3. 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.
5. • Particles less than 2mm is called fine earth ,
normally considered in chemical and mechanical
analysis.
• The component fine earth : sand, silt, clay.
6. SAND:
Consists of quartz but may also contain fragments
of feldspar,mica etc., and can be represented as
spherical.
7. 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.
8. Methods of textural determination
Elutriation method - water and air.
Pipette method.
Decantation method.
Test tube shaking method.
Feel method- feel by fingers , ball formation , stickiness,
ribbon formation.
9. Importance of soil texture
Texture 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.
10. Soil structure
The 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.
11. 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 .
12. • 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.
13. 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.
14.
15. Classes of structure
Depending upon the size of the individual peds:
Very fine or very thin.
Fine or thin.
Medium.
Coarse or thick.
Very coarse / very thick
The term thick and thin are used for platy types,
while the terms fine and coarse are used for other
structural types.
16. Grades of structure
Grades 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.
18. 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
19. Bulk and particle density
TEXTURAL CLASS PARTICLE DENSITY g/cc
Coarse sand 2.655
Fine sand 2.659
Silt
2.798
Clay 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
20.
21. 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
22. % Pore space = 100 – solids
% Pore space = 100 – ( bulk density / particle
density)*100
Porosity = 1- ( bulk density / particle density)
23. Factors influencing pore space
Soil 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
24. Soil colour
Soil 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
25. Measurement of soil colour
Soil 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
26.
27. Factors influencing soil colours
Parent material.
Soil moisture.
Organic matter.
Mixture of organic matter and iron oxides.
Alternate wetting and drying condition
28. Soil consistence
The 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.
29.
30. Atterberg’s constants
He 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
31. Factors affecting Atterberg’s constants
Clay content.
Nature of clay minerals.
Nature of exchangable cations.
Organic matter content.
Cohesion.
Sticky point.
Swelling and shrinkage.
