Soil texture

SOIL TEXTURAL ANALYSIS BY FEEL METHOD

Principle

         Texture is defined as the fineness or coarseness of the soil and varies with the relative proportions of soil particles of varying sizes. Based on the size of the individual particles, they are classified as gravel, sand, silt and clay. As clay particles are very small in size, they are not visible to the naked eye while sand particles can be visually observed. The following are some of the characteristics used for classifying soils into different textural class. 

Name of the particle                  Diameter (mm)                            Visibility                                     Feel by finger                                                    Minerals

Coarse sand                                2.00- 0.20                                 Naked eye                                         Very gritty                                                 Primary minerals

Fine sand                                    0.20- 0.02                                  Naked eye                                         Gritty                                                         Primary minerals

Silt                                               0.02- 0.002                                Ordinary microscope                    Flour like                                                Primary and Secondary

Clay                                               <0.002                                     Electron microscope                     Very sticky                                              Mostly Secondary

The texture of soil is determined qualitatively by the field or feel method and quantitatively by the hydrometer or International pipette method.

Field method (Feel method)

         This method provides an approximate estimation of the texture but in the hands of an experienced worker, the results by this method will be as good as any other laboratory method.

Procedure

         Moisten a small quantity of dry soil with water and mix it thoroughly on a glass or porcelain dish so that a soft ball is formed. Then work it further until stiff, and squeeze it between the thumb and forefingers. Press the thumb forward gradually forming the soil into a ribbon. By observing the feel of the fingers, ease of forming a ball, stickiness or grittiness and whether the soil formed a ribbon or merely crumbles on squeezing, the soil can be classified. The important characters of certain classes are listed below:

Determination of Soil texture (feel method)

      Textural class                        Feel of fingers                               Ball formation                                 Stickiness                         Ribbon                                                                                                                                                                                                                       formation

Sand                                              Very gritty                              Does not form ball                           Does not stain fingers           No                   

Loamy sand                                        Very gritty                    Forms very easily broken ball                   stains fingers slightly                       No

Sandy loam                                    Moderately gritty        Forms fairly firm ball but is easily broken        Definitely stains fingers                   No

Loam                     Neither very gritty nor very smooth                   Forms firm ball                                  Definitely stains fingers                   No

Silt loam                      Smooth or slick ‘buttery’ feel                       Forms firm ball                                  Definitely stains finger    Slight tendency                                                                                                                                                                                                                 to form                                                                                                                                                                                                           ribbon with flaky 

                                                                                                                                                                                                              surface

SOIL TEXTURAL ANALYSIS BY INTERNATIONAL PIPETTE METHOD (OR) ROBINSON’S PIPETTE METHOD

Principle

         The mechanical composition of the soils is assessed in the laboratory by either of the following Methods: a. Hydrometer method b. International pipette method. The hydrometer method is quick and fairly accurate for many purposes. The International method gives very accurate values of each of the soil separates and is useful for most of the fundamental studies but at the same time cumbersome and time consuming.

         This method is based on Stokes’ law. According to this law the rate of fall of a particle in liquid is directly proportional to the square of its radius. V  a r2  

                                                                           V =  2/9 (dp - d ) /n (gr 2)               

Where

         V       =      sedimentation velocity in cm/sec

         g      =      acceleration due to gravity cm/sec 2

         r       =      radius of the particle or sphere (cm)

         dp    =      density of the particle (g/cc)

         d      =      density of the liquid (g/cc)

                 =      viscosity of the liquid

         The soil is first dispersed by destroying the binding agents with hydrogen peroxide and hydrochloric acid followed by treatment with a dispersing agent. Clay and silt are separated by sedimentation and coarse and fine sand by sieving.

Materials required

1000 mL spoutless measuring cylinder                Filter stand                                  500 mL beaker                 Filter paper Whatman No.50                     Funnel  (7.5 cm)    Mechanical  stirrer                                 Rubber stopper for the cylinder              Stop clock                                   Water bath                     150 mL measuring cylinder

Chemical balance                                     Tall form beaker                                Robinson pipette                            Hot air oven                                      Porcelain

6 % Hydrogen peroxide                                 N/5 HCl                                         Normal NaOH                               dish/basin

Procedure  

         Transfer exactly 20 g of air dried soil sample to a 500 ml beaker. Add 60 ml of 6 per cent hydrogen peroxide. Stir it well and keep it on a water bath for 30 minutes till frothing ceases. Treatment with hydrogen peroxide is to destroy the organic matter which is binding soil particles. Hydrogen peroxide treatment is not necessary when the organic matter content of the soil is negligible. Then add 200 ml of N/5 HCl, stir it well and keep it over night. Hydrochloric acid is added to destroy CaCO3 which is also binding agent.

