Structural Elements

A crucial aspect of a soil's capacity to facilitate fluid movement and promote root growth without hindrance is the existence of large, durable aggregates. The soil's ability to withstand the impact of water and external forces like raindrops, treading, and vehicle traffic is commonly known as structural stability or, at a smaller scale, aggregate stability. The presence or absence of various components at microscopic levels significantly contributes to the stability of macro-aggregates.

Macro-aggregate

Macro-aggregates, defined as those larger than 0.25 mm, are typically stabilized by roots and fungal hyphae. Their abundance and stability are directly impacted by soil management practices:

• Tillage and compaction can disrupt macro-aggregates.
• Cropping techniques that reduce the presence of living roots and fibrous root systems for extended periods can result in less stable macro-aggregates.

Micro-aggregate

Micro-aggregates, on the other hand, are typically unaffected by agricultural activities such as tillage and compaction. Composed mainly of silt particles and clusters of silt-sized particles, they are bound together by organic materials including hyphae, strands of glutinous material, and polysaccharides. Soils with abundant organic matter or those supplemented with organic residues tend to have a higher proportion of water-stable aggregates.

Particle Sizes:

• 200 - 2000μm = coarse SAND.
• 20 - 200μm = fine SAND.
• 20μm - 2μm = SILT.
• < 2μm = CLAY.

Cluster

The silt-sized clusters, which form micro-aggregates, comprise packets of clay particles and organic matter such as bacteria, fungal debris, and decomposed plant material. These clusters are strongly bonded together by persistent organic bonds. They can occur as newly formed associations between live bacterial cells and clay packets, and as aged aggregations after decay of bacterial colonies or fungal hyphae, where the fibrous components persist and are bonded to clay particles.

Particle Sizes:

• 200 - 2000μm = coarse SAND.
• 20 - 200μm = fine SAND.
• 20μm - 2μm = SILT.
• < 2μm = CLAY.

Domain

Within their structure, packets or domains of clay particles often protect amorphous organic material from further decomposition. This organic material results from the decay of hyphal fragments or microbial debris. The association of polysaccharides and organic polymers with clay particle surfaces can produce very strong bonds to hold such micro-structures together.

Additional strength can be added by the presence of inorganic cements such as aluminosilicates, carbonates, and oxides of Fe and Al.

Clay plate

Individual clay particles are characteristically in the shape of thin plates. When parallel, these plates can be strongly attracted to each other by electrostatic forces (see flocculation of clay particles in the Aggregation module). The amount of clay in soil can significantly affect soil coherence (refer to the Soil Strength module) and, consequently, aggregate stability.