Soil Structure

What is Soil?

Energy Entropy Functions Decomposition Glomalisation Clay Biome

You know when you jump up and down on soils, they are both very firm and a bit spongy at the same time. If you run on concrete, there is no ‘give’. On tarmac, there is a bit of spring and there are running tracks designed to give more in return. But when you run over ‘the country’, you can feel it is easier to the limbs, despite the mud. It is the aggregates in soil that provide that spring, because they have both strong structures and spaces in between. There is a whole science, called rheology, based on the Greek for 'flow' ,that examines the stresses placed on materials and how they move as a result. The soil structure does not stay still.

Size Range

There are the three classic basic grains based on size ranges (which is different to mineralogical classification):

CLAY particles size range is less than 0.002 millimeters in diameter. Some clay particles are so small that they cannot be seen with a standard microscope. They are small in size and very flaky in shape.

SILT particles range in size from 0.002 to 0.05 millimeters in diameter. Silt particles are only visible under a microscope. They become dusty when dry and are easily brushed off hands and boots.

SAND particles are visible to the naked eye and range in size from 0.05 to 2.0 millimeters in diameter, and their shape can be described by several parameters, including shape and sphericity (surface morphology)

Imagine if you filled a room in your house with footballs, golf balls or pinheads. You will find that the total surface area of pinheads is substantially greater than for footballs and golf balls. Clay particles have about 1,000 times as much external surface area as the particles in an equal weight of sand. The smaller sized particles make a much better place to store plant nutrients, held by the surface tension. This makes clay soils much more fertile than sand

Sponge

It is useful to think of the soil as a ‘sponge’ - or soils as sponges, as they all vary. Like a sponge it has a huge amount of pore space and a lot of surface area. A natural sponge was once a multicellular organism living in the sea and consist of collagen fibres and inorganic components like silica and carbonate to make it flexible and strong. Like these sea sponges, soils are able to absorb vast amounts of water to release when conditions become dry. This is especially important with sandy soils which do not retain water well.

Imagine we were building a town. There would be roads using tarmac, which is gravel and small rocks (called aggregates by builders!) mixed with tar. The pavements would be solid concrete paving slabs. Houses would built with clay-baked bricks and great office blocks made of steel reinforced concrete. In each building there are halls, stairs and rooms. This is where people live and work, in the space between the walls, that are supported by strong structures in the soil. We could add about telephones and other communications systems, but here we are interested in the structures, as something similar goes on below ground.

Lumps of soil particles are properly called ‘peds’ – Greek for ‘soil, earth’ (not to be confused with Latin pedis -foot!). Soil peds are natural, relatively permanent aggregates, separated from each other by voids or natural surfaces of weakness. Peds persist through cycles of wetting and drying. These underground structures are like miracle mud dwellings. They are so important that the dominant science of soils is called ‘pedology’. They form as a result of various processes, called ‘pedogenic’, to produce natural organisations of particles, of discrete units separated by pores or voids. The term ‘ped’ is usually used to describe those aggregates greater than 1 mm in size. These can be seen when you look at a lump of soil to see whether it crumbles well - what we often call ‘friable’.

When you pick up a handful of soil and it crumbles that is a good feeling about soil aggregates. Each aggregate is made up of soil particles of different sizes held together by both the attraction of soil particles and the binding of organic matter, including gels, between soil particles. The glues could be aerobically formed cement in the form of glomalin related products from aerobic digestion or humic substances, the result of anaerobic animal gut digestion. They hold aggregates together, making them stable and structural. At the same time, aggregates and colloidal mineraloid gels, protect the organic matter from decomposition.

A ‘well-structured soil’ will have a continuous sponge with pore spaces for life to move along, and to control drainage of water, free movement of air and unrestricted growth of roots. A ‘good’ soil is thought to consist of around third-half solids, and a third-quarter each of air and water, but this can vary enormously

Friable

'Friable' means the tendency of a soil to crumble easily. Friable soil has a crumbly structure ideal for the underground activity; it is characterised by larger 'clods' that break easily and smaller soil aggregates being harder to break. Soil Fragments and Clods are artificial structural units, formed at or near the surface by cultivation or frost action, and are not peds.

