraction of wells, rivers and dams, land clearance e.g. slash-and-burn, deforestationland clearance e.g. slash-and-burn, deforestation

Excessive and continuous fertilizer, herbicide and Excessive and continuous fertilizer, herbicide and pesticide use, conversion of unsuitable lands to agriculture pesticide use, conversion of unsuitable lands to agriculture e.g. use of marginal lands, clearance of tropical forest for e.g. use of marginal lands, clearance of tropical forest for livestock rearinglivestock rearing