Soil Science

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.

Soil structure and methods of analysis

A typical soil contains approximately 52% soil matter, (50% mineral matter and 2% living organisms) and 48% fluids. The soil components determine the texture, structure, pH and soil fertility. The composition of soil is very important to its productivity (plant growth) and therefore needs to be understood by the grower.

The Earth’s crust is made up of minerals (rock) which have formed over millions of years. The shape of the landscape is the result of kinetic energy through the tectonic plates, erosion and deposition which have caused for example the land to rise up to form mountains and valleys. Soil forms when rocks are broken into small fragments. How this happens and type of soil which forms as a result, depends on:

  • The type of rock being weathered also known as the parent rock.

  • The breakdown of the parent rock by physical or chemical means known as the weathering process.

When mineral matter has been broken down enough to support plant life it becomes soil. Rock matter and climates interact to form a variety of soils in different parts of the globe. As I have shown about 50% of soil comes from parent rocks. There are three main types igneous, sedimentary and metamorphic. Igneous rocks forms when lava cools down and solidified, it takes a long time to weather, usually the largest types, usually the most fertile and examples includes granite and basalt. Sedimentary rocks are fro older rock which were moved and deposited in layers by air, water and ice; these types of rock are soft and quick to weather. Examples include limestone and clay. Finally metamorphic are similar to sedimentary rock which have been change by great physical or chemical process. These rocks are generally harder than original sedimentary rocks and examples include marble, slate and quartzite.

There are three main types of weathering:

  • Mechanical of physical agents – split the rock into pieces by physical forces such as heating and cooling, freezing and thawing, wetting and drying ect.

  • Chemical agents – which cause permanent chemical change in the original rock mineral for example acidic rain.

  • Biological agents – such as plants, animals and humans.

The rock weathering produces the basic soil forming ingredients of:

  • Rock fragments

  • Iron oxides

  • Soluble materials

  • Newly formed secondary minerals such as clay

The mechanical process occurs when rocks or soil are broken down but the mineral characteristics do not change, examples include in upland areas where there is little soil and the minerals are exposed to water and ice. The chemical process forms old rock decomposes to form new minerals. The most important processes are cause by the effect of water through rainfall. The biological process involves both chemical and mechanical processes. Plants such as lichen establish themselves on exposed rocks. Plant roots get into the cracks and mechanically weather the rock. Soil is a dynamic medium changing over time from raw soils containing little more than mineral particles, to highly fertile rich growing media we find in well worked garden situations. Soils maybe formed in situe from the underlying rocks and known as a residual soil or the constituents maybe transported by: wind, water or ice. Other examples are as follows:

  • Residual – Formed in situe, no transport involved.

  • Aeolian – Transported by wind.

  • Alluvial – Transported by water.

  • Glacial – Transported by ice.

Factors affecting soil development included:

  • Parent material

  • Topography

  • Climate/atmosphere

  • Overlying vegetation

  • Animals/humans

  • Time scale

Five most common types of soil found in Britian:

  • Brown earth

  • Podsols

  • Gluey

  • Rendzina

  • Peat

Brown earth is the main type found under deciduous woodland formed in cool, temperate climates on freely drained areas. The texture tends to be a loam, often with a well developed crumb structure. Brown earth provides an ideal soil, therefore large areas of woodland have been cleared in the past for agriculture and finally the pH tends to be weakly to moderate acid. The dark brown colour is cause by the hummus distributed by animals in particle earthworms. To contrasts podsols tends to be formed under coniferous forests. They are poor soils for agriculture therefore are still frequently under forest, (natural or plant). The leaf litter (from pines) is very acidic, too acidic for earthworms therefore not well incorporated. Some is washed down and accumulates in the lower humus. Organic acids from the humus mobilize iron compounds allowing them to be removed from the upper profile leaving a horizon formed mainly from quartzes.

Decomposers in soil

Primary decomposers (Detriovers) – animals e.g. Beetles, Woodlice, Earthworms etc. break up the plant material and corpses into small pieces by chewing and burrowing partially digest some of the “detritus” excreting the more resistant parts. Secondary decomposers (Saprophytes) – fungi and bacteria are able to break down and use tough organic compounds e.g. Wood that cannot be digested by Detriovers. Breakdown complex organic substance to produce simple mineral salts which can be absorbed by plant or “mineralization”.

Primary minerals have not been altered chemically since the time of their crystallization from molten lava and their subsequent deposition. The lower the minerals fall on the chart, the more they resist weathering.

