What is the role of fungi in building stable soil aggregates?

Fungi play a central role in building stable soil aggregates by physically binding soil particles together and secreting sticky compounds that hold those particles in place. Their thread-like structures, called hyphae, weave through the soil like a net, pulling mineral grains and organic matter into clusters that resist erosion and compaction. The sections below unpack exactly how this works, which fungal groups matter most, and what growers can do to support them.

How do fungi physically bind soil particles together?

Fungi bind soil particles together through a network of hyphae, microscopic, thread-like filaments that grow through the soil and physically enmesh mineral grains, organic fragments, and microbial cells into cohesive clusters called aggregates. These hyphal threads act like a biological scaffold, holding particles in contact long enough for chemical and biological bonding to take hold. The result is a porous, crumb-like soil structure that allows roots to penetrate easily and water to move freely.

As hyphae grow, they exert a gentle mechanical pressure on surrounding particles, drawing them together. The threads themselves are tough and resistant to decomposition, meaning their structural contribution persists even after the fungus has completed its active growth phase. Dead hyphal material continues to act as a physical bridge between particles, contributing to what soil scientists describe as macroaggregate stability.

The binding effect is strongest in the rhizosphere, the zone of soil immediately surrounding plant roots, where fungal activity is most intense. Here, hyphae from mycorrhizal fungi form dense mats that interconnect root surfaces with the broader soil matrix, creating a zone of exceptional structural stability that supports both water infiltration and nutrient exchange.

What is glomalin and why does it matter for soil structure?

Glomalin is a glycoprotein, a sticky, carbon-rich compound, secreted by mycorrhizal fungi as they grow through the soil. It coats hyphal threads and soil particles alike, acting as a biological glue that cements aggregates together and makes them water-stable. Glomalin is significant because it is highly persistent in soil, meaning it continues to strengthen aggregate structure long after the hyphae that produced it have died.

The compound was only identified in the 1990s, but research since then has established it as one of the most important contributors to soil organic carbon stocks. Because glomalin is resistant to microbial breakdown, it can remain in the soil for decades, steadily accumulating with each growing season in soils that support healthy fungal communities.

From a practical standpoint, higher glomalin levels correlate consistently with better aggregate stability, reduced erosion, improved water infiltration, and greater drought resilience. Soils rich in glomalin hold their structure under rainfall impact and irrigation pressure, reducing surface crusting and runoff. For growers managing sandy or silty soils prone to structural collapse, supporting glomalin-producing fungi is one of the most effective long-term investments in soil health.

Which types of fungi contribute most to aggregate stability?

Arbuscular mycorrhizal fungi (AMF) are the single most important group for building stable soil aggregates. They colonise the roots of the vast majority of crop plants, extending their hyphae far beyond the root zone and producing large quantities of glomalin. Their contribution to aggregate formation is both mechanical, through hyphal entanglement, and chemical, through glomalin secretion. Saprophytic fungi, which decompose organic matter, also contribute significantly by producing hyphae that bind particles and releasing compounds that feed soil bacteria involved in aggregate cementation.

Arbuscular mycorrhizal fungi (AMF)

AMF form obligate symbioses with plant roots, meaning they cannot complete their life cycle without a host plant. In exchange for photosynthate from the plant, they deliver water and nutrients, particularly phosphorus, directly into root cells. Their hyphae extend several centimetres into the surrounding soil, creating a dense, interconnected network that physically stabilises the soil around the root zone. Crops such as wheat, maize, vegetables, and most horticultural species naturally support AMF communities when the soil is not heavily disturbed.

Saprophytic and ectomycorrhizal fungi

Saprophytic fungi break down crop residues and woody organic matter, and their hyphae bind the resulting fragments into stable micro-aggregates. Ectomycorrhizal fungi, which associate primarily with trees and shrubs, produce particularly robust hyphal networks and contribute meaningfully to aggregate stability in agroforestry systems and perennial crops. While they are less relevant to annual cropping systems, they are important in orchards, vineyards, and mixed farming landscapes.

