PHYSICAL CHARACTERISTICS OF SOIL

 PHYSICAL CHARACTERISTICS OF SOIL

1. Soil Texture

·       Non-technically, Soil texture is defined as “How the soil feels to the touch”.

·       Technically, Soil texture is defined as “The proportions of sand, silt and clay in the soil”.

·       Soil texture refers to the proportion of the soil “separates” that make up the mineral component of soil. These separates are called Sand, Silt and Clay.

a)    Sand

ü  Sand particles are the largest in the soil, ranging in size from 0.05 mm to 2.00 mm.

ü  Soil with high sand content feels gritty and doesn’t hold well in a ball.

ü  Sand particles are highly inactive and play less role in Physicochemical activities but when coated with clay, the sand particles take very active part in chemical reactions.

b)    Slit

ü  Silt particles are moderate size particles and range from 0.002 mm to 0.05 mm.

ü  Silt, when wet, feels plastic but in dry state feels like flour or talcum.

ü  Silt soil has got larger exposed surface area than the sandy soil. Silt soils contain sufficient quantities of nutrients, both organic and inorganic. That is why they are very fertile.

ü  Soils rich in silt possess high water holding capacity. Such soils are good for agriculture. 

c)    Clay

ü  Clay particles are the smallest in the soil, with sizes less than 0.02 mm.

ü  Most clay minerals consist of microscopic layers. These are called Phyllosilicate minerals.

ü  Properties of Clay

§  Target Demonstration - Sticky (adhesion - sticks to other things)

§  Ribbon Demonstration - Plastic (cohesion - sticks to itself)

§  Slinky Demonstration - Shrink swell

§  Block Demonstration - Large surface area, due to layers and size.

2. Soil Plasticity and Consistency

·       Soil plasticity is a property that enables the moist soil to change shape when some force is applied over it and to retain this shape even after the removal of the force from it.

·       The plasticity of soil depends on the Cohesion (sticks to itself) and Adhesion (sticks to other things) of soil materials.

·       Soil consistency depends on the Texture and Amount of inorganic and organic colloids, Structure and Moisture contents of soil.

 

3. Soil structure

·       The Soil separates (Sand, Slit and Clay) can become aggregated together into discrete structural units called “Peds”. These Peds are organized into a repeating pattern that is referred to as Soil structure.

·       The types of Peds are

 

·       Grades of Soil structure are

a)    Structureless – No Peds are observed in the undistributed soil.

b)    Weak – Poorly formed indistinct Peds are found in undistributed soil.

c)    Moderate – Fairly well formed Peds.

d)    Strong - Well formed distinct Peds.

·       Soil structure also reveals the Colour, Texture and Chemical composition of soil aggregates.

·       Soil structure is influenced by Air moisture, Organic matter and Microorganisms.

4. Soil Colour

·       Soils exhibit a variety of colours. Soil colour may be inherited from the parental material or sometimes it may be due to soil forming processes.

·       Red colour in the soil is due to the abundance of Iron oxide under oxidized conditions in the soil.

·       Black or Dark greyish-black colour is generally due to the accumulation of highly decayed Organic matter.

·       Yellow colour is due to hydrated Iron oxides and hydroxide.

·       Black nodules are due to Manganese oxides.

·       Pale yellow mottles coupled with very low pH are indicative of possible acid Sulphate soils.

·       Silica, Lime and some other Inorganic compounds give Light white and Grey tinges to the soil.

·       Soil colour influences greatly the soil temperature. The dark coloured soils absorb heat more readily than light coloured soils.

·       Soil colour is used as an important criterion for description and classification of soil. Many soils are named after their prominent colours, such as Black cotton soil, Red yellow latosol and Grey hydromorphic soils.

5. Soil Temperature

·       The chief sources of soil heat are

ü  Solar radiations

ü  Heat generated in the decomposition of dead Organic matters in the soil.

ü  Heat formed in the interior of earth.

·       The soil temperature greatly affects the physico-chemical and biological processes of the soil.

·       Temperature of soil depends upon the Temperature of Atmospheric air and on Moisture content.

·       Temperature of soil is controlled by Climate, Colour of soil, Slope & altitude of the land and Vegetational cover of the soil.

6. Porosity of Soil

·       Pores are the spaces between Soil particles or Aggregates.

·       Pores are important because they allow Air and Water to move through the soil and also to be stored there.

·       Porosity of soil depends upon the Texture, Structure, Compactness and Organic content of the soil.

·       Porosity of the soil increases with the increase in the percentage of Organic matter in the soil.

·       Porosity of soil also decreases as the soil particles become much smaller in their dimension because of decrease in pore spaces. It also decreases with depth of the soil.

·       The pore spaces are responsible for better plant growth because they contain enough air and moisture.

7. Soil Aeration

·       Soil Aeration is a phenomena of rapid exchange of Oxygen and Carbon-di-oxide between the soil pore and the atmosphere, in order to prevent the Deficiency of Oxygen or Toxicity of Carbon-di-oxide in the soil.

·       The well aerated soil contains enough Oxygen for respiration of roots and aerobic microbes and for oxidation reaction to proceed at optimum rate.

·       Major gases in atmosphere are present in soil. Three important gases are present in soil. They are Nitrogen (79 %), Oxygen (18 – 20 %) and Carbon-di-oxide (1 – 10 %).

·       Soil CO₂ content is higher than Atmospheric CO₂.

·       Soil aeration can be affected by two factors. They are (i) Soil organic matter and (ii) Soil moisture.

·       Soil aeration affects the availability of some nutrients elements to plant roots. Manganese and Iron occurs in the well aerated soil in their higher valent forms (Mn⁺⁺⁺⁺, Mn⁺⁺⁺ & Fe⁺⁺⁺) and in poorly aerated soils in their lower valent forms (Mn⁺⁺, Fe⁺⁺). They are available to plants only in their lower valent forms.

8. Soil Permeability

·       The characteristic of soil that determines the movement of water through pore spaces is referred to as Soil permeability.

·       Soil permeability is directly dependent on the pore size. It will be higher for the soil with large number of macro-pore spaces than that for compact soil with a large number of micro-pore spaces.

·       Permeability of soil also varies with moisture status and usually decreases with the gradual desiccation of soil.