The root-soil interface: one of the most important contacts not in your address book

For plant roots to extract water and nutrients from the soil, roots must maintain contact with soil. Plant roots maintain contact with soil by releasing sticky substances that are formed of long chains of sugars, known as polysaccharides. Some of these polysaccharides cling onto the surface of roots, particularly at their caps, to stick soil to the surface of them. Other polysaccharides that have been secreted are released into the surrounding soil where they can clumps soil particles together, which is a process called aggregation. This aggregation is what maintains the root-soil interface. Root-soil contact is vital for plants to extract the necessary resources needed for plant growth.

The root-soil interface is a two-way exchange, where in exchange for resources that plants need, plants release organic molecules into the soil. These organic molecules create and preserve soil aggregates, which is considered a good indicator of soil health. A healthy soil contains high amounts of these aggregates. These aggregates permit air to penetrate and flow into the soil, and allow water to be held within the soil, which are all crucial for soil-dwelling life. These factors contribute to a productive soil, which is so sort after for agricultural production. As well as released polysaccharides, polysaccharides from decaying plants causes soil to aggregate.

Plants do not have skeletons like us to prop themselves up. Instead, plants have an extensive network of walls, encapsulating each of their cells. These cell walls are formed of various polysaccharides that form a matrix, which is very similar to a network of scaffolding. This scaffolding is embedded within a matrix of pectin, which is comparable to cement. A good analogy of a cell wall would be modern-day concrete, which is used in construction. A network of steel scaffolding are placed into cement before it sets. These steel scaffolds increase the strength of the cement. The same is true for cell walls in plants.

Along with plant polysaccharides, polysaccharides from soil-dwelling bacteria and fungi also aggregate soil, for a very similar purpose. Any lifeform living in soils must form an interface with the soil to extract resources needed for their growth, and even form relationships with other soil-dwelling life. In particular, one relationship between the majority of plants and a special form of soil fungi, mycorrhizal fungi, is critical for plant growth. This symbiotic partnership enables plants to access nutrients from soils that is otherwise inaccessible to them. These fungi can extract a nutrient, which is fast running out, phosphorous, in exchange for carbon, which is used for the fungi’s growth. This carbon forms an energy currency. Without this interface with soil, plants would soon begin to die from a lack of phosphorous. Another example of a symbiotic relationship is between a group of plants known as legumes, which includes pea and lupin, and a bacteria, which can remove nitrogen from the air, and offer it in a form that plants can use.

In addition to beneficial interactions that plants have with other soil life, parasitic and life can highjack this interface to infect plant roots. By controlling and securing this interface, growers could improve crop production. Other than polysaccharides, plant roots release a whole host of organic molecules into the soil, which is known as root exudate. This root exudate has many roles in the root-soil interface from plant defense against infectious lifeforms, nutrient acquisition, water uptake and as a method of competing with other plants. This interface as well as roots themselves are not well understood probably because they are hidden from sight.

Recently, commercial polysaccharides from plants have been shown to greatly increase the abundance of soil aggregates. Although more work is require to understand this effect, it shows promise to developing a soil conditioner. This soil conditioner could be used in tandem with fertiliser to maintain and increase soil aggregates. As an integral method for farmers, field are regularly ploughed to remove weeds and to prepare soil for new crops. This ploughing breakdown these soil aggregates, which in turn leads to weaker less coherent soil, thus contributing to soil erosion. Soil erosion causes millions of hectares of farmable land to turn to dust, which is occurring at an alarming rate. This erosion leads to less farmable land, and famines, causing countless deaths globally. Soil erosion is set to increase as climate change takes hold. A soil conditioner could prevent soil loss due to erosion, by preventing the breakdown of these aggregates. Conditioning the soil with these polysaccharides may even lead to sustainable food production on the moon. By 2037, NASA aims to build a Luna outpost in preparation for a manned mission to Mars. Experiments using moon soil brought back from the Apollo missions, demonstrated that plants can be safely grown using the moon’s soil. However, as the moon’s soil is made of fragments of small dusty rocks, plant’s found it difficult to establish themselves.

