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Structures of colloidal compounds in soil, including organo–mineral and mineral–mineral associations, are considered as composite building units (CBUs) that may combine into soil microaggregates. Despite the ubiquitous occurrence of CBUs, the major formation mechanisms are rather obscure and little is known about whether they form primarily during weathering of the parent rocks or by aggregation processes from the soil suspension. We studied the formation of CBUs from suspensions composed of minerals and organic matter typical for temperate soils (i.e. quartz, goethite, illite and extracellular polymeric substances [EPS]). Without EPS, we found CBUs formed as mineral–mineral associations by hetero coagulation of illite and quartz that is bridged by goethite. The presence of EPS, in contrast, led to the formation of a stable suspension of clay‐sized CBUs with no involvement of quartz. We explained this by the rapid formation of organo–mineral CBUs made of EPS‐associated goethite and EPS‐associated illite. The sorption of EPS to goethite screened its surface charge, thereby reducing the electrostatic attraction between goethite and illite. This interaction effectively impeded the formation of mineral–mineral CBUs. Moreover, interactions of EPS with goethite resulted in a marked decrease of the phosphorus/carbon ratio in the suspension. This suggested a preferred adsorption of phosphorus‐containing EPS constituents to goethite and in turn to a compositional fractionation of EPS constituents between the solid and liquid phase as shown by Fourier‐transform infrared spectroscopy with attenuated total reflection (FTIR–ATR). Laser light diffraction measurements revealed a shift from the fine silt fraction to that of the fine sand that also supports the role of EPS as a ‘binding’ agent. Highlights Composite building units (CBUs) form in suspensions with different mineral and organic components. Both, hetero mineral–mineral and organo–mineral CBUs were formed. The initial composition of the suspension controls type and properties of resulting CBUs. Depending on the mineral surfaces, EPS may serve as a separation or binding agent.

Soil functions are closely related to the structure of soil microaggregates. Yet, the mechanisms controlling the establishment of soil structure are diverse and partly unknown. Hence, the understanding of soil processes and functions requires the connection of the concepts on the formation and consolidation of soil structural elements across scales that are hard to observe experimentally. At the bottom level, the dynamics of microaggregate development and restructuring build the basis for transport phenomena at the continuum scale. By modeling the interactions of specific minerals and/or organic matter, we aim to identify the mechanisms that control the evolution of structure and establishment of stationary aggregate properties. We present a mechanistic framework based on a cellular automaton model to simulate the interplay between the prototypic building units of soil microaggregates quartz, goethite, and illite subject to attractive and repulsive electrostatic interaction forces. The resulting structures are quantified by morphological measures. We investigated shielding effects due to charge neutralization and the aggregate growth rate in response to the net system charge. We found that the fraction as well as the size of the interacting oppositely charged constituents control the size, shape, and amount of occurring aggregates. Furthermore, the concentration in terms of the liquid solid ratio has been shown to increase the aggregation rate. We further adopt the model for an assessment of the temporal evolution of aggregate formation due to successive formation of particle dimers at early stages in comparison to higher order aggregates at later stages. With that we show the effect of composition, charge, size ratio, time, and concentration on microaggregate formation by the application of a mechanistic model which also provides predictions for soil aggregation behavior in case an observation is inhibited by experimental limitations.

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