Useful Notes on the Properties of Soil

Physical:

1. Soil Texture

Image Source: soilbuildingsystems.com

2. Soil Structure

3. Density of soil

4. Porosity of soil

5. Soil Temperature

6. Soil colour

7. Soil water

8. Soil air

Chemical:

1. Soil Colloids

2. Soil Reaction

3. Soil Organic Matters

4. Soil Acidity

5. Soil Alkalinity

6. Neutrality

Dynamic:

1. Soil Consistency

2. Soil Compactions

3. Soil Strength

4. Soil crusting

Biological:

1. Macro-organisms

2. Micro-organisms

Important Properties

Soil Texture:

Can’t be changed/altered.

1. Definition: It is the relative percentage of sand silt and clay in a soil.

2. Fundamental groups of textural classes

i. Sands: > 70% sand separate

ii. Loams: Intermediate

iii. Clays: > 35% clay separate

3. Determination of textural class

i. Feel Method

ii. Laboratory Method (By USDA, based on the mechanical analysis)

4. Important of soil texture

i. State of Hydration and Exchangeable cations

ii. Deflocculation Na

iii. Flocculation Ca²⁺, Al³⁺

iv. Range of Workability

v. Heavy Soils

vi. Light Soils

Soil Structure:

Can be changed/altered.

1. Definition: The mutual arrangement, orientation and organisation of particles in the soil.

2. Types of structure;

3. Classes

i. Very fine

ii. Medium

iii. Very coarse

iv. Fine

v. Coarse

4. Grades

i. Structure less

ii. Moderate

iii. Break

iv. Strong

5. Factors affecting soil structure

(a) Climate

(b) Organic matter

(c) Adsorbed

(d) Tillage

(e) Types of vegetation

(f) Plant roots

(g) Soil organisms

(h) Crop rotation Cations

(i) Alternate wetting and drying

Density of Soil:

1. Definition: Weight per unit volume of a substance.

2. Types of Measurement

i. Bulk Density

ii. Particle Density

(A) Particle density/True density:

1. Definition: The weight per unit volume of the solid portion of soil.

2 Pd = Mass of solids / Volume of solids

3. Unit: gm/cc.

4. Characteristics:

i. Depends on density of individual inorganic and organic constituents of soil.

ii. Particle density decrease with increase in organic matter.

iii. For normal soil particle density is 2.65 g/cc.

(B) Bulk Density:

1. Definition: The mass per unit volume of a dry soil.

2. BD = Weight of soil mass / Soil volume

3. Unit: g/cc

4. Characteristics:

1. Bulk density always smaller than particle density

2. Low Bulk density favourable physical condition.

3. Ideal Bulk density is 1.2 – 1.4 g/cc.

5. Factors affecting: Amount of pore space. Compactness of soil, texture, structure, organic matter.

Porosity of Soil:

1. Definition: The percentage of soil volume occupied by pore space.

2. Types of pore-space

Macro-pores:

i. Large sized pores

ii. Found in between granules

iii. Sandy soils Micro/Capillary Pores

iv. Small sized pores

v. Found within granules

vi. Clayey soils.

Soil Colour:

1. Kinds of soil colour

i. Lithochromic: from Parent Material e.g. Red Soil from Red Sandstone

ii. Pedochromic: during soil formation e.g. Red soils from granite.

Factors affecting soil colour —

i. Oxidation: Reduction.

ii. Alternate wetting and Drying.

iii. Iron compounds

iv. Silica lime and salts.

Soil Temperature:

1. Sources of soil heat:

i. Solar radiation

ii. Bio-chemical reactions

iii. Conduction

iv. Precipitation, Exposure, vegetation

2. Loss of soil heat:

i. Radiation

ii. Conduction

iii. Evaporation

iv. Precipitation

3. Importance on plant growth and Nutrition:

(i) Germination of seeds

(ii) Soil structure

(iii) Decomposition of organic matter in soil.

(iv) Absorption of water

(v) Availability of Nutrients

4. Management of soil temperature

(i) By regulating the amount of water

a. Drain

b. Add

(ii) The colour of surface soil

a. Black: High temp.

b. White: Low temp.

(iii) By providing different types of mulches on the soil surface.

Soil Water:

It is a regular of physical chemical and biological activities in soil.

Soil Air:

1. Definition: The exchange of CO₂ and O₂ gases between soil pore space and atmosphere.

2. Types of pores:

a. Inter crumb pores – between crumbs

b. Crumb pores – pores within crumbs.

3. Reasons for poor Aeration:

i. Excess soil moisture

ii. Gaseous exchange

iii. Water logged conditions

iv. Saturated soil conditions

4. Mechanisms of exchange of gases

(i) Mass flow – due to pressure differences between atmosphere and soil air.

(ii) Diffusion – Molecular transfer of gases.

5. Importance of soil Air in plant growth and biological Activity in soil

(i) Growth of plants

(ii) Development of plant

(iii) Adaptations of plant

(iv) Microbial population and activity

(v) Production of Toxic substances.

(vi) Absorption of water and Nutrients.