The interaction between colloidal mineraloid gels like iron, silicon, and aluminum oxyhydroxides with organic matter plays a vital role in protecting organic carbon in soils. These gels adsorb organic matter onto their surfaces, form stable organo-mineral complexes, and promote soil aggregation, all of which protect organic compounds from microbial degradation. This protection mechanism is crucial for carbon sequestration, soil fertility, and the long-term sustainability of ecosystems..

Colloidal mineraloid gels such as iron, silicon, and aluminum oxyhydroxides can protect organic matter from decomposition in soils. This process is a key part of how soils stabilize organic carbon, leading to long-term carbon storage and influencing soil fertility. These colloidal mineraloid gels, often present in fine soil fractions, play a crucial role in forming organo-mineral complexes that shield organic matter from microbial attack and chemical decomposition.

Here’s how these colloidal gels protect organic matter:

1. Surface Adsorption and Protection of Organic Matter

Iron (Fe), aluminum (Al), and silicon (Si) oxyhydroxides and their colloidal forms are highly reactive and have large surface areas. These materials tend to carry positive charges at certain pH levels, which makes them excellent at adsorbing negatively charged organic molecules, such as humic substances, on their surfaces.
Once organic molecules like humic acids or fulvic acids adsorb onto the surface of these oxyhydroxides, they are physically protected from degradation by soil microorganisms. The close bonding between the organic matter and mineral surface reduces microbial access to the organic molecules.
Microbes are less able to break down the organic matter when it is adsorbed onto mineral surfaces because the enzymes they secrete cannot easily reach the protected organic compounds.

2. Formation of Organo-Mineral Complexes

These mineraloid colloids can form organo-mineral complexes with organic matter. In this process, the organic matter is chemically bound to the surface of minerals through a combination of electrostatic forces, hydrogen bonding, and van der Waals interactions.
The formation of stable organo-mineral complexes further protects the organic matter by making it less prone to microbial decomposition. Once complexed with iron, aluminum, or silicon oxyhydroxides, organic compounds are much more resistant to enzymatic breakdown.
These complexes can persist for hundreds to thousands of years in soils, acting as a long-term carbon sink, which is crucial for carbon sequestration and climate change mitigation.

3. Aggregation and Physical Protection

Colloidal oxyhydroxides help form soil aggregates by binding to clay particles and organic matter. These aggregates, composed of clays, organic matter, and colloidal gels, create microenvironments in soil where organic matter is physically isolated from decomposers.
Iron and aluminum oxyhydroxides are particularly important in forming these stable aggregates. Once inside an aggregate, the organic matter is shielded from microbial enzymes and oxidation, slowing down its decomposition.

4. pH-Dependent Adsorption

The ability of iron and aluminum oxyhydroxides to protect organic matter is influenced by soil pH. At lower pH (acidic soils), these oxyhydroxides tend to have a positive surface charge, which enhances their ability to attract and adsorb negatively charged organic molecules.
In acidic soils, this process is particularly strong, leading to high levels of organic matter protection, as seen in spodosols or podzols, where iron and aluminum-rich horizons can accumulate significant amounts of organic carbon.
Even in more neutral soils, colloidal forms of these minerals still provide protection, although their interaction with organic matter may depend on other factors like the presence of clay minerals or soil moisture.

5. Metal-Oxide Coatings and Encapsulation

In soils, organic matter can become encapsulated by coatings of iron, aluminum, or silicon oxyhydroxides, essentially "trapping" organic compounds within these mineral matrices. This encapsulation reduces the accessibility of organic matter to microbes and their enzymes.
Iron oxides (like goethite and hematite) are particularly effective at forming these protective coatings around organic matter, especially in waterlogged soils where anaerobic conditions slow down microbial degradation.

6. Inhibition of Oxidative Enzymes

Some iron oxyhydroxides, such as ferrihydrite and goethite, can also play a chemical role in protecting organic matter by inhibiting oxidative enzymes that are produced by microbes to break down organic matter.
These enzymes rely on oxidative reactions to degrade organic molecules, but in the presence of iron oxyhydroxides, these reactions can be slowed down or inhibited, further reducing the rate of organic matter decomposition.

