Soil structure refers to the aggregation of primary soil particles into clusters separated by surfaces of weakness. There are different types of soil structure determined by the shape of the peds or aggregates, including plate-like, block-like, prismatic, and granular. Soil structure develops through the flocculation of particles followed by cementation and disruption into fragments. Factors influencing soil structure include climate, organic matter, soil management practices, and biological activity. Proper soil structure is important for factors like porosity, water and air movement, nutrient availability, and microbial populations that influence plant growth.
2. Objectives
• Define: Soil structure
• Types of soil structure
• Genesis of soil structure
• Factors affecting Soil Structure
Role of soil structure in Plant growth
Management of soil structure
3. Soil Structure
• Soil structure refers to the aggregation of primary soil particles
(sand, silt and clay) into defined shaped compound particles or
clusters of primary particles , which are separated from
adjoining aggregates by surfaces of weakness.
Ped – Individual natural soil aggregate is called Ped.
Clod – Artificially formed soil mass such as coherent mass of
soil, broken into any shape by ploughing or digging.
4. • Types of Soil Structure :
The principal types of soil structure is determined by geometric
Shape of the peds and divided into :
a) Simple Structure : Natural cleavage plains are absent or
indistinct.
b) Compound Structure : Natural cleavage plains are distinct .
The types of structure are described according to the relative
lenth of horizontal and vertical axis and shape of peds .
1. Plate like
2. Block like
3. Prism like
4. Sphere like
6. 1. Plate like : Horizontal axis is longer than a vertical axis giving
flattened or plate like appearance of peds . Found in forest
soils commanly . They are subdivided into platy and Laminar.
2. Block like : Horizontal and vertical axis are of similar length
and peds are equally developed along three axis. Found inB
horizons . They are further subdivided into Blocky and Sub –
angular blocky .
3. Prism like : Prismatic peds have longer vertical axis giving a
pillar like shape of ped. Found in soils of arid and sub – arid
regions. They are further subdivided into: Prismatic ( Flat top)
and Columnar ( Rounded top ) .
4. Sphere like : Rounded peds , small in size bounded by curved
and irregular faces. Found in A horizon of grassland soils. They
are further subdivided into Crumby ( More porous) and granular
(less porous). Crumby structure is considered as an ideal
structure for plant growth.
7. • B) Class : On the basis of size of individual peds , they are
divided into :
Very fine
Fine
Medium
Thick
Very thick
C) Grades : Determined by distinctness, stability and strength of
peds .
Structureless : No orderly arrangement of particles or
aggregation and hence, have no noticeable peds.
Structured:
Weak – Poorly formed and non – durable.
Moderate – Well formed and moderately durable.
Strong - Very well formed and durable.
8. Genesis Of Soil Structure :
The genesis of soil structure refers to the mechanism of
formation of the structural units or aggregates. Genesis requires
following steps :
a) Cougulation or Flocculation of the primary fine soil particles
by ions : Soil clays remain in flocculated condition if the
exchangeable positions are saturated by divalent cations like
calcium or magnesium. Among the anions, phosphate ion
plays a significant role. Aggregation begins with the
flocculation of clay particles into microscopic clumps.
b) Cementation of the coagulated materials by adhesive
substances : Three groups of soil colloids such as clay
particles, hydrous oxides of iron and aluminium and organic
substances are responsible for cementation of primary
particles into stable aggregates.
c) Formation of soil mass and its disruption into finer
fragments by water : The water flims hold the oriented
particles together and form a soil mass .
9. Factors affecting Soil Structure
1) Climate : Climate influence the degree of
aggregation and type of soil structure. Temperature
and Rainfall are the main climate factors influencing
structure.
In arid regions, aggregation is very low .
In semi – arid and sub - humid regions, aggregation is
greater.
2) Organic Matter: Organic matter stimulates the
formation and stabilization of most desirable type of soil
structure.
In sandy soils, the sticky and Slimy material produced on
decomposing organic matter , cement the sand particles
together to form aggregates.
In clay soils , it reduces cohesiveness and makes it more
crumby .
10. 3) Adsorbed Cations : Aggregate formation is influenced by
nature of cations adsorbed .
Sodium --- Deflocculation --- Poor structure.
Calcium --- Flocculation --- Good structure.
3) Management Practices : Tillage has both favourable and
unfavourable effects on granulation . Long term effect of
tillage is detrimental to Soil structure because tillage not
only hastens the oxidation of organic matter from soils but
also tend to break down the stable soil aggregates. It also
causes compaction of soils.
4) Biological Processes : Soil fauna like earth worms ,
amoeba and activity of soil flora like microbes influence soil
structure. Soil fauna burrows in the soil and helps in
aggregation . Soil fauna causes decomposition of organic
matter and helps in binding of aggregates.
11. 6) Wetting and Drying : When a dry soil is wetted , the soil
colloids swell on absorbing water. On drying , shrinkage
produces strains in soil mass gives rise to cracks , which break it
up into clods and granules of various sizes .
7) Type Of Vegetation : Grasslands and forest soils have high
stability of aggregates.
12. • Management of Soil Structure :
1. Cultivation of legumes and perennial crops plays a
significant role in improving soil structure through the
impact of granulating effects of root systems and production
of organic materials which promotes biological activities.
2. Application of lime , manures and fertilizers . Liming promotes
greater development of vegetation and production of organic
matter. Application of organic manures like FYM , Green manure
etc. Improves aggregation.
3. In heavy rainfall period , application of surface mulches by
crop residues protect the soil structure from impact of
raindrops.
4. Adoption of minumum tillage and tillage at optimum moisture
content produces aggregates of stable sizes.
5. In alkali soils, application of gypsum or pyrites followed by
green manuring improves structure.
6. Cultivation of plants with large vegetative canopies and
extensive root systems improves aggregation .
13. • Importance of Soil structure :
1. Soil structure influence the amount and nature of porosity.
2. Structure influence the amout of air and water present in
soil .
3. Structure controls the runoff and ersosion .
4. Structure influence the management practices of soil.
5. Structure influence the water holding capacity, drainage and
infiltration rate of soil. For example – Crumby and granular
structure provides Optimum infiltration , drainage and
aeration .
6. It influence availability of plant nutrients .
7. Structure also influence the microbial population in soil .
