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Macroaggregates
Macroaggregates
A later form of aggregation, let's call it 'secondary', involves the addition of humic substances from the process of humification, to the existing smaller aggregates held together with GRSPs. While there would be isolated places where humification would happen it did not become widespread till this period, due to the eating habits of some key characters, earthworms and higher oribatids. They appeared less than 200mya and chewed and pooed their way through soil.
"The accumulation of SOC (Soil organic carbon) in large and small macroaggregates and the accumulation of GRSPs in microaggregates (<0.25 mm)" Xiao 2020, shows how microaggregates (primary) and macroaggregates (secondary) could have come together to produce soil more like we know today.
Properties
Properties
Macroaggregates are larger soil aggregates, typically ranging from 0.25 mm to several mm in diameter.
Primarily composed of sand, silt, clay, and organic matter.
Formed through the binding action of various organic materials, such as plant roots, fungal hyphae, and microbial by-products (GRSPs, humic substances, polysaccharides, proteins, etc.).
More stable and resistant to disruption by water erosion or mechanical forces due to strong binding agents.
Provide pore spaces in the soil, helping water infiltration, aeration, and root penetration and shelter organic matter from decomposition.
Interactions Between Clay microstructures and Humic Substances
Interactions Between Clay microstructures and Humic Substances
Physical Binding and Adsorption:
The negative charge on clay particles allows them to attract and hold onto positively charged ions (cations) and organic molecules, including humic substances. This is called adsorption, where humic substances become bound to the surface of clay particles.
Humic acids, being large and negatively charged, are attracted to the positively charged edges of clay minerals. This helps to form stable organo-mineral complexes, which are key to soil structure.
The large surface area of clays provides a platform for humic substances to attach, creating a matrix that stabilizes organic matter within the soil and prevents it from being washed away by rain or broken down too rapidly by microbes.
Soil Aggregation and Structure:
The interaction between clay particles and humic substances promotes the formation of soil aggregates—clumps of soil that improve the structure, aeration, and water-holding capacity of soils.
Humus acts as a binding agent, gluing clay particles together to form these aggregates. This improves soil porosity, allowing for better root penetration and gas exchange while also reducing erosion.
These aggregates are important for soil tilth, which refers to the physical condition of soil in terms of how easy it is to cultivate and how well it supports plant growth.
Nutrient Retention and Exchange:
Clays and humic substances both have a high cation exchange capacity (CEC), meaning they can adsorb and hold onto nutrient ions like calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), and ammonium (NH₄⁺), making these nutrients available to plants over time.
Clays are particularly effective at holding onto inorganic nutrients, while humic substances play a key role in organic nutrient cycling. Together, they ensure a steady supply of nutrients.
Fulvic acids, due to their smaller size and water solubility, can move more freely through the soil, carrying nutrients from clay particles to plant roots, making these nutrients more accessible to plants.
Water Retention:
Clays have a high water-holding capacity because of their small particle size and large surface area. When combined with humic substances, this water-holding ability is enhanced.
Humic acids have an additional role in increasing the soil’s ability to retain water. Their spongy, porous nature allows them to hold water in the soil matrix, which is particularly important in drought-prone regions.
Together, clays and humic substances create a soil environment that retains moisture better, supports microbial activity, and ensures plants have access to water over longer periods.
Buffering of Soil pH:
Both clay and humic substances help to buffer soil pH. Clays, due to their cation exchange capacity, can hold onto acidic or basic ions, reducing fluctuations in pH.
Humic acids also play a role in buffering, as they can neutralize pH changes by releasing or absorbing H⁺ ions. This interaction is vital for maintaining a stable environment for plant roots and microorganisms.
Microbial Habitat and Activity:
The combination of clays and humic substances creates an ideal environment for microbial life in soil. Clay particles provide physical protection for microbes from predators, while humic substances provide carbon and nutrients to support microbial activity.
Microorganisms, in turn, break down organic material into humus, further enhancing the clay-humus interaction. This positive feedback loop is essential for maintaining a healthy, nutrient-rich soil ecosystem.
HS Structure
HS Structure
Distinctive aggregation behaviours exist in humic substances (HS) Saito et al 2023 The functional groups that contribute most to surface charge and reactivity of humic substances are phenolic and carboxylic groups.
These humic substances would have been added to the existing glomalin-related soil proteins, that had been around for over 200my making micro-aggregates. This addition happened less than 200mya, and so provided a massive boost to building soil structures.
Model structure of a humic acid, having a variety of components including quinone, phenol, catechol, and sugar moieties
Models
Models
"The latest view describes HS aggregates as a hydrogel-like structure comprised by a hydrophobic core of aromatic residues surrounded by polar and amphiphilic molecules akin a self-assembled soft material. A different view is based on the classification of this material as either mass or surface fractals. The former is intended as made by the clustering of macromolecules generating dendritic networks, while the latter have been modelled in terms of a solvent-impenetrable core surrounded by a layer of lyophilic material. (Angelico et al 2023)...
Different sorts
Different sorts
Most of the scientific reports reviewed in literature seem to indicate that humic materials from different sources have different aggregate morphologies (though the aggregation mechanisms seem to be similar), indirectly underscoring the importance of specific chemistry to humic material geometry. So far, the vast physicochemical heterogeneity of HS, mainly due to the extensive variability of the sources from which they are extracted, justifies the current absence of a unified model capable of describing and predicting their dynamic properties and fate both in the aqueous phase and in the solid state." (Angelico et al 2023)
A concept to model the role of HS in biogeochemical cycles is missing and "for this reason there are not carbon sequestration strategies that include also HS and their dynamics. Currently, the fundamental strategy is application of HS from various sources as soil amendments or fertilizers to support the yield of agricultural plants. This strategy, however, leads only to C redistribution as HS are relatively fast metabolized or mineralized." (Angelico et al 2023)
Resilience
Resilience
"Macroaggregates (diameter > 0.25 mm) help the soil surface to resist erosive forces, but their contribution to soil surface stability changes with time because macroaggregate formation and disintegration is a dynamic process. ...Persistence indices are a non-destructive way to describe dynamic changes in macroaggregates at the soil surface, which is complementary to other methods that visually evaluate the soil structure. " (Tian et al 2021)
Ploughing/Tilling breaks apart macroaggregates, limiting their formation and leaving behind tiny soil particles. This creates tight pore spaces and leaves little room for water to infiltrate soil. No-till systems reduce soil disturbance and enhance macroaggregate formation, and also play a part in controlling erosion. Eliminating tillage allows aggregates to grow in size and lengthens breakdown time, providing a long-term source of organic matter for roots, bacteria and fungi. This is recognnised in Regenerative techniques, as I spelled out in the Masters in Regenerative Farming & Food that I wrote for Plymouth University in 2020.
Ponder this
Ponder this
The addition of humic substances, with their sticky properties, mixed with GRSPs, built larger aggregates which provided larger porosphere. These stronger structures were resilient enough to create new architectures for fungi to thrive and loads of creatures to run around. The pores and channels enabled air to get deeper into the soil and allow life to grow deeper down - a great benefit to earthworms and higher oribatids.
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