7. Silicon's Role in Organic Matter Stabilization

Silicon in soils, especially in the form of amorphous silica or silicic acid, can also interact with organic matter. Silicon can coat organic matter, physically stabilizing it and reducing microbial degradation.
In tropical soils, where silica may be more abundant in the weathering zone, it can contribute to long-term stabilization of organic matter, much like iron and aluminum oxyhydroxides.

Examples of Soil Types Where These Interactions Occur:

Spodosols (Podzols): These are soils commonly found in boreal and temperate forest ecosystems, characterized by acidic conditions and accumulations of iron and aluminum oxides in the lower soil horizons. These oxides protect organic matter from decomposition, leading to high organic carbon content.
Andisols: Formed from volcanic ash, these soils are rich in allophane (a type of aluminum silicate) and ferrihydrite, both of which bind tightly to organic matter, stabilizing it over long periods.
Ultisols and Oxisols: In tropical and subtropical regions, these highly weathered soils contain significant amounts of iron and aluminum oxides, which bind and protect organic matter even under high temperatures and moisture conditions.

Structure

Two thirds of healthy soil is made up of air. Voids - or what we call pores! These provide massive surface areas available for taking in water. Because of these surfaces, there can also be a vastly increase the availability of nutrients. All these open up areas for bacterial activity, so other organisms potential increase dramatically. These small homes can provide life to many new forms – and some old ones too. However, to have pores, you have to have peds.

Aggregates (peds) are a key characteristic of soils. They are what distinguish soils from mud. They are mixtures of substances that provide both structure to provide strength and resilience and pores of all sizes and structures. Aggregation is something more than flocculation involving a combination of different factors such as hydration, pressure, dehydration etc. and required cementation of flocculated particles. The cementation may be caused by cations, oxides of Fe and Al, humus substances, GRSPs and products of microbial excretion and synthesis.

The aggregates come in all shapes and sizes, some massively larger than others. They all vary according to circumstances, but have key characteristics.

But how did these peds that define soils come to be? What happened to make those aggregates?

Structure is studied in the field under natural conditions and there are 4 principal forms of soil structure

Spheroid where all rounded aggregates (peds) not exceeding a 2-3 cm in diameter are placed

Plate-like where the aggregates are arranged in relatively thin horizontal plates or leaflets.

Block-like where all three dimensions are about the same size. The aggregates have been reduced to blocks .

Prism-like where the vertical axis is more developed than horizontal, giving a pillar like shape.

From Micro to Macro aggregates

Did this happen overnight, or over several hundred million years?

There has been aggregate formation for well over 400 million years, and has continued to develop to this day. Over hundreds of millions of years micro-aggregates have developed into macro-aggregates.

There would be clay, hyphae, debris and cement all those years ago, which would have been crucial in providing, stable structures, that provide pores for air, resilient enough to withstand the increasing weight of soil above, and the ecosystems above the soils, including dinosaurs, as it is built from below. It is the bacteria, fungi, worms and arthropods, using local materials, and sticky substances like humic substances and glomalin related soil proteins (GRSPs) that build ever more complex aggregates, in ways much the same the world over. Aggregate structures provided both large and small pores. Large soil pores allow water to quickly infiltrate the soil. Smaller soil pores can store plant available water in times of limited rainfall. These pores provide living spaces. These spaces have air for small creatures and water retained on the walls by surface tension.

If aggregates were architecture, we would be marvelling at their structures. All sorts of different materials providing structures for all sorts of functions. Except it would be even more marvellous. These aggregates are not produced following rational design, but produced by random behaviours that result in structures that can provide living spaces underground.

The 'cement' structure illustrated above is oversimplified. Clay minerals can be well or poorly ordered and crystalline include mixed-layer, part swelling part non-swelling clays. These clay particles then cover bits of dead debris, plant or animals.

The sticky clay particles coat bigger bits of debris including fungal filaments, humus and bacteria, getting bigger in size. When water is added, usually by adsorption (see below), bound and unbound water will have a huge effect.

