Physical properties of soil

Physical properties of soil: It includes texture, structure, density, porosity, consistency, temperature, colour, nutrient supplying ability and water holding capacity.

The properties of soil depend on the amount, size, shape, arrangement and mineral composition of its particles, kind and amount of organic matter and the volume and form of its pore and the way they are occupied by water, air at a particular time.

Physical properties of soil

Major properties of soil :

1. Soil texture

2. Soil structure

3. Porosity of soil

4. Densities of soil

5. Soil consistency

6. Soil colour

7. Soil permeability

8. Soil temperature

1. Soil texture

Soil texture refers to the relative percentage of mineral particles (sand, silt and clay) in a soil. The size of particles in mineral soil is not subject to ready change.

Thus it is considered as a permanent feature and a basic property of a soil. Texture is an important physical properties of soil because it will partly determine water intake rates (absorption), water storage in the soil, the ease of tillage operation, aeration status etc.

physical properties of soil

As the soil is a mixture of various sizes of soil separates, it is therefore, necessary to establish limits of variation for the soil separates with a view to group them into different textural classes. Texture is a basic property of a soil and it can not be changed.

Three broad and fundamental groups of soil textural classes are recognized –

Sands – The sand group includes all soils of which the sand separates make up 70 percent or more of the material by weight. Two specific classes are recognized sand and loamy sand.

Loams – Loamy soils containing many sub division does not exhibit the dominated physical properties of any these three soil separates sand, silt and clay. An ideal loam soil may be defined as a mixture of sand, silt and clay particles which exhibited light and heavy in about equal proportions.

Clay – Clay particles commonly are platy in shape and higher plastic when moist. A clay soil must carry at least 35 percent of the clay separate and in most cases not less than 40 percent. They have highest surface area since surface area is inversely related to size.

Rice, cotton, sorghum are grown on heavy textural soils which include clay, loam, silty clay loan, silty clay and clay. Medium textured soils like loams, silt loams, silts and sandy loams are suitable for most of the crops. Sandy, loamy sand and sandy clay are light textured soils. these are suitable for groundnut, potato, tobacco, pearl millet and fodder crops.

Determination of textural class

There are generally two methods employed for the determination of textural class :

Feel method – In the field, texture is commonly determined by the sense of feel. The soil rubbed between thumb and fingers under wet conditions. Sands feel gritty and its particles can be easily seen. The silt when dry feels like flour and talcum powder and is slightly plastic when wet. Clay particles feel very plastic and exhibit stickiness when wet and are hard under dry conditions.

Laboratory method – A more accurate and fundamental method has been devised by the U.S Department of Agriculture for the naming of soils based on a mechanical analysis.

2. Soil structure

The primary soil particles viz., sand, silt and clay are usually grouped together in the form of aggregates. The arrangement of primary soil particles and their arrangement into certain defined patterns is called soil structure.

Natural aggregates are called peds or secondary units and vary in their water stability. Soils that do not have aggregates with naturally preserved boundaries are considered to be structureless. Soil structure is a basic physical properties of soil.

There are two recognized forms of structureless condition :

Single grain – Particles are easily distinguished. They are unattached to each other and each particle acts individually. Movement of the air and capillary water is highest. Such structure is usually found in sandy soil and upper surface of podsols.

Physical properties of soil

Massive – Individual particles adhere closely to each other but mass lack planes of weakness. When clayey soils are ploughed under wet condition, its individual particles adhere closely to each other and soil becomes massive. Such massive soil retains water due to lack of pore spaces and thus is suitable for paddy.

Types of soil structure

It is determined by the general shapes and arrangements of peds. There are mainly four types of soil structure :

Plate like

Prism like

Block like and

Spheroidal.

Grades of soil structure

Grade indicates the degree of distinctness and durability of the individual peds.

Weak – Poorly formed, non-durable, indistinct peds that break into a mixture of a few entire and many broken peds and much unaggregated material.

Moderate – Moderately well developed peds which are fairly durable and distinct.

Strong – very well formed peds are quite durable and distinct.

3. Porosity of soil

Pore spaces in a soil consists of that portion of the soil volume not occupied by solids, either mineral or organic.

The pore space under field conditions, are occupied at all times by air and water. It is a part of Physical properties of soil. Pore spaces directly control the amount of water and air in the soil and indirectly influence the plant plant growth and crop production.

Types of pores in soils

In general there are broadly two types of pores in soils :

(i) Macro pores – Large sized pores are referred to as macro pores which allow air and water movement easily. Sand and sandy soils have a large number of macro-pores. It is found in between the granules.

(ii) Micro pores – Smaller sized pores are generally referred to as a micro pores in which movement of air and water is restricted to some extent. Clays and clayey soils have a greater number of micro-pores. It has got more important in plant growth relationship.

