The study of soil particles, texture, structure, and density are very important parts of soil science. This article will discuss the definition, classification, and comparison of soil particles, texture, structure, and density in detail.
Soil particles:
The particles which constitute the inorganic portion of soil and which are 2 mm or less in diameter are called soil particles. There are 3 soil particles namely sand, silt, and clay.
The particles having more than 2 mm diameter are not included under soil particles. Those are stones, gravels, pebbles, etc.
Soil separate:
Soil particles ranging between specified size limits are called soil separates. Examples –
- Sand: 2.0 – 0.02 mm in diameter
- Silt: 0.02 – 0.002 mm in diameter
- Clay: <0.002 mm in diameter
Mechanical analysis:
The analytical procedure by which the particles are separated into various size groups from the coarse Sand, through Silt, to the finest clay is caused by mechanical analysis.
Classification of soil particles:
Soil particles can be classified into 3 systems –
1. ISSS (International society of soil science) system:
Soil particles Diameter range (mm)
- Coarse sand 2.0 – 0.2
- Fine sand 0.2 – 0.02
- Silt 0.02 – 0.002
- Clay < 0.002
2. USDA (United States Department of Agriculture) system:
Soil particles Diameter range (mm)
- Very coarse sand 2.0 – 1.0
- Coarse sand 1.0 – 0.5
- Medium sand 0.5 – 0.25
- Fine sand 0.25 – 0.1
- Very fine sand 0.1 – 0.05
- Silt 0.05 – 0.002
- Clay < 0.002
3. MIT (Massachusetts Institute of Technology):
Soil particles Diameter range (mm)
- Coarse sand 2.0 – 0.6
- Medium sand 0.6 – 0.2
- Fine sand 0.2 – 0.06
- Silt 0.06 – 0.002
- Clay < 0.002
4. FAA (Federal Aviation Agency):
Soil particles Diameter range (mm)
- Sand 2 – 0.075
- Silt 0.075 – 0.005
- Clay < 0.005
Soil can group 12 textural classes, which are as follows –
- Sand
- Loamy Sand
- Sandy Loam
- Loam
- Silt Loam
- Silt
- Sandy clay Loam
- Clay Loam
- Silty Clay Loam
- Sandy Clay
- Silty Clay
- Clay
Comparative characters of Sand, Silt, and Clay:
| Character/Properties | Sand | Silt | Clay |
| Means of observation | Naked eye | Microscope | Electron Microscope |
| Size(mm) ISSS system | 2.0 – 0.02 | 0.02 – 0.002 | < 0.002 |
| Shape | Round | No definite shape | Plate |
| Water holding capacity | Low | Medium | High |
| Infiltration | Very rapid | Medium | Slow |
| Major minerals | Quartz | Quartz | Vermiculite |
| Minor minerals | Feldspar, Hornblende,Mica | Feldspar, Mica, Hornblende, Oxides of Fe, Al | Oxides of Fe and Al |
| Macro pore space | Large | Medium | Very small |
| Micropore space | Small | Medium | Highest |
| Surface area | Low (0.1 m2/g) | Medium (1 m2/g) | High (10 – 1000) m2/g |
| Plasticity, Stickiness Swelling, Adhesion, Cohesion | Very low | Medium | High |
| Tillage | Easy | Moderate | Difficult |
| Fertility | Very low | Moderate | Very high |
| Chemical activity | Inactive | Slightly active | Very active |
| Nutrient absorption | Nil | Slight | High |
Soil texture:
Soil texture refers to the relative coarseness or fineness of the soil. Specifically, the texture is the relative proportions of sand, silt, and clay or the particle-sized groups smaller than gravel. Sandy texture soil indicates coarse or light texture soil. Similarly, clay texture soil indicates fine texture or heavy texture soil.
Textural classes:
Soil texture is the basic property of soil. Every soil contains 3 particles – Sand, Silt, and clay in different proportions. Soils have been grouped into 12 textural classes depending on the same limits of variations.