Then roots and fungal filaments become part of the building blocks, making macroaggregates

Micro-aggregates

Aggregates are usually classed as micro or macro. The micro sort are less than 250µm and consist of silt and clay particles mixed with organic matter. For many years we have thought organic matter serves as the main binding agent to form and stabilize aggregates. But it is only recently we have worked out which organic matter. The cement for aggregates comes from various sources including colloids, worm mucous, root exudates and arthropod poo. There were no earthworms early on but there were enchytraeid worms.

Think of these aggregates, not as ‘lumps stuck together’, but buildings in a city. A citadel where there are complex constructions creating corridors and catacombs, for creatures to crawl and squirm through.

The structure of these buildings is affected by all sorts of processes – physical, chemical and biological, based on those four building blocks. The natural processes that aid in forming aggregates are wetting and drying, freezing and thawing, microbial and mesofaunal activity

Microaggregates are quite stable. We are increasingly aware that massive machinery over soils smashes these aggregates so the whole soil life is crushed.

Macro-aggregates

There would have been some macroaggregates for the last 400 million years, but it may be 200m years later when they had the most dramatic impact on soil structure.

Macroaggregates are greater than 250um and are a collection of silt/clay particles, humus, microaggregates (micros), and organic matter – dead bits from springtails and oribatids. Along with plant roots and mycorrhizae, they are major contributors to the formation of macroaggregates. GSRP and humic substances provides the ‘glue’ to help bind these larger aggregates.

These larger aggregates have a shorter breakdown time, providing an organic matter source for roots, bacteria, and fungi. Shorter resilience than micro aggregates, which are very stable, they are never the less better at making pores. Aggregates store and supply organic matter in soil; however, they also have structural functions.

Fungi & Soil Aggregation

“Soil structure, the complex arrangement of soil into aggregates and pore spaces, is a key feature of soils and soil biota. Among them, filamentous saprobic fungi have well-documented effects on soil aggregation. However, it is unclear what properties, or traits, determine the overall positive effect of fungi on soil aggregation. To achieve progress, it would be helpful to systematically investigate a broad suite of fungal species for their trait expression and the relation of these traits to soil aggregation.

Here, we apply a trait-based approach to a set of 15 traits measured under standardized conditions on 31 fungal strains including Ascomycota, Basidiomycota, and Mucoromycota, all isolated from the same soil. We find large differences among these fungi in their ability to aggregate soil, including neutral to positive effects, and we document large differences in trait expression among strains. We identify biomass density, i.e., the density with which a mycelium grows (positive effects), leucine aminopeptidase activity (negative effects) and phylogeny (family tree) as important factors explaining differences in soil aggregate formation (SAF) among fungal strains; importantly, growth rate was not among the important traits. Our results point to a typical suite of traits characterizing fungi that are good soil aggregators, and our findings illustrate the power of employing a trait-based approach to unravel biological mechanisms underpinning soil aggregation”(Lehmann 2020)

Microbes & Aggregation

The process of soil aggregation can be considered an important driver of evolution in the soil microbial community. “There are several features that make soil aggregates specifically interesting, and perhaps even unique, in terms of a setting for microbial evolution. Soil aggregation is a continuous and dynamic process in which the formation and disintegration of individual micro- and macroaggregates are separated in time by periods of relative stability. Each individual soil aggregate may provide a unique environmental compartmentalization of the soil microbial community that is, to a large extent, isolated from its surroundings and that can be thought of as an ‘incubator’ for microbial evolutionary change. Because of their isolation, different aggregates can be regarded as ‘concurrent incubators’ that allow enclosed microbial communities to pursue their own independent evolutionary trajectories during their lifetime (‘incubation period’). The huge number of aggregates that exists at any moment in time validates their conceptualization as ‘massively concurrent incubators’ for microbial evolutionary change. Upon disintegration of soil aggregates (‘incubation cycle ends’), formerly enclosed microbial communities are released and allowed to interact with the microbial community of the soil at large.”

The evolution of aggregates itself drives the evolution of bacteria, which we know can relatively quickly adapt to the changed circumstances. Aggregates would provide a wide range of environments from aerobic to nearly anaerobic and different circumstances for redox reactions.

