Physical properties of soils

A.Murugan2014004055

Physical propertiesSoil texture.Soil structure.Surface area.Soil density.Soil porosity.Soil colour.Soil consistence.

Soil texture

• Soil texture refers to the relative proportion of particles.

• It is the relative percentage by weight of three soil separates viz., sand, slit, clay.

• Simply refers to size of soil particles.

• The soil separates are defined in terms of diameter in millimeter of the particles.

Naming soil separates

• USDA• BSI• ISSS• EUROPEAN SYSTEM.

SOIL SEPARATES DIAMETER (mm)

Clay <0.002

Silt 0.002-0.02

Fine sand 0.02-0.2

Coarse sand 0.2-2.0

• Particles less than 2mm is called fine earth , normally considered in chemical and mechanical analysis.

• The component fine earth : sand, silt, clay.

SAND: Consists of quartz but may also contain fragments

of feldspar,mica etc., and can be represented as spherical.

SILT: Intermediate b/w sand and clay. Small size and

more surface area

CLAY: Particle size less than 0.002mm, plate like or

needle like shape .Belongs to alumino silicate group of minerals.

Methods of textural determinationElutriation method - water and air.

Pipette method.

Decantation method.

Test tube shaking method.

Feel method- feel by fingers , ball formation , stickiness, ribbon formation.

Importance of soil textureTexture has good effect on management and

productivity of soil.Sandy soils are poor store house of plant

nutrients.Contain low organic matter.Clay particles play a important roles in soil

fertility.Loam and silt loam are highly desirable for

cultivation.

Soil structureThe arrangement and organization of primary and

secondary particles in a soil known as soil structure.

Structure is described under three categories: Type – shape or form and arrangement pattern

of peds. Class- size of peds. Grade – degree of distinctness of peds.

Types of structure

Plate like: The aggregates are arranged in horizontal plates

or leaflets. When they are thick – platy, thin – laminar.

Prism like : The vertical axis is more developed than

horizontal giving a pillar shape. Length 1-10 cm. Commonly occurred in sub horizons of arid and semi-arid regions .

• When the tops are rounded – columnar. When flat / plane/ level and clear cut - prismatic.

Block like:All the dimensions are about the same size. The

aggregates are reduced to blocks. when the faces are flat and distinct and the edges

are sharp angular- angular blocky. when the faces and the edges are mainly rounded

- sub angular blocky. These types confined to sub soil.

Spheroidal: All rounded aggregates are placed in this category.

The aggregates of this group are usually termed as granular which are relatively less porous.

When the granules are very porous – crumb.

This is specific to surface soil particularly high in organic matter / grassland soils.

Classes of structureDepending upon the size of the individual peds:Very fine or very thin.Fine or thin.Medium.Coarse or thick.Very coarse / very thickThe term thick and thin are used for platy types,

while the terms fine and coarse are used for other structural types.

Grades of structureGrades indicates the degree of the distinctness of

the individual peds.Structure less: There is no noticable aggregation.Weak structure: Poorly developed which are not

durable.Moderate structure: Moderately well developed

peds which are fairly durable ad distinct.Strong structure: Very well formed peds which are

quite durable and distinct.

Factors affecting soil structureClimate.

Organic matter.

Tillage.

Plant roots and residues.

Exchangable cations

Soil density Density of a soil is expressed as weight (mass) per

unit volume.Soil density expressed in two concepts: Particle density / Absolute specific gravity. Bulk density / Apparent specific gravity. The particle density of normal soils are 2.65g/cc. The bulk density of the normal soils ranges b/w

1.00-1.6g/cc

Bulk and particle densityTEXTURAL CLASS PARTICLE DENSITY g/cc

Coarse sand 2.655

Fine sand 2.659

Silt

2.798Clay 2.837

TEXTURAL CLASS BULK DENSITY g/cc PORE SPACE %

Sandy soil 1.6 40

Loam 1.4 47

Silt loam 1.3 50

clay 1.1 58

Soil porosity

• The pore space of a soil is the space occupied by air and water.

• In sandy soil the pore space is low. In clay soil pore space is high.

• Presence of organic matter increases the pore space.

calculating pore space: % solid = (bulk density/ particle density )*100

% Pore space = 100 – solids% Pore space = 100 – ( bulk density / particle

density)*100Porosity = 1- ( bulk density / particle density)

Factors influencing pore spaceSoil texture. Sandy surface soil : 35 to 50% Medium to fine textured soils: 50 to 60% Compact sub soil : 25 to 30%Vegetation.Biological activity.Size of pores. Macro pores (non capillary pores) : >0.05 mm Micro pores (capillary pores) : < 0.05mm

Soil colourSoil colour is due to the reflection of EMR by different soil

constituents like minerals organic matter,water, salts etc.,

The occurrence of two or more patches of colour is called mottling (due to the presence of iron,manganese and copper in soil in patches).

Soil colour is inherited from the parent material and that is referred to as lithocromic . Eg: red soils developed from red sand stone

Measurement of soil colourSoil colour is measured using Munsell colour chart and

expressed in terms of Hue, value and chroma.

Hue denotes the dominant spectral colour.

Value denotes the intensity of colour.

Chroma indicates the purity of colour. 10YR 5/4 means , the Hue value is 10YR, 5 is the value

and 4 is the chroma

Factors influencing soil coloursParent material.

Soil moisture.

Organic matter.

Mixture of organic matter and iron oxides.

Alternate wetting and drying condition

Soil consistenceThe resistance of a soil at various moisture contents to

mechanical stresses or manipulations.It combines with both adhesive and cohesive forces

which determines the soil can be reshaped or ruptures.The soil consistency is described at three moisture

levels : 1.wet , 2.moist , 3.dry soil.Wet soil – consistency is denoted by stickiness and

plasticity.

Atterberg’s constantsHe studied plasticity from the point of view of

moisture range over which plasticity range is maintained.

Shrinkage limit or lower plastic limit.Plastic limit.Liquid limit or upper plastic limit.Plasticity number : The difference b/w the

consistency of upper and lower plastic limits

Factors affecting Atterberg’s constants

Clay content.Nature of clay minerals.Nature of exchangable cations.Organic matter content.Cohesion.Sticky point.Swelling and shrinkage.