Soil as a system
Soil is a complex plant-supporting system consisting of disintegrated rock, organic matter,
water, gases, nutrients, and microorganisms.
o Fundamental to the support of life on this planet and the provision of food fort he
growing human population
The composition of a region’s soil can have as much influence on the regions ecosystems as do
the climate, latitude and elevation.
Soil is composed of living and non living components
Soil formation is slow and complex
Parent material is the base of geological material in a particular location.
o Can include lava or volcanic ash; rock or sediment deposited by glaciers, sediment
doposited by rivers, or bedrock, the continuous mass of solid rock that makes up Earth’s
Weathering, erosion, and the deposition of organic matter were responsible for soil formation.
Weathering describes the physical, chemical, and biological processes that breaks down rocks
and minerals, turning large particles into smaller particles.
Physical Weathering or mechanical weathering breaks down rock without triggering a chemical
change in the parent material
o Wind and rain are two main forces.
o Areas with extreme temperature fluctuations experience rapid rates of physical
o Water freezing and expanding in cracks in rock also causes physical weathering
Chemical weathering results when water or other substances chemically interact with parent
o Eg. Groundwater being unusually acidic.
Biological weathering occurs when living things break down parent material by physical or
o Tree accelerates weathering by physical action of its root growing and rubbing against
Erosion is the movement of soil from one area to another
o This is physical weathering
o A destructive process that reduces the amount of life that a given area of land can
Partial decomposition of organic matter creates humus a dark, spongy, crumbly mass of
material made up of complex organic compounds.
o Soils with high humus content hold moisture well and are productive for plant life.
Soil that are dominated by partially decayed, compressed organic material are called peat.
o Peat is characteristic of northern climates because cool temperatures slow the decay
process, allowing great thicknesses of organic material to accumulate.
5 primary factors that influence the formation of soil (table 7.1 page 192)
Soil forms faster in warm wet climates. Heat speeds chemical reactions and
accelerates weathering, decomposition, and biological growth.
o Organisms Earthworms and other animals mix and aerate soil, add organic matter. Plants
add organic matter
o Topograhpic relief
Steeper slopes result in more runoff as an example
o Parent Material
Chemical and physical attributes of the parent material influence properties of
the resulting soil
Soil formation takes decades, centuries, or mellenia
Multiple factors (including the 4 above)
A soil profile consists of layers known as horizons
Each layer of soil is known as a horizon
the cross section as a whole, from surface to bedrock is known as a soil profile
Five major horizons, O, A, B, C & R
Few soils contain all the horizons
O (organic) horizon is an uppermost layer consisting mostly of organic matter, such as
decomposing branches, leaves and animal waste.
A horizon is under the organic horizon and consists of inorganic mineral components, with
organic matter and humus from above mixed in.
o Refered to as topsoil
o Most nutritive for plans and most vital for ecosystems and agriculture.
o Takes its loose texture and dark colour from its humus contents.
The O and A horizons are home to most of the countless organisms that give life to soil.
Degree of weathering and concentration of organic matter decrease as you go down the soil
profile from the surface
Minerals are carried downward as a result of leaching (the process whereby solid particles
suspended or dissolved in liquid are transported to another location)
o E.g. coffee grounds in a drip fiter.
o Minerals that leach rapidly from soils can be carried into groundwater and can pose
human health threats when the water is extracted
o below A horizon
o Located below B
o Consists of parent material unaltered or only slightly altered by the process of soil
R (rock) horizon lays at the bottom.
Certain soils are characterized by the presence of water (W horizon)
o Some arctic soils for example contain a segregated layer of perennially frozen ice or
Soil can be characterized by colour, texture, structure and pH
10 major groups of different soils.
Soil Colour indicates fertility and composition
o Black or dark brown soils are usually rich in organic matter o Pale grey to white colour indicates leaching or low organic content
Soil Texture is determined by the size of particles and is the basis on which soils are assigned to
one of three categories
o Clay consists of particles < 0.002mm in diameter
Adhere to one another to give it a sticky feeling when moist
o Silt is 0.002-.05
o Sand is 0.05-2
Large enough to see individual and do not adhere to one another
o Soil with a even mixture of the three particles sizes is known as loam
o Important for farmer to know soils workability (its ease or difficulty of cultivation)
Permeability is a measure of the interconnectedness of the spaces and the ease
with which fluids can move around in a material
Porosity is a measure of the relative volume of spaces within the material
o The finer the particles in a sediment or soil, the smaller the spaces between them.
o The smaller the spaces, the harder it is for water and air to travel through and infiltrate.
o materials can have a high porosity but low permeability, common with clay.
o Silty soils with medium sized pores or loamy soils with mixtures of pore sizes are
generally best for plant growth and crop agriculture.