Water holding

Soil aggregates are critical for holding water in the soil for two reasons. First, a well-aggregated soil has large pores between aggregates to let water enter the soil profile. Second, small pores within aggregates hold water tightly enough to keep it around, but loosely enough for plant roots to take it up. It’s critical that soil both let water flow through, and hold water for later.

Wax

When you pour water on dry soil, the first lot runs off. That is called hydrophobic soil, and occurs when a waxy residue builds up on the soil particles. These compounds are naturally present in most soils, created by fungal activity or through the release of a plant's essential oils. They cause any water to repel rather than absorbing it. It is most common in sandy soils, dried out potting mix, and soils containing un-rotted organic matter. In healthy soils, microbial activity continuously breaks these compounds down. But if a soil fully dries out through drought or watering neglect, the microbe population dies off and the oils start to build up to harmful levels.

Leave it a few minutes and when you water again, more of it stays in the soil. And with the next watering, it is even more noticeable that the water stays in the soil. What are the mechanisms that enable this? There is a whole new industry answering this, centred around soil surfactants (that alter the surface tension of liquids so they can interact with solids or other liquids). We come to watering dry soils – an immensely important issue for many parts of the world, the soil does not hold it at first, but as the water soaks in, it can handle more - it is called 'retention'. More organic and healthier soils improve this retention.

Pores

We used to concentrate on peds. Now we are looking at pores more closely. As the aggregates developed and the pores increased in size in those times 350mya, so did the water-holding capacity of the soils. Previously water would run off and evaporate more. Water drying from pores is hard work, as the energy needed to escape from the water tension holding the water to the sides of the pores increased.

These would include the ways water and air move; chemicals interact and also the ways the creatures work together. Imagine the size of these structures, similar but different the world over. Soils can now be six foot deep. Perhaps the nearest other structure is the nest of termites.

Millions of small creatures are building these structures many times larger than themselves, enabling a rich community of life. Some would be building, while others would be burrowing. As the pores increased in number and sizes, and the aggregates became more stable and resilient, the opportunities for many other forms of life were established. The new different but stable environment are ideal conditions for evolution of new forms.

Understanding water-holding properties of soil is becoming more important as it could help millions of lives over the world that increasingly being dominated by changes. Many of the 'climate change'/'global warming' events are water related, and exacerbated by poor soil health. Many areas now suffer drought conditions, and others from flooding where the water runs straight off the land, without being absorbed to help plants grow.

In top1ft depth of soil, increasing Organic Matter by 1% increases available water capacity by about 3,400 gallons per acre in that medium-textured soil, on top of an estimated existing 71,000 gallons available water capacity.

Porosphere

Vannier made diagrams of the state of moisture in the soil. He was looking at what creatures do when it evaporates, showing the importance of pores in creating an environment for creatures to live in.

The title of the paper was fascinating: The poroshere as an ecological medium emphasized in Prof Ghilarov’s work on soil animal adaptation" (Vannier G 1987). This is the paper which introduced me to Ghilarov, who we will find out more about later..

Vannier spells out that there are three different sorts of moisture in these pores. Depending on how water evaporates from soils determines the strength water is held to the particles. The ‘gravitational’ moisture has free water and displaces the small arthropods on to the surface like floods would. ‘Capillary’ moisture is closer to the soil where springtail stay around and mites clear off. While the third is ‘adsorptional’ moisture, the last phase of evaporation, where the water holds tighter, and mites curl up inside.

Porosity

The development of structures that could hold water were absolutely critical for the success of soils and the creatures and plants that developed from them. Soil aggregates are critical for holding water in the soil for two reasons. First, a well-aggregated soil has large pores between aggregates to let water enter the soil profile. Second, small pores within aggregates hold water tightly enough to keep it around, but loosely enough for plant roots to take it up. It’s critical that soil both let water flow through and hold water for later. If our soil doesn’t let water percolate, we have puddles forming, runoff, soil loss and flooding and less water to supply plants. If soils don’t hold water, plants suffer from drought.

Soil Moisture States

There are three main terms to describe soil moisture states - saturation, field capacity and permanent wilting point. These are used to describe water content across different water potentials in soil and are related to the energy required to move water. The maximum amount of water that a given soil can retain is called field capacity, whereas a soil so dry that plants cannot suck moisture from the soil particles is said to be at wilting point. The difference between these two figures is called the Plant available water.When the soil is at or near saturation the direction of the potential energy gradient is downward through the soil profile or laterally down slope.