How does fungal activity affect long-term soil aggregate stability?

Fungal activity improves long-term soil aggregate stability through a self-reinforcing cycle: healthy fungal communities produce hyphae and glomalin that build stable aggregates, and those stable aggregates in turn protect the pore spaces and organic matter that fungi need to thrive. Over time, this cycle increases the proportion of water-stable macroaggregates in the soil, which are the structural units most resistant to erosion, compaction, and waterlogging.

The long-term benefit depends heavily on continuity. In soils where fungal communities are consistently supported, through reduced tillage, organic matter inputs, and diverse cropping, glomalin accumulates season after season. Soils with high glomalin levels have demonstrably better water infiltration, lower bulk density, and greater resistance to surface crusting, all of which translate into more consistent crop establishment and yield stability across variable seasons.

Conversely, soils subjected to repeated deep tillage, high synthetic fertiliser inputs, and bare fallows show declining fungal biomass and aggregate stability over time. The structural degradation that follows is slow but cumulative, and reversing it requires deliberate management changes sustained over multiple growing seasons. The EU Soil Monitoring Directive, which entered into force in December 2025, reflects growing recognition that soil structural health is a long-term asset requiring active protection, not a static resource.

Organic matter inputs play a supporting role here. Materials rich in humic compounds, such as peat-derived soil conditioners, provide the prebiotic substrate that feeds fungal communities and accelerates their re-establishment in degraded soils. By increasing soil organic carbon and creating favourable conditions for microbial life, these inputs help restore the biological foundation on which long-term aggregate stability depends.

What farming and gardening practices support fungi that build soil aggregates?

The practices that most effectively support aggregate-building fungi are those that minimise soil disturbance, maintain continuous organic matter inputs, and keep living roots in the ground for as much of the year as possible. Reduced or no-till cultivation, cover cropping, diverse rotations, and the avoidance of broad-spectrum fungicides are the most impactful changes a grower can make. These approaches work because AMF and other beneficial fungi are highly sensitive to physical disruption and chemical stress.

• Reduce tillage intensity: Deep inversion tillage severs hyphal networks and destroys the pore structure that fungi inhabit. Switching to shallow cultivation or direct drilling preserves existing fungal communities and allows glomalin to accumulate between seasons.
• Maintain continuous plant cover: AMF are obligate symbionts, they require a living host root to survive. Bare fallows eliminate the fungal community. Cover crops, undersown species, and perennial leys keep roots in the ground and sustain fungal populations through non-cropping periods.
• Add diverse organic matter: Compost, green manures, and crop residues feed saprophytic fungi and provide the carbon substrates that support the broader soil food web. Organic inputs with high humic compound content are particularly effective at stimulating microbial and fungal activity.
• Diversify crop rotations: Different crop species support different fungal species. A diverse rotation builds a more resilient and species-rich fungal community, which in turn produces a more structurally stable soil profile.
• Limit high-phosphorus fertiliser applications: High soil phosphorus levels suppress AMF colonisation, because the plant reduces its investment in the symbiosis when phosphorus is already abundant. Matching fertiliser applications to actual crop demand helps maintain the conditions that encourage mycorrhizal activity.
• Avoid broad-spectrum fungicides where possible: Fungicides applied to soil or incorporated into seed treatments can reduce AMF populations. Where disease pressure requires fungicide use, selecting products with lower soil persistence and applying them at the lowest effective rate helps limit collateral damage to beneficial fungi.

For home gardeners, the same principles apply at a smaller scale. Avoiding regular rotavation, mulching beds with organic matter, and planting diverse species, including flowering plants that support mycorrhizal networks, all contribute to a biologically active soil that builds its own structure over time. The European Biostimulants Industry Council highlights the broader role of soil biology in sustainable food production, a perspective that connects directly to the on-the-ground decisions growers and gardeners make every season.

This content was generated with the help of AI and it may contain mistakes