Introducing Soil

Notes from "The Nature and Properties of Soils", Nyle C. Brady Ray R. Weil, Fourteenth Edition



Functions of Soils in our Ecosystem 

(Six Ecological Functions)

    • Soils are crucial to life on earth. All of the world's ecosystems are impacted by soil processes.
    • Food, fibre, feedstock
    • Human population increasing, but soil resources are shrinking
    • Understanding and management of soil essential for survival of humans and other living things

  1. Medium for Plant Growth

    • Medium for plant roots and supply of nutrients
    • Soil properties determine vegetation and animals that can survive on it
    • Competition among roots, rather than shoots, plays a greater role in plant coexistence
    • Plant obtains
      • Physical support
      • Air - Respiration
        • Produces CO2 and uses O2
        • Ventilation through soil pores is important
      • Water
        • Soil pores also hold water
        • Plant leaves exposed to sun draw up water for cooling, transport of nutrients, turgor mainenance, and photosynthesis
        • Soil stores water for plant use even when there is little rain
      • Temperature moderation
        • Protects roots by insulating against extremes of hot and cold
      • Protection from phytotoxins
        • Can be produced by humans, plant roots, microorganisms or by natural chemical reactions
        • Soils protect against toxins by ventilating gases, decomposing or adsorving organic toxins, suppressing toxin producing organisms, or supporting microorganisms which stimulate plant growth and health.
      • Nutrient elements
        • Mainly dissolved inorganic ions, or mineral nutrients
        • Metallic elements like potassium, calcium, iron and copper
        • Non-metallic elements like nitrogen, sulfur, phosphorus and boron.
        • Of 92 naturally occurring chemical elements, 17 are essential elements: plants can not grow without them
          • Macronutrients - used by plants in relatively large amounts
          • Micronutrients - used by plants in relatively small amounts
          • Plants also use minute amounts of organic compounds from soils, but uptake is not necessary for plant growth. Organic metabolites, enzymes, and structural compounds making up plant dry matter consist mainly of carbon, hydrogen and oxygen obtained by photosynthesis from air and water, not from soil.

  1. Regulator of Water Supplies - Hydrologic System

    • Nearly every drop of water in our rivers, lakes, estuaries and aquifers has either traveled through soil or flowed over its surface. (Water falling directly into water bodies is minor.)
    • Soil purifies and cleanses soil, removing impurities and killing disease organisms.

  1. Recycler of Raw Materials

    • Waste products and dead plants and animals are broken down so their essential elements can be reused by future generations
    • Turns organic waste into beneficial humus and converts mineral nutrients into forms that can be used by plants and animals, returning carbon to the atmosphere as CO2, where it becomes a part of living things through photosynthesis
    • Some soil accumulates large amounts of carbon as soil organic matter, impacting the greenhouse effect.

  1. Modifier of the Atmosphere

    • Releases carbon dioxide, oxygen, methane and other gases and contributes dust and re-radiated heat to air.
    • Evaporation contributes water vapour to atmosphere, alters air tem, composition and weather patterns.
    • Soils breath – absorgin O2 and other gases such aas methane, and releaseing CO2  and nitrous oxide. Affects global warming.

  1. Habitat for Soil Organisms

    • Small mammals and reptiles, insects and micro-organisms
    • Billions of organisms, thousands of species in a handful
    • Predators, prey, producers, consumers and parasites – an ecosystem.
    • Niches and habitats – some pores filled with water support roundworms, diatoms and rotifers. Insects may live in other pores filled with air. Some areas enriched with organic material, some acidic or basic. Temperature variations also.

  1. Engineering Medium

    • Some soils not as stable as others – understanding of soil properties important.
    • Bearing strength, compressibility, shear strength and stability are variable.
    • Clays can swell.