The textural classes in order to increase fineness are –
| Common Name | Texture | Textural class |
| Sandy soils | Coarse | · Sand · Loamy sand: Sand (23-52%), Silt (28-50%), Clay (7-23%) |
| Loamy soils | Medium | · Sandy loam · Loam · Silty Loam · Silt · Sandy Clay Loam · Silt Clay Loam · Clay Loam |
| Clayey Soils | Fine/heavy | · Sandy Clay: Sand (45-65%), Silt (0-20%), Clay (35-45%) · Silty Clay · Clay |
Major textural groups: Three major textural groups are –
- Sandy group (Sandy soils): Sand group contains a minimum of 70% sand.
- Loam group (Loamy soils): Loamy group does not contain equal proportions of sand, silt, and clay but it exhibits light and heavy properties in about equal proportions.
- Clay groups (Clayey soils): Clay group contains clay not less than 35% and in most cases not less than 40%.
The loamy type of soil is suitable for agricultural crops because it provides proper aeration, water retention nutrients, holding capacity, and easiness of root penetration.
Importance of soil texture:
1. The texture of the soil is one of the fundamental considerations in soil classification.
2. The rate and extent of many important reactions in soils are governed by soil texture.
3. Some important physical properties of soil like aerations, water movement, root penetration, nutrient holding capacity, and water holding capacity are greatly influenced by soil texture. For example –
- Aeration and water movement – More in sandy soil, less in clay soils.
- Root penetration – Easy in sandy soils, hard in clay soils.
- Water and nutrient holding capacity – Less in sandy soil, more in clay soil.
4. Soil texture indicates the weathering stage of rocks to some extent.
5. The suitability of crops depends on soil texture. For example – Sandy soil is suitable for watermelon, muskmelon, groundnut, etc. Clay soil is good for rice.
6. The nature of tillage operations also depends on soil texture.
Soil Structure:
The term texture is used in reference to the size of soil particles. However, soil particles usually remain together in the form of aggregates. Natural aggregates are called peds while artificial aggregates are known as clods. Soil structure refers to the arrangement of soil particles into a definite pattern.
Classification of soil structure:
1. On the basis of shape peds (known as structural types) –
- Plate-like
- Prism like – Columnar, Prismatic
- Block like – Angular, Sub-angular
- Sphere like – Granular, Crumb
2. On the basis of the size of peds (called structural class) –
- Very fine (very thin): < 1 mm diameter
- Fine (thin): 1-2 mm diameter
- Medium: 2-5 mm diameter
- Coarse: 5-10 mm diameter
- Very coarse (very thick): > 10 mm diameter
3. On the basis of the degree of stability or structural grades –
- Structure less: Particles are not arranged into peds aggregates. Massive: Coherent appearance. eg- compact clay. Single grain: Non-coherent appearance. eg- loose Sand
- Weak: Poorly formed peds that are in distance.
- Moderate: Well-formed and moderately durable peds that are not very distinct.
- Strong: Well-developed peds, quite durable and distinct.
Types of soil structure:
There are four principles types of soil structure which are described below –
1. Plate-like: Peds arranged in the form of horizontal plates, leaflets, or lenses.
Occurrence: Surface even sub-surface layers of virgin soils.
Fig: Plate-like
2. Prism-like: Peds are vertically developed giving a pillar shape. When the tops of the prisms are rounded, the structure is called columnar, and when flat called prismatic.
Occurrence: Sub horizons in arid and semi-arid regions.
3. Block like: Peds are arranged in the form of a cube and all the three dimensions are about the same size. When the faces are sharp, the arrangement is called angular blocky and when rounded called sub-angular blocky.
Occurrence: Sub-soil of the humid region.
4. Sphere-like: Peds are rounded and sized are usually smaller. The aggregates are generally termed granular which are non-porous or relatively less porous. When the granules are more porous, the term crumb is applied.
Occurrence: Surface soil high in organic matter.
