2.2 Soil structure and aggregation

Soil structure and aggregation are crucial aspects of soil health, influencing water retention, aeration, and root growth. They determine how soil particles arrange into aggregates, affecting everything from water movement to microbial activity in the soil.

Different soil structures, like granular or blocky, form in various soil layers. Factors such as organic matter, clay content, and management practices play key roles in shaping and maintaining soil structure, impacting overall soil quality and plant growth.

Soil Structure and Aggregation

Significance of soil structure

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• Soil structure arranges soil particles into aggregates or peds determining pore space distribution and influencing water and air movement
• Affects soil porosity impacting water retention capacity and influencing soil aeration (infiltration rates, drainage)
• Facilitates root penetration enhancing nutrient availability and promoting microbial activity (rhizosphere development)

Types of soil structure

• Granular structure forms small rounded aggregates through organic matter and microbial activity common in surface horizons (topsoil)
• Blocky structure creates cube-like aggregates with angular or subangular edges through wetting and drying cycles typical in subsoil horizons (B horizon)
• Prismatic structure develops vertical columns with flat tops through shrinking and swelling of clay minerals found in deeper soil layers (argillic horizons)
• Platy structure forms thin flat plates through compaction or clay accumulation often occurs in subsurface horizons (E horizon)

Factors influencing soil structure

• Organic matter binds soil particles together enhancing aggregate stability (humus, root exudates)
• Clay content provides cohesion between particles influencing shrink-swell properties (montmorillonite, kaolinite)
• Management practices impact structure:
  1. Tillage disrupts or improves structure
  2. Crop rotation affects organic matter input
  3. Cover crops enhance soil structure through root growth
• Soil fauna activity creates channels and mixes soil (earthworms, termites)
• Climate affects structure through freeze-thaw cycles and rainfall intensity impacts structural stability (cryoturbation, raindrop impact)

Role of soil aggregation

• Soil aggregation binds individual soil particles into stable units mediated by organic and inorganic binding agents (polysaccharides, fungal hyphae)
• Maintains soil structure by resisting erosion and compaction providing structural stability over time (soil resilience)
• Improves infiltration rates enhances water-holding capacity and reduces surface runoff and erosion (water conservation)
• Increases pore space for gas exchange promoting oxygen diffusion to plant roots and facilitating carbon dioxide release from soil (soil respiration)
• Protects organic matter from decomposition creating microhabitats for beneficial microorganisms (carbon sequestration)
• Allows for better root development improving nutrient uptake efficiency (plant productivity)