SOIL FORMATION

SOIL FORMATION

Pedology is soil science.

Pedologist is the one  who studies the soil –

1.Define soil

SOIL is defined as a collection of natural bodies on the earth’s surface containing living matter and supporting or capable of supporting plants.

2. What are the features of soil ?

1. Soil is a dynamic medium in which many chemical, physical and biological activities go on constantly.
2. Soil is a result of decay, it is also the medium for growth.
3. It is a changing and developing body.
4. It has many characteristics that fluctuate with the seasons.
5. It may be alternatively cold and warm or dry and moist.
6. Biological activity is slowed or stopped if the soil becomes too cold or too dry.
7. Organic matter increases when leaves fall or grasses die.
8. The soil chemistry, the amount of organic matter,the soil flora and fauna, the temperature and

                 the moisture, all change with the seasons as well as with more extended periods of time.

.3. What are the soil forming factors? Explain the role of each one of them

Five basic factors control the formation of soils:

(i)                  parent material; (ii) topography; (iii) climate; (iv) biological activity; (v) time.

Parent Material

1.Parent material is a passive control factor in soil formation

2. Parent materials can be any in situ or on-site weathered rock debris (residual soils) or transported deposits

3. Soil formation depends upon the texture(sizes of debris) and structure (disposition of individual grains/particles of debris) as well as the mineral and chemical composition of the rock debris/deposits.

4. Nature and rate of weathering and depth of   weathering mantle are important consideration under parent materials.

5. There may be   differences in soil over similar bedrock and  dissimilar bedrocks may have similar soils above them.

Topography

1. Topography like parent materials is another passive control factor.
2.  The influence of topography is felt through the amount of exposure of a surface covered by parent materials to sunlight and the amount of surface and sub-surface drainage over and  through the parent materials.
3.  Soils will be thin on steep slopes and thick over flat upland areas.

Over gentle slopes where erosion is slow and percolation of water is good, soil formation is very favourable.

Soils over flat areas may develop a thick layer of clay with good accumulation of organic matter giving the soil dark colour.

4. In middle latitudes of northern hemisphere , the south facing slopes exposed to sunlight have different conditions of vegetation and soils and the north facing slopes with cool, moist conditions have some other soils and vegetation.

Climate

Climate is an important active factor in soil formation. The climatic elements involved in soil development are :

 (i) moisture in terms of its intensity, frequency and duration of precipitation - evaporation and humidity;

(ii) temperature in terms of seasonal and diurnal variations.

1. Precipitation gives soil its moisture content which makes the chemical and biological activities possible.
2. Excess of water helps in the downward transportation of soil components through the soil (eluviation) and deposits the same down below (illuviation).

 Eg In climates like wet equatorial rainy areas with high rainfall, not only calcium, sodium, magnesium,

potassium etc. but also a major part of silica is removed from the soil. Removal of silica from the soil is known as desilication. In dry climates, because of high temperature, evaporation exceeds precipitation and hence ground water is brought up to the surface by capillary action and in the process the water evaporates leaving behind salts in the soil. Such salts form into a

crust in the soil known as hardpans.

In tropicalclimates and in areas with intermediate precipitation conditions, calcium carbonate nodules (kanker) are formed.

3  Temperature acts in two ways — increasing or reducing chemical and biological activity.

                                                         Chemical activity is increased in higher temperatures, reduced in cooler temperatures (with an exception of carbonation) and stops in freezing conditions. That is why, tropical soils with higher temperatures show deeper profiles and in the frozen tundra regions soils contain largely mechanically broken materials.

(Q n  Why is the tropical region have deeper soil profiles ? –Ans given above )

Biological Activity

1. The vegetative cover and organisms that occupy the parent materials from the beginning and also at later stages help in adding organic matter,moisture retention, nitrogen etc.
2. Dead plants provide humus, the finely divided organic matter of the soil.
3. Some organic acids which form during humification aid in decomposing the minerals of the soil parent materials
4. Intensity of bacterial activity shows up differences between soils of cold and warm climates.
5. Humus accumulates in cold climates as bacterial growth is slow.

 With undecomposed organic matter because of low bacterial activity,layers of peat develop in sub-arctic and tundra climates.

In humid tropical and equatorial climates, bacterial growth and action is intense and dead vegetation is rapidly oxidised leaving very low humus content in the soil. Further, bacteria and other soil organisms take gaseous nitrogen from the air and convert it into a chemical form that can be used by plants. This process is known as nitrogen fixation.

Rhizobium, a type of bacteria, lives in the root nodules of leguminous plants and fixes nitrogen beneficial to the host plant.

6.  The influence of large animals like ants, termites, earthworms, rodents etc., is mechanical, but, it is nevertheless important in soil formation as they rework the soil up and down. In case of earthworms, as they feed on soil, the texture and chemistry of the soil that comes out of their body changes.

Time

1.      Time is the third important controlling factor in soil formation.

2.       The length of time the soil forming processes operate, determines maturation of soils and profile development.

3.      A soil becomes mature when all soil-forming processes act for a sufficiently long time developing a profile.

4.       Soils developing from recently deposited alluvium or glacial till are considered young and they exhibit no horizons

or only poorly developed horizons.

5.      No specific length of time in absolute terms can be fixed for soils to develop and mature.

Assignments:

Describe the role of Climate as an important  soil forming factor ?

Describe the role of  life forms in the soil forming  ?

Describe  the role of topography  as an important  soil forming factor ?

Describe the role of Parent rock  as an important  soil forming factor ?