Secondary minerals form from the decomposition of primary minerals and a subsequent reprecipitation into a new, chemically distinct mineral. Layer aluminosilicates are the dominant minerals formed in most temperate region soils. These layer silicates are composed of various arrangements of silicon/oxygen sheets in tetrahedral coordination and aluminium/oxygen sheets in octahedral coordination. Kaolinite is composed of one silicon/oxygen tetrahedral sheet and one aluminium/oxygen octahedral sheet and therefore is called a 1:1 mineral. Kaolinite

forms in warm to hot, sub-humid to humid climates. This mineral crystallizes in acid soil where basic cations (positive ions) and some silicon have been leached. Vermiculite is a 2:1 mineral with two silicon tetrahedral sheets surrounding one aluminium octahedral sheet. It forms in sub-humid to humid soils high in mica. Hydrous mica (illite) forms in sub-humid cool areas as mica dissolves and re-crystallizes.

Soil texture is the relative percentages of sand-, silt-, and clay-sized particles in a soil. It is a soil single most influential physical property. Texture influences soil

permeability, water infiltration rate, porosity, and fertility. Soil particles are classified into one of three groups based on size (diameter): clay (<0.002 mm); (0.002 to 0.05 mm); and sand (>0.05 mm). In addition, larger objects may be described as pebbles (2 to 75 mm); cobbles (75 to 250 mm); stones (250 to 600 mm); and boulders (>600 mm). These soil particle size boundaries are not totally arbitrary, as they roughly match changes in properties associated with the differing size fractions. Chemically, sand- and silt-sized particles are relatively inert. They differ in that sand is large enough to resist erosion by wind. Sand-sized particles are predominantly quartz (SiO2) with small amounts of silicate-based primary minerals. Feldspars, hornblende, and micas may total up to 20 percent of the sand fraction in soil. Sand tends to have angular rough surfaces, whereas silt is spherical and more polished. Silt also is predominantly quartz with slightly larger amounts of primary minerals and iron and aluminium oxides. Wind easily erodes the smaller silt grains. Clay particles are chemically active and stick together in aggregates that resist wind erosion and increase soil porosity. The clay fraction in most temperate region soils is dominated by layer aluminosilicates minerals. In the humid tropics, where weathering is more intense, iron and aluminium oxides and hydrous oxides are the dominant minerals present. The textural class can be determined with any two particle size groupings. For example, using the triangle illustrated, the classification of a soil with 30 percent clay and 10 percent silt would be determined in the following way:

1. Find the mark labelled 30 on the left side of the triangle, which indicates the percent of clay.

2. Find the mark labelled 10 on the right side of the triangle, which indicates the percent of silt.

3. Trace a line from the left mark (clay) horizontally and from the right mark (silt) diagonally downward until the two lines intersect. The point of intersection indicates that the soil classification is: Sandy Clay Loam.. (Please refer to the Soil Texture Analysis Triangle below)


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


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.


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


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.


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.


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.

Structure of the soil

Soil structure is the aggregation of primary particles into secondary shapes or forms called pods. Shrink/ swell, freeze/thaw, and other forces in soil bring particles into close proximity, where they can be cemented together. Organic matter forms a weak agent that may eventually give way to stronger bonding by humus. Silica, metal oxides, and carbonates also cement pods. Structure is described by grade, class, and type. Grade represents the stability or distinctiveness of the pod. Because it is moisture dependent, the grade is normally described when the soil is slightly moist. Structural grades are classified as follows:


Pods can be seen in place with careful observation,

however, they cannot be removed intact.


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

will remain intact with gentle handling.


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



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



These soil particles are similar to the granular

class, however, the pods are porous.


These particles are much longer and wider than

tall. The flat pods are arranged around a horizontal


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.


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 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.

Banner of soil science pH chart

Soil modification for growers

How to acidify soil

This is more difficult than liming to raise the pH. The reaction is much slower and more complex making it more difficult to quantify.

    • Addition of sphagnum peat. This used to be common practice though it is no longer to be recommended because of environment concerns over the usage of peat in horticulture. Other forms of organic material may acidify the soil.

    • Addition of sulphur – a cheap method but slow.

    • Addition of aluminium sulphate – very expensive.

    • Addition of iron sulphate – this also helps in correcting iron deficiencies.

    • Addition nitrogenous fertilizers for example ammonium sulphate.

    • Examples of pH for selected trees and shrubs

    • 4.5-5.5 – Rhododendron, Calluna, Erica, Picea and Sciadophytis.

    • 5.5-6.5- Acer, Quercus and Betula.

    • 6.5-7.0- Syringe vulgaris, Sorbus and Salix.

