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Current Category » Principles of Agronomy

Soil- Physical Properties Of soil

The physical properties of soils are dominant factors affecting the use of a soil which determine the availability of O2 in soils, the mobility of water into and though soils and case of root penetration and also the chemical and biological behavior of soil. These depend primarily on the amount, size, shape and arrangement of its inorganic particles, shape and arrangement of it inorganic particles, kind and amount of org. matter, the total volume of pore spaces and the way it is occupied by water and air at a particular time.
Those are: Texture, Structure, Density, Porosity, Consistency, Colour and Temperature.
Soil Textures: It refers to the relative proportions of soils separates i.e. sand, silt and clay in particular soil. It is permanent or static property of soil.
Natural soils are comprised of soil particles of varying sizes. The soil particle size groups are called as soil separates as stone (more than 20mm dia). Gravel (2 – 20 mm dia), Fine earth (less than 2mm dia) coarse sand (0.2 to 2 mm dia), fine sand (0.2 to 0.02 mm), silt (0.02 to 0.002 mm) and clay (less than 0.002 mm dia).

1. Sand: Sand particles are large with very little surface area exposed (0.1 m2/g specific area). These are fragments of quartz, insoluble; nutrients supplying ability are practically nil. Pre space are bigger (macro pores) which facilitates rapid movement of air and water. Sand does not absorb water; do not exhibit properties swelling and shrinkages, stickiness and plasticity. Unless coated with clay or silt, they do not exhibit properties as Cohesions, moisture and nutrient retention, etc. Soils having high percent of sand can be easily cultivated with little or light draft requirements, low water holding capacity, less fertile, dry out quickly. As sand grains are large and coarse, soils dominated by sand are called as coarse textured or light soils.

2. Silt: These particles are intermediate in size to sand and clay. Because of adhering film of clay, they exhibit some plasticity, cohesions adhesion and absorption and can hold more amount of water than sand but less than clay. Soils dominated by silts armid way in properties, workability and productivity between sandy and clayey soils. The average specific area of silt particles is 1 sp. m/g.

3. Clay: It ultra microscopic size and large surface area (10 to 1000 sq. per. g.). The clay particles are smooth and in a colloidal state. It greatly influences the physical and chemical properties of soil. Clay particles absorb and retain water, sweel on wetting and shrink on drying, exhibit properties like flocculation (grouping/clustering)., deflocculating, plasticity and stickiness. Soils with high clay are poor drained, require very heavy draft for cultivation, can be worked in narrow range of moisture regime. Clayey soils are called as heavy soils as they are difficult or heavy for cultivation.

Textural classes: All soils have all the three soil separates in varying proportions. Based on their proportions, the soils can be grouped into textural classes and are named according to the soil separates which is predominant in them as:

Group

Class

 

Ranges (%) of

 

 

 

           Sand

      Silt

    Clay

 

 

 

 

 

 

 

 

Very coarse textured

Sand

 

 

85-100

 

0-10

0-10

 

Loamy sand

 

 

70-90

 

0-30

0-15

Coarse textured

Sandy loam

 

 

43-80

 

0-50

0-20

 

Loam

 

 

23-52

 

 28-50

7-27

 

Silt Loam

 

 

0-50

 

 50-88

0-20

 

Silt

 

 

   0-20

 

 88-100

     0-12

Medium textured

Sandy % clay Loam

 

 

45-80

 

0-28

20-55

Fine textured

Clay loam

 

 

20-45

 

15-53

27-40

 

Silty clay loam

 

 

0-20

 

40-73

27-40

Fine textured

Sandy clay

 

 

40-65

 

0-20

35-45

 

Silty clay
Clay

 

 

0-20
0-40

 

40-60
0-40

40-60
40-60

 

 

 

           
Significance of soil texture:

It influences physical and chemical properties like water holding capacity, nutrient retention and fixation and its availability, drainage, strength, compressibility and thermal regime. Suitability of a soil to a particular crop depends on texture in addition to soil depth, depth of water table, salinity and alkalinity. Loamy soils (Silty) exhibit intermediate properties, so best for agricultural production because they retain more water and nutrients than sandy and have better drainage, aeration and tillage properties than clay soils.

Current Category » Principles of Agronomy