1. Soil structure refers to the arrangement of soil particles into aggregates, which are separated by planes of weakness. The natural processes that form aggregates include wetting/drying, freezing/thawing, microbial activity, root/animal activity, and adsorbed cations. 2. Aggregates are described by their shape, size, and stability. The main types are granular, blocky, prismatic, columnar, platy, single-grained, and massive. 3. Soil structure can be graded on stability from weak to strong, and classified by aggregate size as fine, medium, or coarse. Tillage and traffic can alter soil structure by breaking down natural aggregates.

1. Assignment # 4
Assignment title:
Soil Structure
Submitted to:
Dr. Muhammad Ghulam Abbas
Submitted by:
Muhammad Mohsin Iqbal
Reg #
Sp15-Bes-007
Subject:
Introductory tosoil science

2. Vehari
Soil Structure
1. Introduction
Soil structure refers to the arrangement of soil separates into units called soil aggregates. An
aggregate possesses solids and pore space. Aggregates are separated by planes of weakness and are
dominated by clay particles. Silt and fine sand particles may also be part of an aggregate. The
aggregate acts like a larger siltor sand particle dependinguponits size.
The arrangement of soil aggregates into different forms gives a soil its structure. The natural
processesthat aid in forming aggregatesare:
1) wetting and drying,
2) freezingand thawing,
3) microbial activity that aids in the decay oforganic matter,
4) activity of roots and soil animals,
5) adsorbed cat ions.
The wetting/drying and freezing/thawing action as well as root or animal activity pushes particles
back and forth to form aggregates. Decaying plant residues and microbial byproducts coat soil
particles and bind particles into aggregates. Adsorbed cat ions help form aggregates whenever a cat
ion isbonded to two or more particles.
Aggregates are described by their shape, size and stability. Aggregate types are used most frequently
whendiscussingstructure.
2. Peds
Structural units are called peds, and have distinct boundaries and well-defined planes of
weaknessbetweenthe aggregates.

3. Peds consist of primary particles bound together by cementing agents like organic matter,
clay, and hydrous oxidesof iron and aluminum.
Pedscan take several shapes.
 Granular
 Blocky
 Prismatic
 Columnar
 Platy
 Single-grained
 Massive
2.1 Granular Structure
Resemblescookie crumbsand isusually lessthan 0.5 cm indiameter.
Commonlyfound insurface horizonswhere roots have beengrowing.
2.2 Blocky Structure
Irregular blocks that are usually 1.5 - 5.0 cm in diameter.
Can be sub angular or angular blocky.
2.3 Prismatic Structure
Vertical columns of soil that might be a number of cm long.
Usuallyfoundin lower horizons.
2.4 Columnar Structure
Vertical columns of soil that have a salt "cap" at the top.
Found insoilsof arid climates.
2.5 Platy Structure
Thin, flat platesof soil that lie horizontally.

4. Usuallyfoundin compacted soil
2.6 Single-grainedStructure
Soil is broken intoindividual particlesthat do not stick together.
Always accompaniesa loose consistence.
Commonlyfound insandy soils.
2.7 Massive Structure
Soil has no visible structure,is hard to break apart and appears in verylarge clods.
3. Grade of Soil Structure
The terms weak, moderate, or strong are used to describe the grade or how stable the peds
are and how hard they are to break apart.
4. Class ofSoil Structure
The size or class of the pedsis describedas fine,medium,or coarse.
5. Altering Soil Structure
Unlike texture,structure can be altered by tillage or traffic.
Tilling soils that are too wet or compacting soils with heavy equipment can break down the
natural structural units.