One of the earliest plants to emerge from the primordial sea to colonise the land, over 470 million years ago, have been demonstrated to release polysaccharides. Polysaccharides released by these primitive plants, liverworts, are similar to what modern-day plants release. It is believed that these released polysaccharides helped to form early soils by causing the loose soil particles to aggregate. Prior to plants, early soils would have been formed of small dusty rock fragments. As plants slowly took over the land, they added vast amounts of carbon from generations of decaying plants. By releasing these polysaccharides, early plants formed an early version of this root-soil interface. This enabled them to extract water and nutrients, which had previously been widely available in the primordial sea.

Releasing polysaccharides enables plants to secure and maintain the root-soil interface. This interface ensures that plants can extract water and nutrients from the surrounding soil. The root-soil interface can also enable plants to form symbiotic relationships with soil-dwelling life to extract nutrients previously unavailable to plants. By developing a soil conditioner based on these released polysaccharides, soil erosion could be prevented. Protecting the root-soil interface could also increase nutrient availability for plants, thus increase crop production without the need for genetic manipulation. The root-soil interface was also integral to the evolution of early soils.

Watch a talk on the importance of soil here

Soil Structure and Methods of Analysis

Soil is composed of approximately 52% solid matter—50% mineral matter and 2% living organisms—and 48% fluids. The characteristics of soil, including its texture, structure, pH, and fertility, are crucial for plant growth and productivity. Understanding these components is essential for effective soil management and cultivation.

The Earth's crust consists of minerals (rock) formed over millions of years. Landscape features, such as mountains and valleys, are shaped by kinetic energy from tectonic plate movements, erosion, and deposition. Soil formation begins when rocks break down into smaller fragments. This process and the resulting soil type are influenced by several factors:

1. Parent Rock: The type of rock being weathered, known as the parent rock, plays a significant role in soil formation.

2. Weathering Process: Weathering, either physical or chemical, breaks down the parent rock into smaller particles.

3. Soil Formation: When mineral matter is sufficiently broken down to support plant life, it becomes soil.

There are three primary rock types contributing to soil formation:

• Igneous Rocks: Formed from cooled and solidified lava, igneous rocks, such as granite and basalt, weather slowly but are typically very fertile.

• Sedimentary Rocks: These rocks, including limestone and clay, are formed from older rocks that have been moved and deposited by air, water, and ice. They are softer and weather more quickly.

• Metamorphic Rocks: Resulting from the transformation of sedimentary rocks through intense physical or chemical processes, metamorphic rocks like marble, slate, and quartzite are generally harder and more resistant to weathering.

Weathering processes can be categorized into three main types:

• Mechanical Weathering: Physical forces, such as heating and cooling, freezing and thawing, and wetting and drying, break the rock into smaller pieces without altering its mineral composition.

• Chemical Weathering: Chemical agents, like acidic rain, cause permanent changes to the original rock minerals.

• Biological Weathering: Plants, animals, and humans contribute to weathering through both physical and chemical means. For instance, plant roots can penetrate and break down rocks.

These weathering processes produce the fundamental components of soil, including rock fragments, iron oxides, soluble materials, and newly formed secondary minerals like clay. Mechanical weathering occurs in areas with little soil, where minerals are exposed to water and ice. Chemical weathering, driven by rainfall, transforms old rocks into new minerals. Biological weathering involves plant growth, such as lichen, which mechanically and chemically weather rocks.

Soil can form in place from the underlying rock, known as residual soil, or be transported by natural forces like wind, water, or ice. Transported soil types include:

• Aeolian: Wind-transported soil.

• Alluvial: Water-transported soil.

• Glacial: Ice-transported soil.

• Factors influencing soil development include parent material, topography, climate, vegetation, animals and humans, and time.

