Download Soils 2008

Soils 2009
What is Soil?
 naturally occurring loose material at the surface of the earth that is capable of supporting
plant (growth) and animal life
 human race depends on the top 18cm of 7% of the earth’s surface for its survival. This is
the land on our earth that is fertile enough for agriculture. To demonstrate…
Earth as an Apple
1. Slice an apple into quarters. Set aside three of the quarters. These represent the oceans of
the world. The fourth quarter roughly represents the total land remaining.
2. Slice this land quarter in half, giving you two 1/8 world pieces. Set aside one of the pieces.
This land is inhospitable to people (polar areas, deserts, swamps, very high rocky,
mountainous areas). The other pieces of land are where people live, but not necessarily
grow the foods they need for life.
3. Slice the 1/8 piece into four sections, giving you 1/32nd pieces. Set aside three of these
pieces. These are areas too rocky, too wet, too cold, too steep or with too poor soil to
actually produce food. They also include areas of land that could produce food but are
buried under cities, highways, suburban development, shopping centres and other structures
that people have built.
4. You are left with 1/32nd of the earth. Carefully peel this slice. This tiny bit of peeling
represents the surface, the very thin skin of the earth’s crust upon which humankind
depends. Less than 2 metres (5 feet) deep, it is quite a fixed amount of food producing land.
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Comprised of 4 components in 3 phases
Solid – 45% mineral matter, 5% OM; water (25%); air (25%)
Combination of components determines how suitable a soil is for plant growth and soil
organism activity
Solid component gives soil its’ framework/ structure.
o Mineral matter - Derived from weathering of rock at earth’s surface
 Soil properties (ie. pH and texture) = PM; size of mineral matter - stone, gravel, sand, silt,
clay; abundance of mineral matter; arrangement of mineral matter
o Organic matter - 1-5% of volume in agr. Soil
 dead plant/ animal material broken down by living organisms
 crucial to maintenance of soil productivity
 major source of nutrients for plants/ organisms; role in natural fertility;
 keeps soil loose; holds water; holds particles together / improves structure
o Soil Water - principal medium that essential nutrients are made available to plant roots
 occupies ½ of the soil pores volume
 Size distribution of soil pores determines the ease with which plants may take up
water molecules. Water in the largest pores is taken up by plants with minimal effort
(available water). Water in tiny pores is held tightly to the soil particle and is not
easily released for plant uptake – when soil water can no longer be extracted for
plant use = wilting point. Happens sooner in clay than in sand.
 Nutrients taken up by 3 diff. mechanisms – root interception (directly from soil
solids), mass flow (absorbed along with water), and diffusion.
o Soil air = reservoir of O2 = living organisms need to breathe;
 CO2 = byproduct of respiration must be allowed to escape
 air flow greatly restricted by soil solids and liquids
 Soil air not as uniform as atmospheric air. Higher relative humidity, higher CO2 levels
due to slower air exchange with atmosphere.
Soil Formation
WEATHERING - breaks rocks into fragments of rel. small size - first step in soil formation. Two
types – physical and chemical. PM – regolith - soil
 Physical - breakdown of solid rock into smaller pieces as a result of interaction with air,
water and living organisms while maintaining the same chemistry. Ex. freeze/ thaw of water in
cracks, plant root penetration, transportation by wind, water, ice resulting in cracking, crushing and abrading
forces. Increases exposed surface area helps accelerate weathering.
o Types of Physical Weathering – frost shattering (freeze/thaw), hydraulic action (rocks
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shrink and swell), thermal expansion (expand during day, rapid cool at night – top layer
affected more so peels like a thin sheet), pressure release (pressure on top of a rock
removed and rock fractures), abrasion (aquatic/ rocks tumbling), biotic weathering
(living org. – mosses, lichens, tree roots)
Chemical - leads to change in chemical composition of the original rock. Occurs at surface of
mineral particles therefore, more effective where previous physical weathering has reduced
the parent rock to small fragments.
o Types of Chemical Weathering
 Dissolution – soluble components of rock dissolve in water and wash away. Occurs
when rain water comes into contact with exposed rock (pH 5.6)
 Carbonation – most common type of dissolution – when atmospheric carbon dioxide
reacts with rainwater, it forms a weak carbonic acid which is able to dissolve some
types of rock. Most on calcium carbonate rocks like limestone.
 Hydration – water becomes integrated in the chemical structure of solid rock and
alters the original composition – causes expansion in volume and places stress
within the rock.
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Hydrolysis – water reacts with the rock to create a completely new compound.
Oxidation-Reduction – reactions that occur in presence of oxygen = oxidation;
reactions that occur in the lack of oxygen = reduction. Oxidation-reduction or redox
reactions involve the exchange of electrons between molecules.
Biological Weathering – living org. contribute to process of biochemical weathering
through the release of organic acids – can react with surrounding rock to alter
chemical composition.
 roots often release organic acids to breakdown mineral for nutrient purposes or
kill surrounding plants
5 soil forming factors - parent material, climate, living organisms, time & topography/relief - no
one factor works in isolation - a cyclical process that occurs over long periods of geological time.
