Physical Properties of Soil

The physical properties of soil are characteristics that describe the physical state and behavior of soil.

The physical properties of soil are characteristics that describe the physical state and behavior of soil. These properties influence the ability of soil to support plant growth, retain water, and interact with the surrounding environment.

Before physical properties of soil lets understand some basics :-

Soil -

Is a uppermost, un-consolidate, powdery, porous layer of the earth's crust formed by  weathering of minerals and decomposition of organic matter by soil organism.

It provide stratum for life.

 

S - Stratum or Soul

O - Of

I - Infinite

L - Life

 Pedology - 

G: Pedon-Soil or earth + Logos-Study or discourse

It is study of soil origin, classification, survey and distribution.

It considered the soil as natural body of nature.

It does not focus primarily on soil's immediate use.

 

Edaphology - 

G: Pedon-Soil or ground + Logos-Study or discourse

It deals with influence of soils on living things particularly plant growth.

It is study of soil from the stand point of higher plants.

Considers the properties of soil in relation to plant growth.

 

Soil as a natural body contains 4 major constituents - 

• Inorganic or Mineral matter
• Organic Matter
• Soil Water
• Soil Air.

Inorganic or Mineral matter - 

It varies from soil to soil.

it consist Small rock fragments and minerals of rocks.

Proportion of different size particles called texture.

It determines the nutrients availability for plant growth.

Primary mineral have the more amount of sand and silt particles while secondary mineral are rich in clay particles.

It is original source of nutrients for plant growth in the soil.

Secondary mineral matter has high capacity of water holding while primary has less capacity to hold the water in itself.

Organic matter - 

Comprises the Partially disintegrated and decomposed plant and animal residues.

Beside that Micro-organism also synthesizes organic matter by decaying the residues.

Its maintenance requires continuous addition of organic matter by the decaying process of the residues.

Availability of OM is (↑) in surface soil than in sub-surface soil.

OM plays a significance role in the soil productivity.

Soil Water - 

Soil water occupies aprox. 25% of soil space.

Soil water depends upon the pore space size.

Soil dissolve the nutrients to form the solution which is food of plants.

Presence of soil significantly influence the physical, chemical and biological properties of soil. 

More in soil water of physical properties

 

Soil Air - 

Presence of air directly determines by the availability of soil water.

Air occupies those space or pores not filled by the water.

Air composition of are different from the atmospheric air.

It contains higher amount of CO2 than atmosphere.

Oxygen is less than the atmosphere.

 

Significance - 

Growth of plant and root development - In poor aeration plant root restricted, absorption of water and nutrient decreases, 

Microbes - Microbial activity depends on the aeration of soil, more air provides more O₂

 to microbes for respiration and increases rate of decomposition. 

If CO₂

 increases in the soil anaerobic respiration take place and some toxins releases (lacitc, butyric etc) which have negative influence on the plant growth due to root injury. N2 fixation also depend on air of soil.

Nutrient absorption - Uptake of nutrients is a result of gaseous exchange of air and change the metabolic activity.

Soil Property 

Physical property - 

Texture, 

Structure, 

Density, 

Porosity,

Consistency, 

Colour,

Temperature  and 

Water

 

 

Soil texture - 

Relative proportion or percentage of particles (mineral particles) of various sizes such as sand, silt, and clay.

Almost permanent (Basic) property of soil. 

May change slowly.

Generally size <2 mm diameter considered as soil.

 

 >85% of sand - sandy

>80% of silt - silty

>40 of clay - clay 

 

Loam soils - 23-52% of sand, 28-50% of silt, 7- 27% of clay.

Significance of soil texture - 

To access the value of land.

Land capability and management of soil 

Suitability to grow the different crops.

Suitability for foundation of building and road construction.

Total pore space for water and air availability.

Ability to hold and supply nutrient.

Most stable property

Evaluate the amount of amendment needed for reclamation.

Determined the temp. of soil.

 

Soil Structure - 

Studying the soil texture or separates is not enough to understand the soil body, but the various soil structure in which soil particle held to gather by complex network is necessary to understand the soil.

Indiavidual primary particles not exist separately but combine in various larger units or aggregates called secondary particles.

arrangement of primary particles (sand, silt and clay), secondary particles (aggregates) and pores in to a certain definite pattern under field conditions’

 

 When we discuss about Soil structure should keep in mind all these 3 - 

size, shape and arrangement of particles and aggregates

size, shape and arrangement of the spaces separating particles and aggregates

combination of pores and aggregates in to various types of structures

Shape of Soil Structure - 

Classification of soil structure based on shape of peds.