In Britain, the five most common soil types are:

• Brown Earth: Typically found under deciduous woodland in cool, temperate climates, this soil has a loamy texture and well-developed crumb structure. It is ideal for agriculture and has a weakly to moderately acidic pH. Its dark brown color results from humus distributed by earthworms and other animals.

• Podsols: Formed under coniferous forests, podsols are poor for agriculture due to their acidic nature. Leaf litter from conifers creates highly acidic conditions, preventing the incorporation of humus by earthworms. Organic acids leach iron compounds from the upper soil layers, leaving a horizon predominantly composed of quartz.

• Clayey Soil: Characterized by high clay content, this soil retains water well but can be challenging to manage due to its tendency to become compacted.

• Rendzina: Found on calcareous rock substrates, rendzina soils are rich in lime and support diverse vegetation.

• Peat: Accumulated from partially decayed organic matter, peat soils are typically waterlogged and highly acidic.

Understanding soil structure and its formation processes is vital for optimizing soil management practices and enhancing agricultural productivity.

Decomposers in soil

Decomposers are essential to soil ecosystems, playing a key role in breaking down organic matter and recycling nutrients. Primary decomposers, such as beetles, woodlice, and earthworms, physically break down plant material and animal remains. These organisms chew and burrow through the material, partially digesting some of it and excreting the more resistant parts. Secondary decomposers, including fungi and bacteria, further decompose these tougher organic compounds, such as wood, which primary decomposers cannot digest. They break down complex organic substances into simpler mineral salts through a process known as mineralisation, making these nutrients available for plant uptake.

Soil minerals are classified into primary and secondary types. Primary minerals, which have not been chemically altered since their formation from molten lava, are more resistant to weathering. As primary minerals decompose, they form secondary minerals through a process of reprecipitation. Secondary minerals, such as layer aluminosilicates, are predominant in most temperate soils. These minerals consist of silicon/oxygen sheets in tetrahedral coordination and aluminium/oxygen sheets in octahedral coordination. For instance, kaolinite, a 1:1 mineral, has one silicon/oxygen tetrahedral sheet and one aluminium/oxygen octahedral sheet, and forms in warm to hot climates with acidic soils where basic cations and some silicon have been leached. Vermiculite, a 2:1 mineral, features two silicon tetrahedral sheets surrounding one aluminium octahedral sheet and forms in sub-humid to humid soils rich in mica. Hydrous mica (illite), formed in cooler, sub-humid areas, results from the dissolution and re-crystallisation of mica.

Soil texture, which refers to the relative proportions of sand, silt, and clay-sized particles, is a crucial physical property of soil. It affects soil permeability, water infiltration, porosity, and fertility. Soil particles are classified into three main categories based on size: clay (<0.002 mm), silt (0.002 to 0.05 mm), and sand (>0.05 mm). Larger particles are also classified as pebbles (2 to 75 mm), cobbles (75 to 250 mm), stones (250 to 600 mm), and boulders (>600 mm). These classifications are not arbitrary, as they align with changes in soil properties associated with different particle sizes. Chemically, sand and silt particles are relatively inert, with sand being resistant to wind erosion due to its larger size. Sand particles are predominantly quartz (SiO2) with minor amounts of silicate-based primary minerals such as feldspars, hornblende, and micas. Sand particles tend to have rough, angular surfaces, whereas silt particles are more spherical and polished. Silt is mostly quartz but contains higher proportions of primary minerals and iron and aluminium oxides. Wind easily erodes smaller silt grains.

Clay particles are chemically active and form aggregates that resist wind erosion and enhance soil porosity. In temperate soils, the clay fraction is typically dominated by layer aluminosilicates. In the humid tropics, where weathering is more intense, iron and aluminium oxides, along with hydrous oxides, become the dominant minerals. Soil texture can be classified using a triangular diagram. For example, to classify soil with 30% clay and 10% silt, locate the 30% clay mark on the left side of the triangle and the 10% silt mark on the right side. Draw lines horizontally from the clay mark and diagonally downward from the silt mark until they intersect. The intersection point indicates the soil classification, which in this case would be Sandy Clay Loam.

