from "The Nature and Properties of Soils", Nyle C. Brady Ray
R. Weil, Fourteenth Edition
(Six Ecological Functions)
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 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.
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.
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.
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.
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.”
· 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.
· 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.
§ 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.
· 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...)