Soil: Definition and Concepts

• Origin of the word “Soil”:Derived from Latin word ‘Solum’, meaning floor.
• Definition: Soil is a dynamic natural body on the surface of the Earth in which plants grow, composed of mineral and organic materials and living forms.

Two Basic Concepts of Soil

Pedology

1. • Considers soil as a natural entity.
   • Soil is a biochemically weathered and synthesized product of nature.
   • Involves: origin, classification, and description of soil.

Edaphology

1. • Origin: Greek words ‘Edaphos’ = soil and ‘Logos’ = discourse.
   • Considers soil as a habitat for plants.
   • Focuses on soil properties as they relate to plant growth, nutrition, and yield.

Composition of Soil

Soil is composed of four major components:

Sub-soil: Layer below topsoil; compact and less fertile.

Physical Properties of Soil

• Depend on: particle size, shape, arrangement, mineral composition, organic matter content, and pore space.
• Important physical properties:

• • Soil texture
  • Soil structure
  • Soil density
  • Soil porosity
  • Soil color
  • Soil temperature
  • Soil consistency

1. Soil Texture

• Refers to relative proportion of sand, silt, and clay.
• Inalterable property of soil.
• Soil Separates (Diameter in mm):

• Minerals:
  • Sand: Quartz
  • Silt: Feldspar, Mica, Hematite, Laminate
  • Clay: Kaolinite, Montmorillonite, Illite
• Determination in field: Feel method.

2. Soil Structure

• Definition: Arrangement of soil particles and their aggregates into defined patterns.
• Natural aggregates: Peds
• Artificial aggregates: Clods
• Can be changed by agronomic practices.

Types of Soil Structure:

i) Plate-like (Platy) Structure

• • Thin horizontal plates; horizontal dimension > vertical.
  • Thick unit: Platy, Thin unit: Laminar
  • Common in virgin surface soil, may occur in subsoil.

ii) Prism-like Structure

• Vertical axis > horizontal; pillar-like.
• Top rounded: Columnar, Top flat: Prismatic

iii) Block-like Structure

• All dimensions similar.
• Angular blocky: Flat faces, sharp edges

iv) Spheroidal (Granular / Crumb) Structure

• • Rounded aggregates.
  • Granular: Fine, less porous, good for crops.
  • Crumb: Highly porous, best for cultivation.

Factors Affecting Soil Structure

Summary:

• Soil is a natural body essential for plant growth, composed of minerals, organic matter, water, and air.
• Its physical properties, especially texture and structure, are critical for agronomic practices and crop productivity.
• Proper management, including organic matter addition and reduced intensive tillage, improves soil structure and fertility.