The genesis of soil structure:
The mechanism of structure formation is very complex. For the formation of aggregates the soil particles should coagulate or flocculate and should be bound together by some binding or cementing materials like plant root exudates, organic matter, oxides of Fe of Al, and carbonates of Ca and Mg.
Aggregate formation in the soil is largely a function of –
- Clay content: The higher amount and smaller clay particles have high base exchanging capacity in a soil form aggregate very effectively and readily.
- Organic matter: During the decomposition of organic matter humic acid and other sticky substances are produced which help to form aggregates.
- Absorbed cations: Aggregate formation is influenced by the name of cations absorbed by soil colloids Ca++ ion has a flocculating effect on clay which helps for the formation of stable granulation of soil particles while Na has a deflocculating effect.
- Inorganic colloids: Iron and aluminum oxides and hydroxides are irreversible colloidal materials that help to form water-stable aggregates.
- Plant roots: Secretary products from plant roots may act as a cementing agent and help for the formation and good soil structure.
- Microbial activity: The soil organisms take part in the aggregation of soil separates through their slimy and other secretary products.
- Vegetation: Grasses are most effective in promoting granulation as well as soil aggregation.
- Alternate wetting and drying: Variation in the moisture content of soils affect the formation of aggregates and the development of different types of soil structure.
Distinguish between Soil texture and Soil structure:
| Soil texture | Soil structure |
| Soil texture refers to the relative proportion of sand, silt, and clay. | Soil structure refers to the arrangement of sand, silt, and clay into a definite pattern. |
| Examples – Loam, Sandy, Clay, etc. | Examples – Strong Coarse, Angular Blocky. |
| It is a basic property of soil and can’t be altered easily. | It is easily liable to change under different management practices such as plowing, liming manuring, etc. |
| It can be identified by the finger feel method. | It can be identified by its physical appearance. |
| Soil texture is formed due to weathering. | Soil structure is formed due to physical, chemical, and biological activities. |
| Based on soil texture soils have been grouped into 12 textural classes. | Based on soil structure soils have been grouped into 5 structural classes. |
| Loam and silt loam textures are good for agriculture. | Granular and crumby structures are good for agriculture. |
Significance of soil structure in agriculture:
- Soil structure regulates the amount and nature of pore space in the soil.
- It influences aeration and water movement in the soil.
- Platy structure hinders aeration and free drainage.
- Granular and crumby structures of soil facilitate aeration and water movement and permit germination and plant growth.
- Structure less soil is liable to be eroded easily.
- It influences microbial activity and the decomposition of organic matter.
- Pudding destroys soil structure and may cause a bad tilth for the next crops.
Soil density:
Soil density refers to the mass per unit volume of soil. i.e. D = MV g/cc
Where,
- D = Soil density
- M = Mass of soil
- V = Volume of soil
Soil density types
The soil density is of two types –
1. Particle/Solid/True/Actual density: Particle density may be defined as the mass per unit volume of soil solids.
Particle density (Pd) = MₛVₛ g/cm2
Here,
- Ms = Mass of the solid portion of soil
- Vs = Volume of the solid portion
Generally, the particle density of normal soils is 2.65 grams per cubic centimeter. Particle density is also termed true density. It is higher if a large number of heavy minerals such as magnetite, limonite, and hematite are present in the soil. With the increase in organic matter the particle density decreases.
2. Bulk/Apparent density: Bulk density may be defined as the mass of a unit volume of soil. This volume includes both soil solids and pore space.
Bulk density (Bd) = MₛVₜ g/cm2
Here,
- Vt = total volume of soil
The oven-dry weight of a unit volume of soil inclusive of pore space is called bulk density. The bulk density of a soil is always smaller than particle density. The bulk density of sandy soils is about 1.6 gm/cc. Where the organic matter is about 0.5. Bulk density normally decreases as mineral soils become finer in texture. Generally, soils with low bulk density have favorable physical conditions.
