Notes from "The Nature and Properties of Soils", Nyle C. Brady Ray R. Weil, Fourteenth Edition
CHAPTER 1 – THE SOILS AROUND US
(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
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.
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.
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.
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.
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.
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.
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.
Examples:
· 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.
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.
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
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.
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.
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.
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.
Humus
· 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.
· 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
· 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
· 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
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.
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.
... 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...)