Definition of Soil Structure
Soil structure is the arrangement of soil particles and their aggregates into specific patterns or shapes.
- Aggregates = Grouped particles
- Peds = Natural aggregates
- Clods = Artificial aggregates (often from tillage)
Structureless Soils
|
Type |
Description |
|
Single Grain |
Particles are loose and unaggregated (common in sandy soils) |
|
Massive |
Particles stick tightly without visible peds (e.g., wet clay or puddled paddy soil) |
Importance of Soil Structure
Soil structure affects:
- Plant growth
- Water retention and drainage
- Aeration
- Heat transfer
- Microbial activity
- Workability and tillage response
Most agronomic practices (e.g., ploughing, liming, manuring) affect structure, not texture.
Soil Structure Formation
Influencing Factors:
- Clay colloids
- Nature of cations:
- Ca²⁺, H⁺ promote aggregation
- Na⁺, K⁺ promote dispersion
- Organic matter & humus
- Microorganisms & fungi
- Sesquioxides (Fe/Al oxides): Act as cementing agents
Organic matter > Silicate clays in forming stable aggregates
Lime (Ca²⁺) + Organic Matter = Crumb structure
Types of Soil Structure (Based on Shape)
|
Type |
Description |
|
Platy |
Thin horizontal layers; found in surface of virgin soils or compacted subsoils |
|
Prismatic |
Vertical aggregates with flat tops; found in arid/semi-arid clayey B horizons |
|
Columnar |
Vertical aggregates with rounded tops; indicate sodic soils |
|
Blocky |
Irregular cube-shaped; common in B horizon of humid climates |
|
Subangular Blocky |
Softer edges than true blocks |
|
Granular |
Small, rounded aggregates; common in A horizon; good for root growth |
|
Crumb |
Very porous granular aggregates; ideal for plant growth |
Crumb and granular structures are promoted by organic matter, clay-humus complexes, and lime.
Soil Structure Classification Summary
|
Category |
Sub-Type |
Occurrence/Properties |
|
Structureless |
Single Grain |
Sandy soils (loose) |
|
Massive |
Puddled clay (compact) |
|
|
Aggregated |
Platy |
Surface/compacted zones |
|
Prismatic/Columnar |
B horizon in dry zones |
|
|
Blocky/Subangular |
B horizon in humid zones |
|
|
Granular/Crumb |
A horizon, cultivated soil |
Soil Structure Grades (Stability)
|
Grade |
Description |
|
Structureless |
Single grain / Massive |
|
Weak |
Poorly formed peds |
|
Moderate |
Well-formed, moderately durable |
|
Strong |
Distinct, stable peds visible in undisturbed soil |
Soil Structure Classes (Size of Aggregates)
- Very Fine
- Fine
- Medium
- Coarse
- Very Coarse
Factors Affecting Soil Structure
- Soil Management: Crop rotation, tillage, and manuring maintain structure
- Adsorbed Cations:
- Ca²⁺, Ba²⁺ → promote flocculation
- Na⁺, K⁺ → promote dispersion
- Microbial Activity: Earthworms, insects, fungi help aggregate formation
- Soil Moisture Variation: Wet-dry cycles cause shrink-swell (esp. Vertisols)
- Organic Matter: Acts as binding agent and improves porosity
Only practical way to improve soil structure = Add organic matter
Particle Density (P.D.)
Mass of solids per unit volume of soil solids only
- Symbol: Dp
- Unit: g/cm³ or Mg/m³
- Average for mineral soil: 2.65 g/cm³
- Organic matter: 1.1 – 1.4 g/cm³
✅ Not affected by particle size or pore space
Bulk Density (B.D.)
Mass of oven-dry soil per unit total volume (solids + pores)
- Symbol: Bd
- Unit: g/cm³
- Typical value: ~1.33 g/cm³
- Clay soils: Lower Bd due to higher pore space
- Sandy soils: Higher Bd due to lower porosity
✅ B.D. increases with depth due to compaction and less organic matter
Pore Space & Porosity
Total volume of pore space as % of total soil volume
Formula: Porosity = 100 − (B.D.P.D.×100
|
Soil Type |
Typical Porosity (%) |
|
Sandy |
30–50% |
|
Loamy |
40–60% |
|
Clayey |
50–60% |
- Macropores (>0.06 mm): Air and water movement
- Micropores (<0.06 mm): Water retention
- Clay soils = ~25 × 10⁶ pores/m²
- Sandy soils = ~25,000 pores/m²
Bulk Density vs Particle Density Summary
|
Property |
Particle Density (P.D.) |
Bulk Density (B.D.) |
|
Includes pore space |
❌ No |
✅ Yes |
|
Affected by structure |
❌ No |
✅ Yes |
|
Typical Value |
2.65 g/cm³ |
1.1–1.6 g/cm³ |
|
Affected by OM |
Slightly |
Significantly |
Quick Revision Points
- Structureless = Single grain or massive
- Favorable structure for crops = Crumb or granular
- Sodic soils = Columnar structure
- Clayey soils = Blocky or prismatic
- Soil with most stable structure = Aggregated with Fe/Al oxides
- Best way to improve structure = Add organic matter
- Bulk density increases with soil depth and cultivation
- Porosity and B.D. are inversely related
