Lecture: 7       Soil Texture 

    Physical properties (mechanical behaviour) of a soil greatly influence its use and behaviour towards plant growth. The plant support, root penetration, drainage, aeration, retention of moisture, and plant nutrients are linked with the physical condition of the soil. Physical properties also influence the chemical and biological behaviour of soil. The physical properties of a soil depend on the amount, size, shape, arrangement and mineral composition of its particles. These properties also depend on organic matter content and pore spaces.

 Important physical properties of soils.

 1. Soil texture,.                    2. Soil structure 3. Surface area.                    4. Soil density,.      5. Soil porosity,                     6.Soil colour, 7.Soil consistence 

Soil texture

 Definition 

Soil texture refers to the relative proportion of particles or it is the relative percentage by weight of the three soil separates viz., sand, silt and clay or simply refers to the size of soil particles. Soil texture is a basic property of soil which cannot be change or modified thus it is a static property of soil (Except kitchen garden). 

Classification of Soil particles

 A. Based on their size- 4 systems 

1. USDA (United State Department of Agriculture) 

2. ISSS (International Society of Soil Science) 

3. BSI (British Standards Institute)

 4. Mohr’s 10 fraction system

 1. USDA- Follow in world, 7 fraction system

B. Based on tillage hindrance:

1. Light texture-

                              Easy to cultivate or require light draft/ less energy for ploughing. Light texture does                                not mean that soils are light in weight. Ex. Sandy, Loamy Sand

2. Heavy texture- 

                              Difficult to cultivate or require more/heavy energy for ploughing. Heavy texture                                      does not mean that soils are heavy in weight. Ex.Sandy clay, silty clay, clay.

LABARATORY METHOD: PARTICLE SIZE ANALYSIS / MECHANICAL ANALYSIS:

 The determination of relative distribution of individual soil particles below 2 mm diameter

is called as Particle size analysis or Mechanical analysis. There are two steps in mechanical analysis.

a. Dispersion     b. Sedimentation

 a. Dispersion:-

                          Dispersion of soil particles is breaking down the cementing agents viz., organic matter,                           calcium carbonate and iron oxides; and flocculating agents like salts and ions.                                           Removal of organic matter is usually oxidation with hydrogen peroxide and calcium                               carbonate can be dissolved by dilute HCl. Sodium bicarbonate and sodium dithionate                               does remove iron and aluminum oxides. Dispersion or deflocculation is carried by                                   sodium hexametaphosphate.

Now particles of 2 to 0.2 mm (coarse sand ) are separated by sieving through 0.2 mm sieve. For separation of remaining particle sedimentation technique is used

 b. Sedimentation: -

                                It is based on measuring the relative settling velocities of particles of various sizes                                   from aqueous suspension. In a given medium larger particles settle more quickly                                     than smaller ones of the same density. The relation between particle density and rate                                 of fall is expressed by Stokes’ law.

There are two widely accepted methods:

 1. Robinson’s Pipette Method:-

                                                     Also called as International pipette method. It is based on the sedimentation principle.

2. Bouyoucos Hydrometer Method:-

                                                             It is based on the principle that there is a continuous decrease in                                                                 the density of soil suspension over a period of time. The pipette                                                                 method is regarded as the standard method for particle size                                                                         analysis because of its accuracy. It is very time consuming and                                                                   difficult to analysis large no. of samples. The hydrometer method                                                              is rapid but less accurate than the pipette method. This method                                                                  gives erroneous results in soils having high CaCO3 and organic                                                                matter contents and high salinity.

Stoke’s law:-

                       Given by G.G.Stoke (1851) The velocity of fall of particles with same density is directly                         proportional to the square of the radius and inversely proportional to the viscosity of                               liquid medium.

                             𝐕 =2gr2*(𝐝𝐬 − 𝐝𝐰)/ 𝟗𝒏gr2

Where, 

V - Velocity of settling particle (cm/sec.) 

g - Acceleration due to gravity cm/ sec2 (981)

 ds - Density of soil particle (2.65) 

dw - density of water (1 )

 n - Coefficient of viscosity of water (0.0015 at 4oC)

r - Radius of spherical particles (cm). 

Assumptions:

                       1. Particles must be spherical, smooth and rigid 

                       2. Particles must be of uniform density 

                       3. Fall must be unhindered 

                       4. Size of the particle must be large compared to the size of molecule of the liquid 

                       5. The suspension must be still without any turbulence

Limitations: 

                   1. Some colloidal particles are plate shaped and fall slower than spherical particles of the                             same mass

                    2. Soil particles are not all of the same density 

                    3. Particles >0.08 mm diameter settle quickly and cause turbulence 

                    4. Maintain a constant temperature

The textural triangle

                                   It is used to determine the soil textural name after the percentages of sand, silt,                                         and clay are determined from a laboratory analysis. Since the soil's textural                                              classification includes only mineral particles and those of less than 2mm                                                  diameter, the sand plus silt plus clay percentages equal 100 percent. (note that                                          organic matter is not included.) Knowing the amount of any two fractions                                                automatically fixes the percentage of the third one. To use the diagram, locate the                                    percentage of clay first and project inward parallel to sand line. Do likewise for                                        the per cent silt and project inward parallel to clay line and for sand, project                                              inward parallel to silt. The point at which the projections cross or intersect will                                        identify the textural class name.