Sand

Sand is the largest textural class. Sandy soils are

dominated by the properties of sand: weak structure,

rapid infiltration rate, slight erosion potential, loose

consistence, and low fertility. When the soil is moist

and moulded into a ball, it will easily crumble when

touched. Sands contain 85 to 100 percent

sand, 0 to 15 percent silt, and 0 to 10 percent clay.

Coarse Sand

More than 25 percent of sand particles are 0.50

mm diameter in size or larger, and less than 50

percent are between 0.05 and 0.50 mm.

Medium Sand

Twenty-five percent of the particles are larger

than 0.25 mm. Less than 50 percent measure between

0.25 and 0.05 mm.

Fine Sand

More than 50 percent of the particles are between

0.10 and 0.25 mm or less than 25 percent are

greater than 0.25 mm and less than 50 percent

range between 0.05 and 0.10 mm.

Very Fine Sand

More than 50 percent of the particles are between

0.10 and 0.05 mm.

Loamy Sand

This category contains 70 to 85 percent sand, 0 to

30 percent silt, and 10 to 15 percent clay. Because

loamy sand contains more clay than does sand, it is

slightly cohesive and can be moulded into a ball that

Silt

Silts are highly erodible, relatively infertile soils.

They contain 80 to 100 percent silt, 0 to 20 percent

sand, and 12 percent or less clay. They can be moulded

into a ball that keeps its shape under gentle pressure.

The low percentage of clay precludes the formation

of a ribbon. Silts are distinguished from

loamy sands by placing a small amount of excessively

wet material in the palm of your hand and

rubbing the wet soil. Silt feels floury, whereas loamy

sand feels gritty.

Clay

Clayey soils have a very slow infiltration rate, drain

slowly, are very sticky and plastic when wet, and

form hard clods when dry.

Silty Clay

Silty clays are similar to clays. They contain 40

to 60 percent clay, 0 to 20 percent sand, and 40 to

60 percent silt. They form a ribbon greater than 5

cm in length and are very smooth when excessively

wet.

Sandy Clay

This category contains 35 to 55 percent clay, 45

to 65 percent sand, and 0 to 20 percent silt. Like

the other clayey soils, sandy clays form long ribbons.

When excessively wet, however, the higher

sand content gives them a gritty feel.

Loam

Loamy soils have characteristics intermediate between

those of sandy and clayey soils. These soils

can be moulded, and, as clay content increases, the

mould becomes firm and resists deformation under

moderate to strong hand pressure. Also, as the clay

content increases, the infiltration rate slows and the

soil forms hard clods when dry.

Sandy Loam

These loams contain 85 to 43 percent sand, 0 to

50 percent silt, and 0 to 20 percent clay. They are

slightly cohesive and can form ribbons less than

2.5 cm in length. When wet, they have a very

gritty feel. Sandy loams are further divided into

the following categories:

Coarse Sandy Loam

This group contains more than 25 percent

sand-sized particles greater than 0.50 mm in

diameter and less than 50 percent between 0.05

and 0.50 mm.

Medium Sandy Loam

More than 30 percent of this group is made of

particles greater than 0.25 mm in diameter; less

than 25 percent measures between 1 and 2 mm;

and less than 30 percent falls between 0.05

and 0.25 mm.

Fine Sandy Loam

More than 30 percent of the fine sandy loams

have particles that range in size between 0.05

and 0.10 mm; 15 to 30 percent are greater than

0.25 mm.

Very Fine Sandy Loam

More than 30 percent of these loam particles

range between 0.05 and 0.10 mm in diameter

or more than 40 percent range between 0.05

and 0.25 mm (half of which are less than 0.10

mm) and less than 15 percent are greater than

0.25 mm.

