NRS 111: Exercise

Exercise

Aim: Study of Soil Profile

The vertical section of the soil from the surface extending in to the parent material is called soil profile.
The various layers, composing the soil, are called horizons. A soil horizon may be defined as a layer (of varying thickness) approximately parallel to to the soil surface.The horizons in profile (Fig-1) can be designated as follows:

The organic horizon on the top is called ‘O’ horizon which is formed above the mineral soil. This results from the addition of any organic matter like litter derived from plants, roots, rootlets, dead bodies of insects, animals, etc. This layer ordinarily occurs in cool, humid temperate and subtropical climates.
The ‘A’ horizon (zone of eluviation)is the mineral horizon which lies below ‘O’ near the surface and is characterised usually as the zone of intensive leaching.
The ‘B’ horizon (zone of illuviation) occurs immediately below the ‘A’ horizon in which maximum accumulation of materials such as iron and aluminium oxides, salts and clays has taken place. These might have been washed down from upper horizons. The ‘A’ and ‘B’ horizons together are called ‘solum’. These are further divided into A₁, A₂, A₃, B₁,B₂ and B₃ horizons depending upon identifying characters. The A₃ and B₃ are transition horizons between A and B, B and C horizon, respectively.
The ‘C’ horizon is the parent material underlying the solum. It is less weathered than solum and consequently has no horizon differentiation. However in certain cases like alluvial soils, black soil, the horizon differentiation may not be there. The ‘R’ layer is the underlying consolidated bed rock but in alluvial soils this is not likely to be present.

Significance:

The study of soil profile and its characteristics determines the agricultural value(orchard, forest & crop) of land.
It also gives an insight in to the problems, such as salinity and alkalinity, waterlogging, presence of hard pans etc of soil and explain its behaviour towards use and management.
It is pre-requisite for classifying soils.

Fig1: Soil Profile with common horizon designation.

Materials required:

Procedure:
A. Selection of soil profile:

It is representative of the normal conditions prevailing in the area.
It is away from tree an irrigation channel/ditch/rivers, roads,human inhabitation etc.
It shows minimum of human interference, if, possible virgin area should be selected.

B.Digging of a soil profile

Dig out a pit of size 2m x 2m to a depth of 2m or upto the parent material. Provide steps, if necessary, on one side for getting into the pit.

C.Examination and Description of soil profile

Identify the horizons of the profile on that side of the pit which receives maximum daylight.
Demarcate the horizons on the basis of characteristics that can be seen or measured in the field such as colour, texture, structure, consistence, presence of carbonates etc.
If the horizons are not properly differentiated as in case of alluvial and black soils, examination of soil should be done at a depth intervals of 15 cm down to 1 m depth and thereafter, at intervals of 15 cm up to 2m or up to the parent material, whichever occurs earlier.
Describe each horizon for the following:

Horizon symbol
Depth of from the top to bottom of a particular horizon in centimeters
Nature of boundary with the horizon below
Colour : Moist and dry –Munsell colour chart
Colour of mottling ( if any)
Texture: Feel method which will be explained in the field
Structure: Three feature of structure viz. grade, class and type.
Consistence : wet, moist and dry
Cutans(ped coatings), pressure faces, slickensides etc.
Roots traces : The quantity, size and location of roots in each horizon
Nodule concretions and cementation (if any)
Pores
Lithorelics: The content of rock and mineral fragments in each horizon, note down their percentage.
Hard pans
Content of carbonates, soluble salts etc.
Artefacts : The activity of man’s activity
Soil reaction

Complete the profile description in the given proforma, while studying the profile.

Collect the soil samples in marked sample bags by making a vertically uniform slice of soil for further examination in the laboratory.

D.General description of profile site:
The general description of soil profile should include

Profile number e.g. P₁, P₂ etc.
Location of the profile: Give longitude and latitude with distance and direction from a nearby bench-mark reference site.
Date of examination
Climatic conditions e.g. monthly and annual rainfall and temperature data.
Vegetation and landuse e.g. grasses,shrubs, forest etc. and landuse i.e. wheat, paddy or fallow
Parent material e.g. alluvium, mica schist, granite etc.
Physiographic position of the site e.g. terrace, flood plain, depression, plateau, valley bottom etc.
Land form of surrounding area
Elevation ( in meters above the mean sea level )
Hydrology. Drainage condition, depth of ground water, moisture condition in the profile and artificial drainage.
Biological activity, root development etc
Evidence of erosion and its intensity
Presence of salts or alkali
Human influence like tillage, levelling, use of amendments, heavy manuring etc.
Presence of surface stones or rock out crop

Precautions:

Expose the profile at such a place that it is representative of the whole area.
Do not expose it at elevated or depressed locations in field or in a direction directly facing the sun.
Dig the profile at such a place that maximum visibility is obtained.
Remove the plant cover if any before digging the pit in a manner so that the soil surface is not disturbed.
Mark the soil sample bags properly indicating location, depth, horizon details, etc.
Dig the profile pit in such a way that one side of it has steps for movement of the personnel.

Last modified: Tuesday, 13 March 2012, 11:06 AM