🌱 Soil & Its Physical Properties, Soil Formation, Soil Colloids & Silicate Clay Minerals

Definition of Soil: Soil is a natural body developed from weathered rocks that consists of minerals, organic matter, water, air and living organisms, and supports plant growth.

Component	Percentage
Mineral Matter	45%
Water	25%
Air	25%
Organic Matter	5%

PYQ Note: Ideal soil contains 5% organic matter.

Subject	Definition
Pedology	Study of soil as a natural body
Edaphology	Soil in relation to plant growth

Scientist	Contribution
Vasily V. Dokuchaev	Father of Modern Soil Science

Definition: Soil formation is called Pedogenesis. It is the process through which rocks gradually change into soil.

Steps of Soil Formation

Jenny’s Soil Forming Factors (1941)

Equation: S = f(Cl, O, R, P, T)

Symbol	Factor	Type	Role
Cl	Climate ⭐⭐⭐	Active Factor	Supplies energy (Most Important)
O	Organisms	Active Factor	Biological activity
R	Relief	Passive Factor	Drainage
P	Parent Material	Passive Factor	Mineral composition
T	Time	Passive Factor	Soil maturity

Process	Meaning
Addition	Organic matter, fertilizers
Loss	Leaching, erosion
Transformation	Humification, mineralization
Translocation	Movement of clay & nutrients

Horizon	Characteristics
O	Organic matter layer
A	Topsoil
E	Eluviation (Leaching out)
B	Illuviation (Accumulation layer)
C	Parent material
R	Bedrock

1. Soil Texture ⭐⭐⭐

Definition: Relative proportion of Sand, Silt, and Clay.

Fraction (Soil Separates)	Size (mm)
Gravel	> 2.0
Coarse Sand	2.0 – 0.2
Fine Sand	0.2 – 0.02
Silt	0.02 – 0.002
Clay	< 0.002

Texture Classes: Sandy, Loamy, Clayey, Silty.

Best Agricultural Soil: Loam ⭐⭐⭐

2. Soil Structure ⭐⭐⭐

Definition: Arrangement of soil particles into aggregates.

Structure	Shape
Granular ⭐⭐⭐	Rounded (Best for agriculture)
Blocky	Cube-like
Prismatic	Vertical columns
Columnar	Rounded top vertical columns
Platy	Plate-like horizontal units

3. Soil Colour ⭐⭐⭐

Colour	Controlled By / Reason
Black	Organic matter / Basalt
Red	Iron oxide
Yellow	Hydrated iron oxide
White	Lime / Silica
Grey	Poor drainage (Waterlogging)

Measurement Instrument: Munsell Colour Chart ⭐⭐⭐

4. Soil Density

Property	Formula	Standard Value / Normal Range	Includes Pores?
Particle Density	Mass of Soil Solids / Volume of Soil Solids	2.65 g/cm³ ⭐⭐⭐	No
Bulk Density	Dry Soil Mass / Total Soil Volume	1.2 – 1.6 g/cm³ (Normal: 1.3 g/cm³)	Yes

5. Soil Porosity ⭐⭐⭐

Formula: Porosity (%) = [1 - (Bulk Density / Particle Density)] × 100

Ideal Soil Porosity: 50% ⭐⭐⭐

6. Soil Consistency

Resistance of soil against deformation. Types: Loose, Friable, Firm, Plastic, Sticky.

Best Consistency for Tillage: Friable ⭐⭐⭐

7. Soil Air & Water

Soil Air Contains: Oxygen, Carbon dioxide, Nitrogen. (Well-drained soils contain more oxygen).

Form	Availability
Gravitational Water	Not available (drains freely)
Capillary Water ⭐⭐⭐	Available to plants
Hygroscopic Water	Not available (held tightly)

Constant	Tension
Saturation	0 bar
Field Capacity	0.1 – 0.33 bar
Permanent Wilting Point	15 bar

Definition: Finest particles of soil with high surface area and negative charge.

