SOIL TEXTURE

The soil separates are defined in terms of diameter (in millimeters) of the particles. Soil particles >2 mm in diameter are excluded from soil textural determinations.         

Particles less than 2 mm are called fine earth, normally considered in chemical and mechanical analysis.

The size limits of these fractions have been established by various organizations. There are a number of systems of naming soil separates.

•   The American system developed by United States Department of Agriculture(USDA)

•   The English system or British system developed by British Standard Institute (BSI )

•  The International system developed by International Society of Soil Science (ISSS)

•   European  system 

i) USDA

Soil Separates                                          Diameter (mm)

Clay                                                                <0.002

Silt                                                                   0.002-0.05

Very Fine Sand                                                0.05-0.10

Fine Sand                                                         0.10-0.25

Medium Sand                                                   0.25-0.50

Coarse Sand                                                     0.50-1.00

Very Coarse Sand                                            1.00-2.00

 ii)BSI

Soil separates                                     Diameter (mm)

Clay                                                      < 0.002

Fine Silt                                                 0.002 – 0.01

Medium  silt                                          0.01  – 0.04

Coarse Silt                                            0.04   – 0.06

Fine Sand                                              0.06  -  0.20

Medium Sand                                        0.20 -  1.00

Coarse Sand                                         1.00 – 2.00

iii) ISSS

  Soil separates                     Diameter (mm)

  Clay                                      < 0.002

  Silt                                         0.002 – 0.02 

  Fine sand                              0.02 – 0.2

  Coarse sand                         0.2 – 2.0

(v) European System

          Soil separates                    Diameter (mm)

            Fine clay                          < 0.0002

          Medium clay                      0.0002 – 0.0006

          Coarse clay                        0.0006 – 0.002

             Fine silt                            0.002   - 0.006

           Medium silt                           0.006   - 0.02

           Coarse silt                             0.02  - 0.06

            Fine sand                              0.06 - 0.20

           Medium sand                         0.20 -  0.60

            Coarse sand                         0. 60 -  2.00

Particle size distribution/ determination

         The determination of relative distribution of the ultimate or individual soil particles below 2 mm diameter is called as Particle size analysis or Mechanical analysis.

Two steps are involved

i) Separation of all the particles from each other ie., Complete dispersion into ultimate particles

ii) Measuring the amount of each group

Separation of particles

S. No       Aggregating agents                           Dispersion method

1              Lime and Oxides of Fe & Al                Dissolving in  HCl

2              Organic matter                                     Oxidizes  with H2O2

3              High concentration of                 precipitate and decant or            

                electrolytes (soluble                               filter with suction   salt)                                                                                                                                                                                                                                         

4           Surface tension                                 Elimination of air by stirriring with water or boiling                                                                                                                                     

After removing the cementing agents, disperse by adding NaOH

Measurement 

Once the soil particles are dispersed into ultimate particles, measurement can be done as follows

i)  Coarser fractions – sieving – sieves used in the mechanical analysis corresponds to the desired particle size separation

•   For 2 mm, 1 mm and 0.5 mm – sieves with circular holes

•   For smaller sizes, wire mesh screens are used ( screening)

ii) Finer fractions – by settling in a medium 

The settling or the velocity of the fall of particles is influenced by

•  Viscosity of the medium

•   Difference in density between the medium and falling particles

•   Size and shape of object

Methods of Textural determination

Numerous methods for lab and field use have been developed

i)                    Elutriation method – Water & Air

ii)                  Pipette method

iii)                Decantation/ beaker method

iv)                Test tube shaking method

v)                  Feel method – Applicable to the field – quick method – by feeling the soil between thumb and fingers

Soil Textural Classes

         To convey an idea of the textural make up of soils and to give an indication of their physical properties, soil textural class names are used. These are grouped into three main fractions viz., Sand, Silt and Clay.

According to the proportion of these three fractions a soil is given a name to indicate its textural composition. Such a name gives an idea not only of the textural composition of a soil but also of its various properties in general.

On this basis, soils are classified into various textural classes like sands, clays, silts, loams etc.

Sands:

The sand group includes all soils in which the sand separates make up at least 70% and the clay separate 15% or less of the material by weight. The properties of such soils are therefore characteristically those of sand in contrast to the stickier nature of clays. Two specific textural classes are recognized in this group sandy and loamy sand.

Silt:

The silt group includes soils with at least 80% silt and 12% or less clay. Naturally the properties of this group are dominated by those of silt. Only one textural class - Silt is included in this group.

Clays:

To be designated clay a soil must contain at least 35% of the clay separate and in most cases not less than 40%. In such soils the characteristics of the clay separates are distinctly dominant, and the class names are clay, sandy clay and silty clay. Sandy clays may contain more sand than clay. Likewise, the silt content of silty clays usually exceeds clay fraction

Loams:

The loam group, which contains many subdivisions, is a more complicated soil textural class. An ideal loam may be defined as a mixture of sand, silt and clay particles that exhibits the properties of those separates in about equal proportions. Loam soils do not exhibit dominant physical properties of sand, silt or clay. Loam does not contain equal percentage of sand, silt and clay. However, exhibit approximately equal properties of sand, silt and clay.

Determination of Textural Class:

 In the American system as developed by the United State Department of Agriculture twelve textural classes are proposed. 

Importance of Soil Texture

•   The presence of each type of soil particles makes its contribution to the nature and properties of soil as a whole

•   Texture has good effect on management and productivity of soil. Sandy soils are of open character usually loose and friable.

•   Such type of texture is easy to handle in tillage operations.

•  Sand facilitates drainage and aeration. It allows rapid evaporation and percolation.

•   Sandy soils have very little water holding capacity. Such soils can not stand drought and unsuitable for dry farming.

•   Sandy soils are poor store house of plant nutrients

•   Contain low organic matter

•   Leaching of applied nutrients is very high.

•   In sandy soil, few crops can be grown such as potato, groundnut and cucumbers.

•   Clay particles play a very important role in soil fertility.

•   Clayey soils are difficult to till and require much skill in handling. When moist clayey soils are exceedingly sticky and when dry, become very hard and difficult to break.

•   They have fine pores, and are poor in drainage and aeration.

  •   They have a high water holding capacity and poor percolation, which usually results in water logging.

  •   They are generally very fertile soils, in respect of plant nutrient content. Rice, jute, sugarcane can be grown very successfully in these soils.

  •   Loam and Silt loam soils are highly desirable for cultivation

  •   Generally, the best agriculture soils are those that contain 10 – 20 per cent clay, 5 – 10 per cent organic matter and the rest equally shared by silt and sand