Particle density vs Bulk density
| Particle density | Bulk density |
| The weight per unit volume of a solid portion of soil is called particle density. | The weight per unit bulk volume of oven-dry soil is called bulk density. |
| This volume includes only soil solids. | This volume includes both soil solids and pore spaces. |
| It is expressed by the following formula, P.D = MₛVₛ g/cc. Where, Ms = Mass of the solid portion of the soil. Vs = Volume of the solid portion of the soil. | It is expressed by the following formula, B.D = MₛVₜ g/cc. Where, Ms = Mass of dry soil.Vs = Total Volume of soil |
| Particle density of mineral soil varies/ranges from 2.6 – 2.75 gm/cc. | Bulk density of mineral soil range from 1.0 – 1.8 gm/cc. |
| Particle density of organic soil varies from 1.2 – 1.7 gm/cc. | Bulk density of organic soil is about 0.5 gm/cc. |
| It does not depend on soil texture, structure, and porosity. | It depends on soil texture, structure, and porosity. |
| Particle density of a soil is always higher than bulk density. | Bulk density of a soil is always lower than particle density. |
Comparative characteristics of Sand, Silt, Clay
| Character | Sand | Silt | Clay |
| Size | (2.0 – 0.02) mm | (0.02 – 0.002) mm | < 0.002 |
| Visibility | Visible by the naked eye | Visible by microscope | Visible by ultra-microscope |
| Shape | Round | No definite shape | plate |
| Water holding capacity | Low | Medium | High |
| Infiltration | Very rapid | Medium | Slow |
| Aeration | Very rapid | Medium | Slow |
| Macro pores | Large | Medium | Very small |
| Micro pores | Small | Medium | Very high |
| Surface area | Low | Medium | Highest |
| Adhesion, Cohesion, Plasticity, Swelling, Stickiness | Very low | Medium | Very high |
| Total porosity | Low | Medium | Very high |
| Tillage | Easy | Moderate | difficult |
| Fertility | Very low | Medium | Very high |
| Chemical activity | Inactive | Solid/medium active | Very active |
| Mineral | Quartz and mica dominate | Feldspar, Mica Hematite, Limonite | Kaolinite, Smectite, Vermiculite |
| Feel on rubbing between thumb and finger | Gritty | Like flower, talcum powder | Feels very plastic and sticky when wet and becomes hard when dry. |
Stokes’ Law:
G.G. Stokes (1851) suggested the relationship between the radius of a particle and its rate of fall in a liquid. He stated that the resistance offered by the liquid to the fall of the particle varied with the radius of the sphere and not with the surface.
According to the formula, the velocity of a particle, with the same density is directly proportional to the square of the radius and inversely proportional to the viscosity of the medium.
Stokes’ Law Formula:
V = 2 dp-d gr²9 η
Where,
- V = Velocity of fall (cm/sec)
- g = Acceleration due to gravity (cm/sec2)
- dp = density of the particle
- d = density of the liquid
- r = radius of the particle
- η= absolute viscosity of the liquid (poise)
Assumptions of Stokes’ Law:
- The particles must be large in comparison to liquid molecules so that Brownian movement will not affect the fall.
- The extent of the liquid must be great in comparison with the size of the particles. The fall of the particle must not be affected by the proximity of the wall of the vessel.
- Particles must be rigid and smooth.
- There must be no slipping between the particle and the liquid.
- The velocity of fall must not exceed a certain critical value so that the viscosity of the liquid remains the only resistance to the fall of the particle.
- Particles greater than silt size fractions of a soil mass cannot be separated accurately with the help of this law.
Limitation of the Stokes Law:
- All the soil particles are not the same shape.
- Since the rate of the fall of the particles varies inversely with the viscosity of the liquid. It is necessary to maintain a known constant temperature during analysis.
- The density of soil particles is not always the same.
- Density depends on the mineralogical and chemical constituents as well as the hydration of soil particles. Ex: A hydrate particle will have low density than a non-hydrated particle.