Diameter: Less than 0.001 mm ⭐⭐⭐

Types: Inorganic (Clay minerals), Organic (Humus).

Key Characteristics & Importance: High surface area, negative charge, high adsorption, nutrient storage, water holding capacity, buffering capacity, and soil fertility.

Definition: Ability of soil to hold exchangeable cations.

Units: cmol(+)/kg (Old unit: meq/100 g)

High CEC: Clay & Humus | Low CEC: Sand

Clay minerals consist of silica tetrahedral sheets (SiO₄) and alumina octahedral sheets (Al(OH)₆).

Mineral Name	Lattice Type	Expansion / Swelling	CEC (cmol(+)/kg)	Key Feature / Occurrence
Kaolinite	1:1 (1 Silica : 1 Alumina)	None (Non-expanding)	3 – 15 (Low)	Highly weathered tropical/red soils
Montmorillonite ⭐⭐⭐	2:1 (2 Silica : 1 Alumina)	Very High	80 – 150 (Very High)	Dominant in Black Soils
Illite	2:1 (Non-expanding)	Low	20 – 40 (Medium)	Rich in Potassium (K)
Vermiculite	2:1	Medium	100 – 180 (Highest) ⭐⭐⭐	Highest CEC among common clays
Chlorite	2:2	Low	10 – 40 (Moderate)	Magnesium-rich layer

Property	Kaolinite	Illite	Montmorillonite	Vermiculite
Type	1:1	2:1	2:1	2:1
Swelling	None	Low	Very High	Medium
CEC	Low	Medium	High	Very High
Surface Area	Low	Medium	High	Very High

Question	Answer
Best agricultural texture	Loam
Best soil structure	Granular
Standard particle density	2.65 g/cm³
Soil colloid size	< 0.001 mm
Highest CEC clay	Vermiculite
Swelling clay	Montmorillonite
Black soil clay	Montmorillonite
Non-expanding clay	Kaolinite
Best available water	Capillary water
Soil colour chart	Munsell
Soil forming equation	Jenny’s equation
Father of Soil Science	Dokuchaev
Ideal soil organic matter	5%
Major colloids	Clay & Humus

#	Fact / Concept
1	Soil formation is called Pedogenesis.
2	Pedology is the study of soil as a natural body.
3	Edaphology studies soil-plant relationships.
4	Dokuchaev is the Father of Soil Science.
5	Climate is the most important soil-forming factor.
6	Active factors are Climate and Organisms.
7	Passive factors are Parent Material, Relief, and Time.
8	Weathering is the first step of soil formation.
9	Ideal soil contains 45% minerals.
10	Organic matter in ideal soil is about 5%.
11	Loam is the best agricultural soil.
12	Granular is the best soil structure.
13	Munsell chart measures soil colour.
14	Particle density is 2.65 g/cm³.
15	Bulk density is lower than particle density.
16	Porosity decreases as bulk density increases.
17	Capillary water is available to plants.
18	Hygroscopic water is unavailable.
19	Gravitational water drains freely.
20	Field capacity is about 0.1–0.33 bar.
21	Permanent wilting point is 15 bar.
22	Soil colloids are < 0.001 mm.
23	Humus has a very high CEC.
24	Sand has a very low CEC.
25	Kaolinite is a 1:1 clay mineral.
26	Montmorillonite is a 2:1 clay mineral.
27	Illite is rich in potassium.
28	Vermiculite has the highest CEC among common clay minerals.
29	Montmorillonite shows maximum swelling.
30	Kaolinite shows no swelling.
31	Black soils are rich in Montmorillonite.
32	Red soils are generally rich in Kaolinite.
33	Clay has the highest surface area.
34	Soil air is rich in oxygen in well-drained soils.
35	Addition, loss, transformation, and translocation are pedogenic processes.
36	Eluviation occurs in the E horizon.
37	Illuviation occurs in the B horizon.
38	Topsoil is the A horizon.
39	Bedrock is the R horizon.
40	Soil fertility largely depends on clay and humus (soil colloids).