Silt Loam

Silt loams contain 0 to 50 percent sand, 50 to 88

percent silt, and 0 to 27 percent clay. They are

slightly cohesive when wet and form soft clods

when dry. Silt loams feel smooth when wet and

can form a ribbon less than 2.5 cm in length.

Loam

Loams contain 23 to 52 percent sand, 28 to 50

percent silt, and 7 to 27 percent clay. Slightly cohesive,

they form ribbons less than 2.5 cm long,

and feel moderately smooth when wet.

Sandy Clay Loam

Containing 45 to 80 percent sand, 0 to 28 percent

silt, and 20 to 35 percent clay, these loams are

moderately cohesive, forming ribbons between

2.5 and 5.0 cm in length. When wet, they have a

gritty feel.

Silty Clay Loam

This group contains 0 to 20 percent sand, 60 to

73 percent silt, and 27 to 40 percent clay. Ribbons

2.5 to 5.0 cm long can be formed. When

wet, the soil has a moderately gritty feel.

Clay Loam

Clay loams contain 20 to 45 percent sand, 15 to

53 percent silt, and 27 to 40 percent clay. These

soils are sticky and plastic when wet and hard

when dry. They form ribbons 2.5 to 5.0 cm in

length and are moderately gritty when wet.

Soil structure

Soil structure refers to the way primary soil particles aggregate into secondary shapes or forms known as peds. Various forces, such as shrinkage and swelling, freeze-thaw cycles, and other environmental factors, can bring soil particles close together, allowing them to bond. Initially, organic matter acts as a weak binding agent, which may later be replaced by stronger bonds formed by humus. Additionally, silica, metal oxides, and carbonates contribute to the cementation of soil peds.

Soil structure is characterised by grade, class, and type. The grade indicates the stability or distinctiveness of the soil peds. Since soil moisture affects structure, grade is typically assessed when the soil is slightly moist. The structural grades are categorised as follows:

Weak

Pods can be seen in place with careful observation,

however, they cannot be removed intact.

Moderate

Pods can be readily seen in place and, once removed,

will remain intact with gentle handling.

Strong

Pods are distinctive in place and will withstand

considerable handling.

Class refers to the size of the pod. Since some structural types are inherently larger than others, a size range for each structural type has been determined. The class designations are: very fine or very thin, fine or thin, medium and coarse. Type refers to the shape of an individual pod. Structural types are classified as follows:

Single Grain

Individual soil particles do not form aggregates;

soil tends to have a sandy texture very low in organic

matter.

Granular

These spheroids or polyhedrons are of roughly

equal size in all dimensions and have plane or

curved surfaces with slight or no accommodation

to the faces of surrounding pods. Nonporous

pods are generally found in sandy, low-organic matter

soils.

Crumb

These soil particles are similar to the granular

class, however, the pods are porous.

Platy

These particles are much longer and wider than

tall. The flat pods are arranged around a horizontal

plane.

Angular block pods are of roughly equal size in all dimensions; blocks or polyhedrons have plane or curved surfaces that are casts of the moulds formed by the faces of the surrounding pods. Faces are flattened, and most vertices are sharply angular. These particles tend to occur in B horizons or where moderate amounts of clay are present.

Sub-angular Blocky

Basically the same as the angular blocky particles,

the sub-angular blocky faces are mixed, rounded,

and flattened with many rounded vertices.

Prismatic

These particles, with two horizontal dimensions,

are smaller than the vertical and taller than long

or wide. They are arranged around a vertical line

with vertical faces well defined and angular vertices

without rounded caps. They are generally

found in arid regions below the surface in horizons

with moderate to high clay content.

Columnar

Columnar particles are like the prismatic particles

but with rounded caps.

Massive or Structure-less

The shape of these particles cannot be determined;

they cling together in huge masses with no definite

arrangement along lines of weakness. They

are normally very hard.

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