Plate Like

Horizontal dimensions are more developed than the vertical dimensions.

Looks like as lens or flat peds.

When units are thick called platy and thin called laminar.

Inherited from parent materials carried by water or ice.

Found in surface layers of some virgin (soil not cultivated before) soils.

Prism Like

Vertical dimensions are more developed than the horizontal dimensions.

It looks like pillars.

When top of peds are round called columnar and when plan are called prismatic.

Block Like

All the three dimension are more or less same size.

Blocky structure when faces are flat and edges of cube are sharp angular - Angular Blocky and 

when faces and edges are rounded called - sub-angular blocky.

It found in sub-surface soil influences soil drainage, aeration and root penetration.

Spherical

All dimensions developed equally  in irregular faces.

All rounded aggregates forms such type of structure.

Found in horizons O and E horizons.

Aggregates of such soils called granule and less porous

When more porous called crumb.

It is best for plant growth and cultivation.

Spherical soils are greatly found in clay soils and less in sandy soils.

Semi-arid regions - "E" horizon are granulated and "I" horizon prismatic (how)

Humid temperate - "E" horizon are granulated but "I" horizon are platy or block (how)

Factors affecting soil structures-

Climate - 

Arid Region less aggregation

Humid region more aggregation

 

Organic matter -

OM is major agent for aggregation and formation of granular structure of soil.

During the decomposition of OM many organic compound and slimy material helps to formation of aggregates by binding the particles.

 

Cations - 

Na⁺ and K⁺ plays deflocculation role

Ca⁺ and ---- favours the flocculation process.

 

Tillage - 

For the development of good structured soil tillage should be done at proper moisture otherwise if soil is moist then big clods are forms.

 

Micro-organism - 

Microbial decomposition of plant and animal residues binds the soil particles.

Earthworm and other insects makes soil more porous which directly affect the soil structure.

 

Soil moisture - 

In well irrigated soils, good structure develops

Dry soil have influences formation of bad soil structure ===Clods, cracks 

 

Vegetation - 

Grassland, forest and legumes having the high aggregation ability 

while corn have opposite effect.

 

Roots -

Root secretion have the binding effect for the particles of soil to form good soil structures

Roots hairs also holds granule of soil on the surface favores granular structure of soil.

 

Manures and Crop rotation -

Green and organic manures improves the soil structures.

Crop rotation like wheat-jowar and wheat maize improves the soil structures.

 

Soil moisture -

Drying of soils forms crack in the soil and poor drainages have poor soil structure.

 

 

Soil Density - 

Generally density is weight substance available in per unit volume of that substance.

Soil density means weight of soil in per unit volume of soil.

Expressed in g/cm³

 

S.D. grouped into   two types  - 

Particle density - 

Bulk density - 

 

Particle Density - 

Weight or Mass in per unit volume of soil. 

It depends on Chemical composition and crystals of minerals.

It has no impact of soil structures

PD is also a permanent character of soil like texture of soil.

Particle density of most minerals are = 2.60-2.75 g/cm³

 

Humus   1.3-1.5 g/cm³

Clay        2.2-2.6 g/cm³

 

Bulk Density - 

mass per unit volume of dry soil (volume of solid and pore spaces). 

Always smaller than the PD.

Fine textured soil - Clay have low BD than the sandy soils.----

Loose soil have less B.D. than the compact soil.

Soil pores and BD is inversely related.

Generally it ranges from 1.33 g/cm³.

 

BD = Weight of soil/Volume of soil and pores.

 

Factors affecting BD - 

Pore space (↑) = BD (↓)

Compactness (↑) = BD (↑) 

Depth (↑) = BD (↑)

Fine Texture = BD (↓)

O.M. (↑) = BD (↓)

Crumby soil structure (↓) = Platy (↑)

Tillage reduces BD

Croppin increases the BD of top soils.

 

Importance of BD - 

BD determines = Pores spaces = Helps in Soil-Water-Air  =  Facilitate plant growth.

It also determines - 

Infiltration

Percolation

Permeability and 

Water retention capacity of soils.

 

Soil Porosity -

Percentage of Soil volume occupied by pore space known as SP.

Voids or pores constitute the soil which is not occupied by solids like mineral or organic.

Void are filled generally by air and water.

