The process of aggregation refers to the binding together of soil particles. The resulting aggregates can have different sizes and influence not only the structure of the soil but also the accessibility of soil organic matter that is packed inside these particles. Earthworms, enchytraeids and soil fungi primarily promote aggregation. Bacteria, fungi and plant roots (through exudates) contribute to this process by producing a glueing agent (mucilaginous polysaccharides) that help stick soil particles together.

Aggregation determines how the building blocks of the soil (soil particles and aggregates) are arranged. The structure of this arrangement is extremely important because it determines how much space (or soil pores) there is between the building blocks. This has a large influence on the flow of water through the soil (infiltration and percolation) and how much water can be stored by the soil overall (like a sponge). In this way, aggregation supports the Water Regulation function. The way organic matter and soil particles are packed together provide soil microbes with easy or difficult access to the carbon and nutrients contained in the aggregates. Large aggregates contain more organic material and let in more oxygen than small aggregates, and thus aggregate size determines how easily microbes can consume the organic matter inside. Therefore, aggregation is also important for the Carbon and Climate Regulation and Nutrient Cycling functions.

Aggregation can be measured as a direct effect of biological activity, such as earthworm burrowing and cast formation[1]. Aggregation in (small) volumes of soil can also be classified manually based on their morphology or near-infra red reflection signatures[2]. Other methods to measure aggregation are based on physical fractionation[3].

[1] Shipitalo MJ and Le Bayon R. 2004. Quantifying the Effects of Earthworms on Soil Aggregation and Porosity. Earthworm Ecology 10: 183–200.

[2] Velasquez E et al. 2007. This ped is my ped: Visual separation and near infrared spectra allow determination of the origins of soil macroaggregates. Pedobiologia 51: 75–87.

[3] Poeplau C et al. 2018. Isolating organic carbon fractions with varying turnover rates in temperate agricultural soils–A comprehensive method comparison. Soil Biology and Biochemistry 125: 10-26.