Pedosphere as Environmental Interface

    • Lithosphere – Rock
    • Hydrosphere – Water
    • Biosphere – Living things
    • Atmosphere – Air

Environments where all of these interact are the most complex and productive. NOTE: THIS REMINDS ME OF PERMACULTURE DISCUSSIONS ABOUT THE IMPORTANCE OF MAXIMIZING EDGE.



·         An estuary where shallow water meets land and air – more complex than deep ocean trenches where hydrosphere is isolated, or upper atmosphere.

·         The soil environment where these interact is called the pedosphere


In soil, these interactions take place at all scales:

·         Kilometres – Channelling water from rain to rivers and transferring mineral elements from bed rocks to oceans, removing vast amounts of atmospheric gases

·         Metres – Transition zone between rock and air, holding water and oxygen for roots. Transfers mineral elements from Earth’s crust to vegetation. Processes dead plants and animals

·         Millimetres – Microhabitats for organisms, channelling water and nutrients to roots, biochemical reactions

·         Micrometres and smaller – Mineral and organic surfaces for chemical reactions and interactions with water and solutes. Microzones of electromagnetic charge – attract bacterial cell walls and proteins and water molecules.

Soil as a Natural Body

    • Many different types of soils, just as there are many different types of trees.
    • Regolith
      • Rock exposed at Earth’s surface, crumbled and decayesd into a layer of debris over the unweathered rock.
      • Varies in thickness, from non-existent in some places, to tens of metres in others
      • Can sometimes be transported many kms from initial site of formation
      • Where underlying rock is loose enough to be dug with a spade, the term saprolite is used.
      • The regolith is altered by living organisms such as bacteria, fungi and roots. It transforms inorganic rock and debris into living soil.
      • Soil is a product of destructive and creative (synthetic) processes.
      • The most striking result of synthetic processes is the creation of soil horizons in the upper regolith.
    • Pedology – study of soils as natural bodies, properties of horizons and relationships among soils in a landscape.
    • Edaphology – study of soil as a habitat for living organisms, especially plants.


The Soil Profile and its Layers (Horizons)

    • Soil profile – Vertical section exposing a set of horizons int the wall of a soil pit (usually several meters deep and about a meter wide)
    • Parent material – The original rock from which the regolith is composed.
    • O horizon – Layer of organic material at the surface: fallen leaves, plant and animal remails, undergoing physical and biochemical breakdown. Includes fresh debris and partially decomposed material.
    • A horizon – Soil organisms and water transport organic material downward, mix with regolith and decomposing roots, darkens upper mineral layers.
      • Can leach clay or other weathering products into horizons below
      • Dominated by mineral particles but darkened by organic matter
      • Often referred to as topsoil.
    • E horizon – In some soils, intensely weathered and leached horizons tha have not accumulated organic matter, jus below A horizon.
    • B horizon – Much less organic matter than surface horizons. Silicate clays, iron and aluminum oxides, gypsum, calcium carbonate may accumulate. May have washed down from above or formed in place through weathering.
      • Plant roots can extend below B horizon, esp in humid areas – can cause chemical changes in soil water, biochemical weathering of the regolith, and formation of C Horizons.
    • C horizon – Least weathered part of soil profile

Soil horizons can be very distinct in color with sharp boundaries, or changes can be very gradual. Delineations can also be determined by feel, smell and hearing (when soil rubbed together) as well as chemical tests.

Topsoil and Subsoil

    • Topsoil
      • Plowing and modifying topsoil (A horizon) homogenizes and modifies top 10 to 25 cm of the profile to form a plow layer.
      • In cultivated soils, the majority of plant roots are found in the topsoil. This is the area that can be enhanced with nutrients, air and water.
      • More conducive to plant growth than subsoil.
      • Productivity correlated with thickness of topsoil.
    • Subsoil
      •  Underlying the topsoil
      • Much of water needed by plants is stored here.
      • Can supply plant nutrients
      • Can impede plant growth if too wet, dense or acidic.