A further term ‘Drainable porosity’ is the amount of water that drains by gravity (through macropores) between saturation to field capacity – representing a few days drainage in a field. This Is not considered available to plants as it soon runs away. As the soil dries, field capacity is reached after free drainage of macropores has occurred. Field capacity represents the soil water content retained against the force of gravity by matric forces - in both micropores and mesopores. Further drying and water is held more strongly to mineral surfaces due to cohesive forces between water molecules and adhesive forces associated with water and mineral particles (capillary forces.

Holding Capacity

Soil texture and organic matter are the key components that determine soil water holding capacity and are a reflection of the aggregate building capacity. Soil texture, arises from the mixture of particle sizes.

Small ones such as silt and clay, have larger surface area making it easier for the soil to hold onto water, giving a higher water holding capacity. Clay only slowly adsorbs water but holds more than sand. Clay-rich soils have the largest internal pore space, hence the greatest total water holding capacity but this does not describe how much water is available to plants, or how freely water drains in soil. These processes are governed by potential energy. Water is stored and redistributed within soil in response to differences in potential energy. A potential energy gradient dictates soil moisture redistribution and losses, where water moves from areas of high- to low-potential energy. When clay soils are wet, they are often too sticky to work and when they are dry they are too hard to work. Sand in contrast has large particle sizes, with smaller surface area, so the water holding capacity for sand is low.

Soil organic matter (SOM) and inorganic minerloid & amorphous gels can help increase water holding capacity. SOM has a natural attraction to water, and helps create pores in a range of sizes. Exactly how much more water is stored due to soil organic matter will depend on soil texture, though. One very rough estimate is that for an increase of 1%SOM, an acre of soil can hold 3000 gallons more.

Adsorption & Absorption

The distinction between the two processes and how they relate to clay:

Adsorption (Primary Mechanism):

Adsorption refers to the process where water molecules adhere to the surface of clay particles.
Clay particles have a large surface area and are made up of layers with a net negative charge. This negative charge attracts water molecules, which are polar, causing them to stick to the surface of the clay.
Water molecules are held in thin layers around the clay particles, often forming what’s known as an adsorption layer.
This adsorbed water is relatively difficult to remove because it is bound tightly to the charged clay surfaces.

Absorption (2ndary Mechanism):

Absorption occurs when water is taken into the interior of the clay structure.
Certain types of clay, especially expanding clays like montmorillonite, can absorb water between their crystal layers, causing the clay to swell. This happens because water enters the interlayer spaces of the clay mineral structure.
In this process, the clay takes in water, leading to an increase in its volume. This swelling capacity is a key property of some clays and makes them useful for specific applications, such as in sealing or water retention.

Types of Clays and Water Interaction:

Kaolinite: Adsorbs water on its surface but does not swell significantly because its layers are tightly bound.
Montmorillonite: Both adsorbs water on the surface and absorbs water between its layers, leading to swelling. It can hold much more water than kaolinite.
Illite: Similar to kaolinite in that it adsorbs water but doesn’t exhibit much swelling.

Water movement in soil

Some will evaporate, some used by plants will transpire, some is held in the aggregates and the rest percolates through the subsoil. This will be at rates depending on the composition and structure of lower layers. The depth water is found in the ground varies from a few centimetres to many metres. Deeper down it seeps into the rocks, although some are impermeable. Subsurface water not absorbed by the soil is called groundwater and is found at the level, called the water table.

The ease with which water drains from soil is equally as important as storage. For example, most terrestrial plants need to assimilate oxygen through roots, but oxygen is scarce in saturated soils. Moreover, microbial decomposition of organic matter is greatest (by orders of magnitude) under aerobic conditions. Poorly drained soils limit this decomposition, as we shall soon see.

Water storage dynamics and flow facilitate the four basic soil forming processes: translocations, transformations, additions and losses of soil constituents in a soil profile. These processes determine the chemical, morphological and physical properties of soil such as the variation of texture with depth. Because of the new water holding capacities, those forces would have undergone massive changes in this period.

