Macroaggregate

Macroaggregate refers to a soil aggregate larger than about 0.25 millimeters in diameter. They are visible to the naked eye and form the major structural units that build the soil’s pore network for drainage and aeration. Macroaggregates consist of mineral grains bound together by organic matter, clay minerals, iron and aluminum oxides, and biological substances such as root exudates and fungal hyphae, including glomalin produced by arbuscular mycorrhizal fungi.

Formation and dynamics occur through a combination of physical processes, biological activity, and organic matter accumulation. Root growth and mucilages, fungal hyphae, microbial byproducts, and the binding action of clay and mineral surfaces help fuse particles into larger clumps. Wet-dry and freeze-thaw cycles promote aggregation, while persistent organic matter input and microbial activity reinforce the bonds that stabilize macroaggregates.

Function and importance: Macroaggregates create stable pore networks that promote infiltration, drainage, and aeration, while protecting soil organic matter from rapid mineralization. They influence soil fertility by shaping nutrient mineralization, retention, and availability, and affect erosion resistance, crust formation, and overall soil structure resilience. Macroaggregation also contributes to soil carbon storage by physically protecting organic matter within larger aggregates.

Measurement and management: The proportion of macroaggregates and their stability are assessed by wet sieving or dry sieving methods in soil testing and research. Agricultural practices that promote macroaggregation include reduced tillage, cover crops, residue retention, and organic amendments; conventional tillage and soil compaction tend to disrupt macroaggregates and reduce soil structural quality.

Related concepts: Macroaggregates are part of the broader soil aggregate hierarchy, alongside microaggregates and ped structure, with the distinction between macro- and microaggregates used to describe soil structure and its response to management.