Forms of lime

  1. Calcium Carbonate (CaCo3) – Sometimes referred to as carbonate of lime and is obtainable in three forms. Ground Limestone – the product of several processes involving crushing, drying and grinding. It should pass through a one hundred mesh sieve. Suitable of all soils, particularly those rich in organic matter since it causes less rapid decomposition than other forms. Limestone Dust – An impure form of calcium carbonate which consists partly of lumps up to 3mm in diameter and partly of fine powder which rapidly dissolves in the soil. The lumps break down over a long period proving a long availability time. Chalk – Contains much water when quarried and must be dried before grinding. More expensive than ground limestone but more soluble and therefore faster acting. Suitable for sandy soils and light loams. Often included in seed and potting compost.

  2. Magnesium limestone (CaMg (Co3) 2) – Supplies magnesium as well as calcium. Widely used in nursery stock composts.

  3. Hydrated Lime (Ca (OH) 2) – Obtained from calcium oxide when it is slaked with water under controlled conditions. A finely divided powder which has a very high pH 12. More expensive than lime but more effective.

Sign of lime deficiency

The appearance of certain weeds e.g. Sheep’s Sorrel, Corn Spurry, Woodrushes and Corn Marigold. The appearance of club root in Brassica. The absence of earthworms and beneficial bacteria. Signs of calcium deficiency in crops.

Signs of over-liming

Development of chlorosis in certain plants e.g. Hydrangea. The high pH making iron and other nutrients unavailable. This is often called “lime induced chlorosis”.

Some diseases become more prevalent e.g. scab in Potatoes.

Cation exchange processes regulate soil pH

The best single index of potential soil fertility is its capacity to exchange cations. Exchangeable cations can be artificially changed to improve soil for crop production. Cation exchange capacity (CEC) is also an index of the soil’s ability to absorb biocides such as herbicides of the traizine class and radioisotopes from thermonuclear fallout. The unit of measurement is the milliequivalent, the amount of matter which will replace or combine with 1mg of hydrogen per 100g of dry soil. Virtually all exchangeable cations can be absorbed on the negatively charge surfaces of clay particles. Different clay minerals vary in their CEC’s, for example Kaolinite is low at 3-15 me/100g whereas montmorillonite has a high CEC in the order of 70-100me/g. Mineral particles larger than clay are usually negative contributors to the CEC of a soil though larger particles of minerals such as feldspars and micas will contribute. A significant part of the supportive capacity of many soils for cations and for proton-accepting molecules resides in the humus fraction of the soil. This increases with the degree of humification and very high values of CEC can be found at (150-400me/100g).

Soil pH is an important consideration for horticulturists and/or agriculture for several reasons:

  • Many plants and soil life forms have a preference for either alkaline or acidic conditions, affecting the choice of crop or plant that can be grown without intervention to adjust the pH

  • Diseases affecting plants also tend to thrive in soil with a particular pH range

  • The pH can affect the availability of nutrients in the soil.

A pH level of around 6.3-6.8 is also the optimum range preferred by most soil bacteria, although fungi, moulds, and anaerobic bacteria have a broader tolerance and tend to multiply at lower pH values. Therefore, more acidic soils tend to be susceptible to souring and putrefaction, rather than undergoing the sweet decay processes associated with the decay of organic matter, which immeasurably benefit the soil.

  • Acid soil of pH 4.5-5.0 Some plants will not tolerate higher pH such as Blueberry, Heather, Cranberry, Orchid, Azalea and for blue Hydrangea (less acidic for pink).

  • Acid soil of pH 5.0 - 5.5 Plants for acid soil in this range include Parsley, Potato, Conifers, Pine, Sweet Potato, Maize, Millet, Oars Radish, Ferns, Iris, Rhododendron and Camellia.

  • Acid soil of pH 5.5 - 6.0 Plants for a moderately acid soil include Bean, Brussels Sprouts, Carrot, Peanuts, Rhubarb, Soya bean, Crimson Clover, Aster, Begonia, Canna, Daffodil, Larkspur, Petunia, Primrose, Violet and most bulbs.

  • Acid soil of pH 6.0 - 6.5 Plants that prefer this soil include Broccoli, Cabbage, Cauliflower, Cucumber, , Pea, Sweet Corn, Pumpkin, Squash, Tomato, Turnip, Red Clover, Sweet Clover, White Clover, Candytuft, Pansy, Rose, Snapdragon, Viola, Wallflower, and Strawberry.

  • Acid to neutral soil of pH 6.5 - 7.0 Plants that favor very mildly acid soil are Asparagus, Beet, Celery, Lettuce, Melons, Onion, Parsnip, Spinach, Lucerne, Dianthus, Chrysanthemum, Dahlia, Stock, Sweet Pea and Tulip.

  • Neutral to Alkaline soil of pH 7.1 - 8.0 Clematises and Lilacs.

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