It seems that anaerobic processes may be becoming more dominant in agricultural soils, as there seems to be more compaction and flooding. The implications of shifting to less efficient anaerobic processes may have a significant impact concerning global warming.

“The creation of anaerobic soil conditions is predicated in the situation where demand exceeds the supply of oxygen. Once a soil becomes saturated, the supply of oxygen is immediately reduced owing to the displacement of oxygen contained in the available pore space. Following consumption of the relatively small amount of available oxygen in the pore water, oxygen can only be supplied to respiring organisms through the process of diffusion from the nearest aerobic zone. This process is comparatively slow under saturated soil conditions as oxygen diffusion in water is approximately 10,000 times slower than through air. Under these conditions, even moderate rates of soil or root respiration can quickly deplete available oxygen and result in anaerobic soil conditions"

Architecture

Wouldn’t it be great if we could look into these soil cities, and see the architecture, the large and the small of it. If only we could but a microscope into the ground and have a look. But we can’t because of the basic laws of light. What about one of those endoscopes? They all disrupt the very life we want to see, much the same as in other sciences, like histology, where the tissues and cells studied under microscopes are poor facsimiles of living, pulsing, metabolising tissues and cells. But at least they get a look in!

We are increasingly talking more about soil architecture, rather than ‘structure’. ‘Architecture’ emphasises the close relationship between the arrangement of soil physical constituents in space and the functions that such arrangement enables.

Non-disturbing imaging techniques has led to significant progress in the description of soil architecture, which is leading to an ongoing battle between the ped gangs and the pore gangs as to how we should approach soil architecture.

It is this architecture which determines the life in soils, in the many different buildings, that go to make up soil cities.

Latest on soil architecture

Perhaps you can see how mixing the cement to bind together the bricks, that hold steel structures in place, to provide windows and doors, corridors and rooms, is similar to what goes on underground. It will not have the preciseness of Frank Lloyd Wright or Corbusier designs. Perhaps Gaudi gives some idea, although it would be interesting to discuss whether his inspiration was aesthetic, rather than a scientific appreciation of the architectural nature of natural phenomena. What did he know about clay?

Chips

So it was wonderful to find – quite recently – that somebody had worked out a way to use microchips – those in all our phones, in a novel way. Apparently, some of these chips can work in liquids – but not the ones that went in my phone with me when I fell into the pond. They’re called ‘microfluid chips. And somebody had the very bright idea to stick some in the soil and see what they detected. They could register what arrived and when where and how.

This is what these chips can do..

A study revealed the dark, dank cities in which soil microbes reside. They found labyrinths of tiny highways, skyscrapers, bridges and rivers which are navigated by microorganisms to find food, or to avoid becoming someone’s next meal. “All major groups of soil microorganisms (bacteria, fungi, protists), as well as invertebrates such as nematodes and microarthropods, colonized the chips and explored their internal structures, both when the chips were incorporated into soil and when they were incubated with soil in the laboratory. Soil mineral particles and soil solution also entered the chips via water movements. The transparency of the chips allowed us to observe the primary colonization of a pristine pore space and soil microbial interactions in real time: microhabitat formation, interactions of fungal hyphae with other soil organisms and components, and microbial food-web interactions.”

This would have gone on for millions of years, and in the process, the next steps would be to use these aggregates, as building blocks, to build other structures. Imagine building the Taj Mahal only many times larger. There would be a mixture of building blocks, and ways to arrange them. None of this would have been done with prior thought or planning, but by the laws of nature. And the results are all the more impressive because of that.

Primary Aggregation Early Soils
Clay soil

It was during this period 360-300mya that these new earthly structures were developing stability and improved water-holding capacities, some based on sand, others on mud. These would have been first near the surface, leading to less runoff and more stable living conditions, but with the roots going down and more burrowing going on, the increase in aggregates provided improved water holding abilities. This new underground architecture was created by organisms and creatures working with each other to provided their living space too. We’ve seen the outline of a whole new architecture of aggregate development between the existing players – roots, fungi, worms, bacteria and bugs that now created strong structures with pores in between that enabled new living spaces for organisms to find and evolve into. Let’s look at the various players in more detail.

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