         Filter the contents through what man No.50 filter paper and wash it with water till the filtrate runs free of chloride (collect about 10 mL of the filtrate in a test tube and add 2-3 drops of silver nitrate solution. Formation of curdy white precipitate indicates the presence of chloride). When the filtrate runs free of chloride, transfer the soil material from the filter paper to another 500 ml beaker and add about 400 ml water. Then add 8 ml of normal sodium hydroxide and stir it well for 10 minutes with a mechanical stirrer. Transfer the contents to a 1000 ml spout less measuring cylinder and make up to 1000 ml mark with water. Cover the cylinder tightly with a rubber stopper and shake the contents thoroughly by repeated inversions holding the rubber stopper tightly so as to avoid spilling of the soil water suspension.

Clay and silt

Remove the rubber stopper and place the cylinder under Robinson pipette and start a stop clock simultaneously. Note down the temperature and the settling time for clay plus silt from the table. Till the settling time is over do not disturb the suspension. First lower the pipette in such a way that the tip of the pipette just touches the surface of the suspension. At the end of the stipulated settling time for clay and silt, lower the pipette to 10 cm depth and draw 20 mL suspension and deliver it to a weighed clean porcelain dish. This suspension contains clay plus silt. Evaporate this first by keeping in on a water bath and dry it in an air oven at 105°C. Cool it in a desiccator and determine the weight of clay plus silt and calculate the percent.

Clay alone

         Shake the contents of the cylinder well and allow undisturbed till the stipulated settling time for clay alone corresponding to the suspension temperature. Withdraw 20 mL of the suspension at the end of the period as done in the case of clay plus silt and determine the weight as clay alone after evaporating and drying.

Coarse sand and fine sand

         Pour out major portion of the suspension from the cylinder after withdrawing sample for clay alone. While pouring out care should be taken to see that no sand fraction is lost. Then wash the sediment with water and transfer the contents to a tall form beaker. Add water to a height of more than 10 cm. Stir well and allow it to stand for 4 minutes. Then pour off the supernatant liquid. Repeat this process till the water poured off is no longer turbid. Transfer the residue to a porcelain basin, dry it in an oven and weigh as coarse sand plus fine sand. Sieve the coarse sand and fine sand in a seventy mesh sieve. The material passing through the sieve will be fine sand while the coarse sand fraction will be retained on the sieve. Weigh it as coarse sand.

Calculation

Clay + Silt

         Weight of soil taken                         =      20 g

         Volume of suspension                     =      1000 mL

         Volume of suspension pipetted out =      20 mL

         Weight of empty porcelain dish       =      a g

         Weight of silt + clay + dish + NaOH =      b g

         Weight of clay + silt + NaOH           =      (b-a) g

         Weight of NaOH alone                               

         (Present in 20 mL of suspension)    =      0.0064 g

         Weight of clay + silt alone               =      b - (a + 0.0064) g

         Per cent clay + silt                            =      b-(a + 0.0064) x  

Per cent clay + silt                                      =

Clay alone

         Weight of empty porcelain dish               = p g

         Weight of dish + clay + NaOH                       = q g

         Weight of clay + NaOH                               = (q-p) g

Weight of clay alone                                        = q - (p + 0.0064) g  

Weight of NaOH alone

(in 20 mL of suspension)                             =   0.0064 g

         Per cent clay                                  =   q – (p + 0.0064) x

Per cent silt                                                 =   (Per cent clay + silt) – (Per cent Clay)

Per cent clay                                               =

Per cent silt                                                  =

Coarse sand + fine sand

         Weight of porcelain basin                            = x g

         Weight of dish + coarse sand + fine sand    = y g

         Weight of coarse sand + fine sand alone                   = y - x g

         Per cent coarse sand + fine sand                          = (y – x) x                                                                                                                        

Coarse sand alone

         Weight of porcelain basin                =      c g

         Weight of basin + coarse sand         =      d g

         Weight of coarse sand alone            =      (d-c) g

         Per cent coarse sand                   =      (d – c) x                                                                                                                          

             Per cent fine sand                                   =   (Per cent coarse sand + fine sand) –

                                                                                          (Percent coarse sand)

Per cent coarse sand                       =

Per cent fine sand                            =