 

Soil Pores - 

Macropores - >0.06 mm, Generally in sand size soils.

Micropores - <0.06 mm, Imp. for crop growth, Found more in clay soils.

 

Factors affecting porosity - 

Soil Structure - Granular structure (↑) pores while Platy (↓), Aggregated structure (↑), single grain (↓)

 

Soil texture - Sandy (↓), Fine textured clay and clay loam (↑)

 

O.M. - (↑) aggregation, (↑) porosity.

 

Depth - (↑) depth, (↓) porosity.

 

Organism - (↑) Organism, (↑) Porosity.

 

Cultivation - Intensive cultivation, (↓) in porosity due to loss of organic matter content.

 

Soil Consistency - 

Resistance of soil material to rupture.

Manifestation of the physical forces of the cohesion and adhesion acting within the soil at various moisture constant.

Soil consistency depends on the texture, structure, inorganic and organic colloids and moisture content.

It describe at 3 moisture content levels wet, moist and dry.

 

Soil Color -

Soil color not directly affect the plant growth but describe the genesis of soil and climatic condition of a regions.

Color of soil is due to organic or inorganic matter content of soil.

Different inorganic and organic matter imparts the different color like - 

Organic matter / humus - black to dark grey

 Iron - Red

Iron - Brown 

Iron - Yellow

Reduced Iron - Blue

Quartz - White

Lime stone - White, Grey

 

Significance of soil color -

Tells climatic condition, parent material of a region - 

Basic rock - Darker than the acidic rock.

Humid temperate - Grey

Tropical and sub-tropical - Red and yellow

 

Dark color  more productive.

Black > brown > rust brown > gray brown > red > gray >yellow > white

For soil profile description.

Soil formation process.

Depression (Dark) and adjacent upland area (Light)

 

 

Soil Temperature -

Degree or measurement of energy are called as temperature.

It is coldness and hotness of any things.

 

Significance of Temp. - 

 

High temp and low temp not good for crop development - 

Germination - ----℃

Root growth - 10-27 ℃

Crop growth - 15-45 ℃

Nutrient availability - Weathering of minerals.

Organic matter decomposition - Temp. for microbial development = Rest-<10℃, Ceases-<5℃.

Nitrification and ammonification affected by too High and too low temp.

 

Soil Water -

Water is a universal solvent and hence necessary for nutrients dissolution in soil for better plant growth.

Water remain soil in place of air, Soil pores are filled by soil water, if water decreases air increases and vice versa.

 

Gravitational Water - 

Held at tension of -0.1 to -0.3 atm.

Free water drains out.

Drainage water.

Present in macropores.

Undesirable because nutrient leaching takes place.

Downward movement due to gravitational force hence called gravitational water.

 

Capillary Water - 

Present in soil capillaries hence called capillary water.

Held between field capacity and hygroscopic coefficient in micropores.

Soil water tension varies from 0.1-31 atm.

Functions as soil solution.

Moves from thick to thin film.

Related to organic matter content and texture of soil.

 

Hygroscopic Water -

Held at hygroscopic coefficient.

Tension varies from 31-10,000 atm.

Held by soil colloids.

Non-liquid and biologically inactive.

Moves in vapor form.

Related to organic matter content and texture of soil.

 

Biological classification of water

In this classification water is grouped on the basis of availability of waater for plants utilization.

1. Available water - 

Available water are those water which is held between field capacity (-1/3 bar) and permanent wilting coefficient (-15 bar).

It is easily available for plants hence called available water.

 

AW=FC-PW

 

2. Un-Available water - 

Held at >-15 bars.

It is unavailable to plants.

 

UW=Water below the -15 bars

 

3. Superfluous water - 

Water held above the field capacity (-15 bars).

Unavailable to plant.

If retain for longer period in soil causes harmful effect on the plant growth due lack of water.

 

SW=Above the FC+some CW.

Why water combines with other molecules?

Different forces are responsible for attraction of molecules.

Water molecules have two types of charged - one is positive hydrogen side and another is negative oxygen side.

When like molecules attracts to each other called cohesion and when unlike attracts called adhesion.

Attraction between colloids and water - adhesion

Attraction between water and water - cohesion.

 

Water Potential - 

Water potential means potential energy and kinetic energy of water in the soil to do  some work with reference to pure water.

Potential energy is greater than kinetic energy in water potential of soil due to low movement of water.