Soil: The Interface of Air, Minerals, Water and Life

    • Four components of soil: Air, water, mineral matter and organic matter.
    • Relative proportions greatly affect soil behaviour and productivity.
    • Approximate proportions by volume of components in loam surface soil in good condition for plant growth: (Only about ½ of soil volume is solid (mineral and organic). The rest consists of pore spaces filled with air and water.
      • 50% soil solids – If more than this, soil will be too compacted for good plant growth
        • Mineral – 45% of volume.
        • Organic – 5% of volume . Influence of organic component is far greater than its small proportion would suggest. Far less dense than mineral matter, so only accounts for about 2% of weight of soil.
      • 50% pore space
        • Water filled – 20% - 30% of soil volume, fluctuates as soil wetter or drier. If much more, soil will be waterlogged.
        • Air filled– 20% - 30% of soil volume, fluctuates as soil wetter or drier. If much more, plants will suffer from drought.
      • Subsoils contain less organic matter, less pore space and a larger proportion of small pores (micropores,) which tend to be filled with water rather than air.


Mineral (Inorganic) Constituents of Soils

Larger soil particles (stones, gravel and coars sands) are generally rock fragments made up of several minerals. Smaller particles tend to be made of a single mineral.

    • Sand – 2 mm to 0.05 mm – visible with naked eye, feels gritty
    • Silt – 0.05 – 0.002mm – visible with microscope, feels smooth, even when wet
    • Clay – 0.002mm and smaller – feels sticky when wet, clods when dry
      • 0.001 and smaller clay particles and similar sized organic particles have colloidal properties – can only be seen with electron microscope – have tremendous surface area per unit of mass
        • Surfaces of colloids exhibit electromagnetic charges that attract positive and negative ions as well as water. This fraction of soil is the seat of most of soil’s chemical and physical activity.

Proportionof particles in different size ranges describes soil texture ie. sandy loam, silty clay, clay loam.


Texture has a profound effect on many soil properties.


Soil Minerals

Primary minerals – have persisted with little change in copmposition since they were extruded in monten lave (e.g. micas and feldspars)

Secondary minerals – ie. silicate clays and iron oxides, formed by breakdown and weathering of less resistant minerals – dominate in clay and in some cases, silt fractions.


Soil structure

The way sand silt and clay are put together. Commonly they are associated together in aggregates of different size particles, ie. granules, blocks, plates. Fundamentally influences many processes, just like texture.


Soil Organic Matter


Consists of a range of organic (carbonaceous) substances, including living organisms (biomass), carbonaceous remains of organisms that once occupied the soil and organic compounds produced by soil metabolism.


Microbial respiration results in loss of organic matter as CO2. When conditions favour plant growth over microbial decay, atmospheric CO2  used by plants in photosynthesis are sequestered in plant tissue that becomes part of soil organic matter. The balance between accumulation of soil organic matter and its loss through microbial respiration affects global warming. More carbon is stored in soil than in plant biomass and atmosphere combined.


“Organic matter binds mineral particles into a granular soil structure that is largely responsible for the loose, easily managed condition of productive soils. Part of the soil organic matter that is especially effective in stabilizing these granules consists of certain gluelike substances produced by various soil organisms, including plant roots.”


Organic matter

·         Increases amount of water soil can hold.

·         Major source of nutrients phosphorus and sulphur and is the primary source of nitrogen.

·         Is the main food of carbon and energy for soil organisms.



·         Complex organic compounds that accumulate because they are resistant to decay.

·         Is the colloidal fraction of soil organic matter.

·         Charged surfaces – like clay – act as contact bridges between larger soil particles. Both play an important role in soil structure. Attract nutrient ions and water, though humus capacity to hold nutrients and water is far greater than clay. Hums can have a hormone like stimulatory effect on plants. Small amounts of humus can greatly increase soil capacity for plant growth.



Soil Water: A Dynamic Solution

·         Held within soil pores - attracted to soil surfaces – restricts flow – soil in smaller pores is strongly attracted to particles – not all water is available to plants. “Depending on the soil, one-sixth to one-half of water may remain in the soil after plants have wilted or died for lack of moisture.”

