This unit is about Soil profile and structure.


As soil profile is the vertical arrangement of various soil layers or horizons. The layers can clearly be seen in a well developed or mature soil since they differ from each other in colour, humus content particle size and mineral composition.

soil profile

The organic matter layer; contains relatively un decomposed organic matter in various stages of decay.

The top soil horizon;  is the one immediately below the organic material and is characterized by:-

  • Loose decaying organic matter which gives a black colour.
  • Has many micro and macro organisms that interact with each other and break down organic matter.
  • It is porus so it can easily allow free movement of all, water, heat and root penetration.
  • A lot of leaching of mineral nutrients goes on and that is why it is called the Eluvia layer.
  • It has numerous plant roots.
  • This horizons is most suitable for crop production an it is the one that is most interfered with by man through digging, additional of manure, irrigation e.t.c

The subsoil horizon; the minerals leached from the top soil are deposited here that is why it is called the illuvial layer. It is usually reddish organs in colour due to high content of calcium and silca.

  • It has very little organic matter and therefore a low population and activities of soil organisms.
  • It is closely packed so there is limited movement of air, water, heat and root movement.
  • A few roots go down to this horizon mainly tap roots.
  • Some times a hard pan forms especially where machinery is used for cultivation and this will limit down ward root development.

The substratum; these are course particles under going a slow weathering process. They are too compact that water can rarely pass through and there is no organic matter. Only a few of trees can go down to this layer.

The Bed stratum; this is the solid rock that has not been weathered and sometimes under ground water accumulates on top to form ponds. Sometimes the sub and bed stratum are known to form parental material or rock strata.


The exact thickness and composition of each soil horizon changes from place to  place and from time to time depending on:-

  • Topography
  • Climate especially rainfall and temperature.
  • Vegetation affect type of humas formed.
  • Parent material which influences drainage and leaching.

Importance of soil profile on crop production

  • The top soil contains most of the plant nutrients so the deeper it is, the more suitable a soil is for crop rotation.
  • A loosely packed sub soil will allow easy penetration of crop roots, free drainage aeration and heat transfer.
  • A loosely packed sub soil reduces the degree of surface runoff and ensures erosion does not take place.
  • The nature and composition of the bedrock influences minerals composition of soil.
  • A deep tap soil provides the optimum environment for soil organisms activities.



Soil structure is the way individual soil particles are packed , arranged or aggregated to form lumps or clouds.

Types of soil structure

Plany structure

There are horizontally arranged plates like structures on top of each other. They may occur in any part of the soil profile on the surface they occur as surface.


Crust but are also common in sub soils.

structure 1

Columnar structure

When the prismatic structure is eroded, the corners are founded to form the columnar structure. It’s also common in sub soils of arid and semi arid regions.

structure 2

Block structure

structure 3


Irregular four or six faced three dimensional aggregates common in sub soils of heavy clay soils in humid area.

Granular Structure:

Those are around aggregates which are porus common in the tap soil horizon and subjected to rapid changes.

structure 4

Crumb Structure

Rounded soil aggregates that loosely and can be shaken apart very porous, common in top soils that are cultivated and rich in organic matter.

structure 5

Factors that influence the types of soil structure

  • Physical and chemical nature; of parent rock.
  • Presence of organic matter; will stabilize soil structure.
  • Nature of cultivation; over cultivation destroys soil structure.
  • Surface vegetation cover; grass cover with a abundant fibrous roots makes soil more porous.
  • Living organisms in the soil; are responsible for organic matter decomposition and larger ones tunnel so make soil more porus.
  • Soil forming processes;
  • Climate especially rainfall temperature and wind; which causes soil aggregates to swell and shrink creating porousity.
  • In organic compound; g. iron oxides have binding properties that help formation of stable granule.

The influence of soil structure on crop production

  • Determines air and water movement in soil therefore presence of soil organisms.
  • Determines water holding capacity and therefore how much water is available for plant use.
  • Since it determines air movement in soil, carbondioxide will not build up toxic levels for crops.
  • Influences penetration of plant roots to obtain food and emergency of shoots during germination.
  • Preparation of a suitable seed bed if its too compact cultivation will be hard.
  • Allows proper heat transfer, a warm soil is desirable for soil organisms, germination and proper root development.

Ways through which soil structure may be destroyed.

  • Continues cropping which uses up all organic matter so that soil aggregates breakdown readily so it becomes hard to cultivate.
  • Cultivation of soil, especially if it’s rich in day when very wet so that it becomes puddles and all pore spaces collapse.
  • Burning since it destroys organic matter.
  • Drainage during which pore spaces may be closed.

Ways through which good soil structure may be maintained.

  • Regular addition of organic matter so that the granular structure remains staple and air and water can move through freely.
  • Minimum tillage so that dangers of erosion are minimized.
  • Crop rotation so that when land is under bush fullow fibrous roots of grass can rebuild soil structure.
  • Mulching to prevent soil aggregates being broken down by impact of rain drop.
  • Liming lime aggregates soil particles.
  • Inter planting with leafy cover crops that reduce surface water flow and hold soil particles together.



ASSIGNMENT : SOIL PROFILE Agriculture assignment MARKS : 10  DURATION : 24 hours

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