Free energy - Results of all energy having the Ability to do work.

Energy status of water - strength with which water is held.

Pressure required to remove the water from the soil is called atmospheric pressure.

Atmospheric pressure is opposite to surface water tension because surface tension essential for holding the water in the soil while pressure is responsible for removal of water from the soil.

Free energy less (water potential) in adsorbed water (impure water) and more in pool water (water in a chamber).

When TP of water is constant at every place in the soil, water called as in static equilibrium.

In such scenario acting force on water to move is "0" 

Movement of water is happens when force on water is not equal to zero.

 

Units of soil water potential - 

Height of water column in cm - higher the  water column greater the potential

Standard atmospheric pressure at sea level - Weight of 1020 cm of water column

Bar also used as unit - 1 bar = 0.9869 atm aprox. 1 atm= 1020 cm of water column.

 Soil water potential = Matric potential + osmotic or solute potential + pressure potential

Total water potential = Soil water potential + soil gravitational potential.

 

Matric potential = 

It is combined effect of adsorption and capillary action of water.

In this phenomenon water attracts towards soil solids and exchangeable ions and loses the energy. 

It causes reduction of free energy.

It is always negative due to loss of free energy.

 

Osmotic potential = 

It is because of solutes available in soil.

When any solutes dissolves in water, reduced the free energy of water.

Osmotic potential is the work to be done to detach these ions from water molecules.

It is important process involves in the absorption of water by the  plants from the soil.

Negative in nature.

When osmotic potential of solutes increased due to excess salts then plant unable to absorb the water from the soil.

 

Pressure potential = 

it is due to weight of water or atmospheric or gas pressure effect at certain point.

 

Soil gravitational potential = 

The force of gravity acts on soil water, the attraction being towards the

earth’s center.

GP always positive Point of reference always chosen at below the soil surface.

Hence always water moves down due to GP.

It is very beneficial for removal of excess rain  water from the soil.

 

Soil pF - 

the logarithm of 8 with base 2 is 3.

2³=8

“logarithm to the base 10 of the numerical value of the negative pressure of the

soil moisture expressed in centimeters”

pF=log₁₀(-h)

p=Logarithmic value

F= Free energy

-h=Height of water column

Height of water column (cm)	atm	pF
1	0.001	0
10	0.01	1
100	0.1	2
346	1/3	2.53 FC
103	1.0	3
104	10.0	4
15,849	15.0	4.18 WP
31,623	31.0	4.50 HW
105	100.0	5
106	1000.0	6
107	10000.0	7

Water of constitution and inter layer water  = pF >7.0

Hygroscopic water  = pF 7.0 – 4.5

Capillary water  = pF 4.5 – 2.5

Gravitational water  = pF 2.5 – 0.0

Ground water  = Tension free

Soil Moisture Constants - 

It is a reference point or amount of water that soil contains at each equilibrium points.

Like some moisture tension with reference point are given blow - 

 - Reference point with different moisture tension and pF - 

Reference point	Height of water column (cm)	atm	pF
	1	-0.001	0
	10	-0.01	1
	100	-0.1	2
Field Capacity	346	-1/3	2.53 FC
	103	-1.0	3
	104	-10.0	4
Wilting Coefficient	15,849	-15.0	4.18 WP
Hygroscopic Coefficient	31,623	-31.0	4.50 HW
	105	-100.0	5
Air Dry Soil	106	-1000.0	6
Oven Dry Soil	107	-10000.0	7

1. Field Capacity - 

Capacity of the soil to retain water against the downward pull of the force of gravity

Held at -1/3 atm pressure.

At this stage, only micropores or capillary pores are filled with water

It is available soil water to the plants and microorganism.

2. Wilting Coefficient - 

Plants start wilting due to lack of water is termed the Wilting Point

Percentage amount of water held a this point is known as wilting coefficient.

It held strongly with soil.

Held at -15 atm.

At primary stage like wilting point plants start wilting but if water not supplied to field then plant dies and can’t recover it is called as permanent wilting point.

3. Hygroscopic Coefficient - 

Hygroscopicity is the ability of a body to adsorb moisture from the atmosphere.

It s ability of soil to absorbs the moisture from the atmosphere at 50% relative humidity and 15℃ temp.

It held at about 31 atm.

Not available to plants but may be available to some microorganism.

Rest point are not much important for agriculture optional point of view.

 

Almost all the important physical property of soil completed here but soil profile provided in another blog.

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