Soil Structure is a measure of the organization or ‘clumpiness’ of soil.
Soil PH is the degree of acidity or alkalinity influences a soil’s ability to support plant growth
o Plants die in soils that are too acidic (low pH) or alkaline (high Ph)
Cation exchange is vital for plant growth
Plants gain many nutrients through a process called cation exchange
o In cation exchange, plant roots donate hydrogen ions in exchange for nutrient ions
Cation exchange capacity expresses a soil’s ability to hold cations (preventing them from
leaching and thus making them available to plants
Soils with fine texture (e.g. clay) and soils rich in organic matter have the greatest cation
As soil pH becomes lower (more acidic), cation exchange capacity diminishes, nutrients leach
away, and soil instead may supply plants with harmful aluminum ions.
Soil: The Foundation for Feeding a Growing Population
Productive soil is renewable, if it is degraded or washed away at a rate faster than it can be
renewed, it will deplete
Agriculture is the practice of raising crops and livestock for human use and consumption.
We obtain most of our food and fibre from cropland, land used to raise plants for human use,
and rangeland, land used for grazing livestock.
As population and consumption increase, soils are being degraded
We can’t simply keep expanding agriculture into new areas, the extensification / spreading of
resource extraction because land suitable and available for farming is running out.
o We must find ways to improve the efficiency of food production in areas that are
already in agricultural use.
Mismanaged agriculture has turned grasslands into deserts
o Has diminished biodiversity o Polluted soil, air, water with toxic chemicals.
Soil Degradation , damage to or loss of soil, around the globe has resulted from roughly equal
parts of forest removal, cropland agriculture, and overgrazing of livestock, with a much smaller
contribution from industrial contamination.
Agriculture began to appear around 10 000 years ago
Began as hunter-gatherers brought back to their encampments wild fruits, grains, and nuts.
Agriculture is thus a form of intensification – a way to increase the productivity of a given unit
o Intensification can increase the carrying capacity of a land area (up to a point)
A hunter-gatherer lifestyle requires a very large land area to support a given
For thousands of years, the work of cultivating, harvesting, distributing was perfoemd by human
and animal muscle power.
o This biologically powered agriculture is known as traditional agriculture
Subsistence agriculture is when farming famalies produce only enough food for themselves and
do not make use of large scale irrigation, fertilizer or teams of labouring animals.
Industrialized agriculture is newer STILL
Industrialized agriculture demands that vast fields be planted with single types of crops.
o Distinct from polyculture approach of much traditional agriculture.
o Spread from developed nations to developing nations with the advent of the green
Green revolution was in 1950, it introduced new technology, crop varieties and
farming practices to the developed world.
Despite its success, it is pricy
Soil Degradation: Problems and Solutions
Most desirable soil for agriculture is a loamy mixture with a pH close to neutral that is workable
and capable of holding nutrients.
Human impact has degraded many once-excellent soils.
Regional Differences in Soil Traits can affect agriculture
Characteristic of soil and soil profiles can vary from place to place.
Although rainforest ecosystem have high primary productivity, most of their nutrients are tied
up in plant tissues and not in soil.
o A lot of rain means the rain leaches minerals and nutrients out of the topsoil.
The traditional form of agriculture in tropical forested areas is swidded agriculture, in which the
farmer cultivates a plot for one to a few years and then moves on to another plot, leaving the
first to grow back to forest.
When there isn’t much rain, leaching is reduced and nutrients remain high in the soil profile.
Plants take up nutrients and then return them to the topsoil when they die, maintaining the
soils fertility. Erosion can degrade ecosystems and agriculture
Deposition is the arrival of eroded material at its new location
Erosion and deposition are natural processes that in the long run can help create soil
Erosion becomes a problem because it almost always takes place more quickly than soil is
o Erosion removes topsoil
Topsoil is the most valuable soil layer for living things.
People have increased the vulnerability of fertile lands to erosion through
o 1. Overcultivating fields through poor planning or excessive ploughing, disking, or
o 2. Overgrazing rangelands with more livestock than the land can support
o 3. Clearing forested areas on steep slopes or with large clear-cuts
Erosion can be gradual and hard to detect as it removes a penny’s thickness of soil per year.
Soil erodes by several mechanisms
Wind erosion can occur and four principal kinds of water erosion
Rill erosion has greatest potential to move topsoil, followed by sheet erosion and splash erosion.
Pictures on pg 201 with images of different erosion