  • Medium pores – 0.2-0.02 mm or 200-20 microns average diameter.
  • Fine pores – 0.02-0.002 mm or 20-2 microns average diameter.
  • Very fine pores – Less than 2 microns average diameter.
Factors affecting porosity of soil

Wide difference in the total pore space of various soils occurs depending upon the following several factors :

Soil structure – A soil having granular and crumb structure contains more pores spaces than that of prismatic and platy soil structure. So well aggregated soil structure has greater pore space as compared to single grain soil.

Soil texture – In sandy soils the total pore space is small whereas in fine textured clay and clayey loam soils total pore space is high and there is a possibility of more granulation in clay soils.

Arrangement of soil particles – When the sphere like particles are arranged in columnar form It gives the most open packing system very low amount of pore spaces. When such particles are arranged in the pyramidal form it gives the most close packing system resulting high amount of pore spaces.

Organic matter – Soil containing high organic matter possesses high porosity because of well aggregate formation.

Macro-organism – Macro organisms like earthworm, rodents, insects etc. increase macro pores in the soil.

Depth of soil – With the increase in depth of, the porosity will decrease because of compactness in the sub-soil.

Puddling – Due to puddling under sufficient soil moisture, the soil surface layer is made dense and compact. Eventually, the porosity of this surface soil is reduced by the infiltration of muddy surface materials.

Cropping – At last, intensive crop cultivation tends to lower the porosity of soil as compared to fallow soils. The decrease in porosity may be due to reduction in organic matter content.

4. Density of soil

It is the weight per unit volume of a substance. It is expressed in gram per cubic centimeter or pound per cubic foot or mega gram per cubic metre (Mg m-3).

Two density measurements — particles density and bulk density are common for soils.

Density (D) = Mass (M) / Volume (V). gm/c.c or Mg m-3

Particle density – It is the mass of a unit volume of soil solids. It is denoted by P.D. and expressed in gm/c.c. and was previously known as Real specific gravity.

It depends upon the accumulative densities of the individual inorganic and organic constituents of soil. Generally in the normal soils the particle density is 2.65 grams per cubic centimeter or mega grams per cubic centimeter.

Particle density depends on the chemical composition and crystal structure of the mineral particle. The size of the particles of a given mineral and the arrangement of the soil solid have no effect on particle density.

The particle density is higher if large amount of heavy mineral such as magnetite, limonite, hematite and zircon are present. It is one of the most effective physical properties of soil.

Bulk density – It is the mass of a unit volume of dry soil including both solids and space (pore). It is denoted by B.D and expressed in gm/c.c or mega gram per cubic metre.

The bulk density of a soil is always smaller than its particle density. Bulk density normally decreases as mineral soils become finer in texture. It is of greater importance than particle density in understanding the physical properties of soil and behavior.

5. Soil consistency

Soil consistency is an important physical properties of soil which varies with the variation of soil moisture and applied stress. Consistency is the behaviour of soil under stress.

This stress is commonly evidenced by feeling the soil, manipulating it by hand, or by tillage operation. Soil consistency is considered a combination of soil properties dependent upon the forces of attraction between soil particles adhere influenced by soil moisture.

In the soil science, soil consistency is defined as the manifestations of the physical forces of cohesion and adhesion acting within the soil at various moisture constants. These manifestation include the behaviour towards gravity, pressure, thrust and pull etc.

Soil consistency depends on the texture, nature and amount of inorganic and organic colloids, structure and moisture content etc. With decreasing moisture content, the soils lose their stickiness and plasticity and become friable and soft and finally when dry become hard and coherent.

6. Soil colour

It is inherited from its parent material and that is referred to as lithochromic. Besides soil colour also develops during soil formation through different soil forming processes and that is referred to as acquired or pedochromic colour.

It indicates many physical properties of soil. A change in soil colour from the adjacent soils indicates a difference in the soil’s mineral origin (parent material) or in the soil development. Soil colour varies among different kinds as well as within the soil profile of the same kind of soil.

Physical properties of soil

Organic matter imparts black to dark grey tinges, iron compounds for red, brown and yellow tinges: and silica lime and other salts give light, white and grey tinges. Red colour is related to unhydrated ferric oxide although manganese dioxide and partially hydrated iron oxide may also contribute to it.

The yellow colour is due to oxides of iron. Yellow colour indicates more moist soils than red colour. Soil colour directly influences soil temperature.

7. Soil permeability

Soil permeability is the ability of the soil to transmit air and water.

Soil is good for aquaculture because water loss through seepage is low. The permeable differs from one layer to another layer as the soil layers vary in their characteristics.

The permeability of the soil is measured in the laboratory by measuring the rate of flow of water from a constant head of water through the soil.

8. Soil temperature

Temperature of soil has an effect on the growth of plant, it’s germination, nutrient uptake transformation. Heat flow in the soil is slower than the atmosphere.

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