·         Contains hundreds of dissolved substances – called soil solution

o   Buffering capacity – Soil solution tends to resist change to its composition even when compounds are added or removed from soil. Dependednt on many chemicaland biological reactions, including attrattion and release by colloidal particles.

o   Ph

§  Soil acidity or alkalinity

§  Relative levels of hydrogen ions (H+) and hydroxyl ions (OH-)

§  Of great significance to nearly all aspects of soil science


Soil Air: A Changing Mixture of Gases


·         ½ of soil volume consists of pores filled with air or water

·         Therefore air content is inversely related to water content


Different from atmospheric air in these repects:

·         Composition of soil air varies greatly from place to place due to variety of reactions of roots and microbes, approaching 100% humidity unless very dry

·         Soil air has higher moisture content

·         CO2 content much higher, and oxygen lower


Interaction of Four Components to Supply Plant Nutrients

·         Inter-related – minerals, air, water and organic matter interact with each other in all processes

Essential Element Availability

o   The most important interactive process involving the four soil components

o   Nutrients and water provided by soil solution, but it can only provide for plant needs for a few hours or days. Nutrients therefore must be repleniwhed from inorganic or organic parts of the soil or from fertilizers or manures.

o   These nutrients can be released from soil solids (organic and inorganic) through chemical and biochemical processes. Ie. Colloidal particles – clay and humus – exhibit negative and positive charges which tend to attract or adsorb oppositely charged iions from soil solution and hold them as exchangeable ions. “Through ion exchange, elements such as calcium and potassium are released from this state of electrostatic adsorption on colloidal surfaces and escape into the soil solution.”

o   Some scientists consider this ion exchange process as among the monst important of chemical reactions in nature.


Adsorption – Attraction of ions to the surface of particles. Adsorbed ions are exchangeable with ions in the soil solution.

Absorption – Process by which ions are taken into plant roots.


Nutrients also released to soil soilution from decomposition of organic tissues by microorganisms.

Most nutrient elements in soil are held in structural framework of minerals and organic matter. Only small amounts are present in forms that are readily available to plants.


Plant roots do not ingest soil particles – only nutrients



Nutrient Uptake by Plant Roots

To be taken up by a plant, a nutrient element bus be in a soluble form and must be located at the root surface.


Direct exchange can take place between nutrient ions on surface of soil colloids and H+ ions from the surface of root cell walls.


Three other mechanisms by which concentration of nutrient ions at root surface is maintained.

1.      Root interception – Roots grow into undepleted soil

2.      Mass flow – Movment of nutrient ions in soil solution dissolved with flowing soil water towards root.

3.      Diffusion – From greater areas of concentration towards depleted areas of lower concentration around root surface.


Soil compaction, low temperature and low moisture can result in poor nutrient uptake even when adequate soluble nutrients are available.


Nutrients do not enter roots through passive diffusion. They react with specific chemical binding sites on larege protein carrier molecules. Different nutrients are taken up by different types of carrier molecules.


Conditions that inhibit root metabolism may also inhibit nutrient uptake.


Soil Quality, Degradation and Resilience

            Most soil profiles are thousands of years in the making.

Soil quality

·         Measure of the ability of a soil to carry out particular ecological functions

·         Combination of chemical physical and biological properties

·         Some are inherent and unchangeable. Some can be changed by soil management.


Some soils have sufficient resilience ro recover from minor degradation. Some require effort, with vegetation, amendments, physical alterations (tillage or grading) or removal of contaminants.


The science of restoration ecology and the job of soil restoration require in-depth knowledge of all aspects of the soil system.

Soil Carbon - putting it back where it belongs

... but here Tony Lovell explains this in some detail, starting with how & why we generally find this quite difficult to understand, for lots of systemic reasons.  One of the truly great TED talks ... long but very important, explains why grazing animals could be so vitally important for our future (vegetarians might not like this...)