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Parent Material - rocks from which soil originates
o Consists of rock fragments that have been weathered in place or material that has been
transported or deposited by glaciers, wind or water.
o Type of parent material influences rate of soil formation – some rock break down faster
than others.
o PM determines mineral content & physical /chemical properties of soil.
 Physical - texture, water-holding capacity, fertility, particle size
 Chemical – pH and nutrient availability
 Ex. Limestone vs. granite and plant growth. Limestone is rich in minerals for plant growth
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Climate - specific temp. & precip. regime, humidity, sunshine and wind velocity
o Provides energy that drives physical, chemical and biological reactions on PM.
o Precip. & temp. most important variables
 As temp. & precipitation increase so does rate of weathering, freq. and mag. of soil
chemical reactions and rate of plant growth.
 Temp increases biological activity of soil organisms
 Moisture affects soil pH, rate of decomposition, horizon development through
leaching and capillary action.
o comparison of desert (little soil formation) and temperate forest (well developed )
o Ex. The higher the temp, the faster the decay of organic matter by soil organisms, the faster the rate of
chemical weathering of mineral fragments = faster rate of soil formation.
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Topography - shape of the landscape wrt altitude and slope degree
o modifies other soil factors like climate and vegetation due to creation of microclimates
o Slope steepness - Degree of slope affects rate of erosion and moisture accumulation
 Steep slopes more prone to erosion due to gravity and precipitation washing away
topsoil. Steeper topography inhibits accumulation and development of soil due to
constant removal of surface sediments.
 Valleys – soils differ - thicker and wetter due to deposition of eroded sediments and
drainage of water down into valley.
o Direction of a slope – amount of sun exposure –
 northern Canada - south-facing slope may support grasses, cooler north slope may
support a coniferous forest - production of 2 diff. soils
Shaded slope
vs.
Colder soils
Wetter soils
Restricted soil fauna
Surface accumulation of acid OM
Sunny slope
warmer soils
drier soils
varied soil fauna
OM incorporated
o Vegetation affected by moisture - south-facing slopes receive more sun t/f dry faster north-facing slopes more moisture = coniferous growth
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Living Organisms
o Vegetation
 provide a protective covering that buffers erosion
 reduce runoff & evaporation of soil water
 trap water at the surface and allow it to infiltrate
 push rocks apart with roots, release chemicals to aid in decomposition of rocks and
release of nutrients needed for growth.
 Replenish organic matter through dead leaves, etc.
o Soil animals
 Decomposers - turn OM into a form of nutrients that plants can use
 Fungi – increase process of decay
 Bacteria capture N from atmosphere
 Other animals mix / turn soil helping in profile differentiation
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Time -four factors work together to determine the kinds of changes that occur in parent
material and the rate at which these changes take place. Soil profile will reveal the
environmental conditions under which formation has taken place and the amount of time
the soil has been in formation.
Soil Structure
Over time soils begin to differentiate vertically and display distinct horizons.
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Horizons - layer of soil that run parallel to ground surface and exhibits diff. properties from
the horizons above and below – vary in colour, texture and structure.
o Evolve with the addition (OM, water, air, energy from sun); loss (evaporation,
leaching, erosion); translocation (movement of material up and down profile) and
transformation (formation of structural aggregates) of soil particles.
Soil Profile - two or more horizons could be distinguished from one another by differences
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in properties such as colour, texture and structure - well developed soil normally has 3-4
major horizons that can be identified.
Type and sequence of horizons help to classify them.
O horizon - dark organic, dead plant and animal material with humus underneath
o decomposition of organic matter by animals, fungi and microorganisms
o further distinguished by level of decomposition – i (slightly decomposed), e (moderate
decomposition), a (most decomposition where you cannot tell the original material)
o usually exists were there is permanent vegetation
A horizon - most productive, darker due to accumulation of organic matter.
o Nutrients and minerals such as clay are lost with downward movement of water aka
leaching (light grey or white with significant leaching) or eluviation t/f often referred to
as the eluvial zone in a profile.
o O & A referred to as topsoil layer - most important layer since growth medium for
plants. Topsoil not productive w/o subsoil.
E Horizon – zone of max. leaching or eluviation. Made up mostly of sand/ silt. Usually only
present in older soils.
B Horizon - deposition layer (clay), zone of accumulation or illuviation b/c receives leached
materials from above.
C Horizon - partially weathered parent material or regolith. Least affected by soil forming
processes. *** where little change has taken place in the parent material and there are no
defined horizons whole soil is referred to as C horizon.
R Horizon – hard bedrock.
**more mature the soil, the more differentiated the horizons are**
Soil Classification – Soil Taxonomy
 based on properties and arrangement of horizons which are products of the soil forming
environment
 Canada - soils divided into orders, great groups, subgroups, families and series
o 10 main orders: luvisols, podzols, brunisols, regosols, cryosols, chernozems, gleysols,
organic, vertisols and solonetzic (most common in ontario are in bold type)
Physical Nature of Soils
Texture - Determined by the relative proportion of each of the three particle sizes (sand, silt,
clay). Ex. clay soils = fine-textured soil. Has sig. influence on properties ie. drainage, waterholding capacity, ease of cultivation; Difficult to change texture
o Gravel, sand, clay, silt = terms used to classify mineral particles according to their size.
o Gravel, sand - gritty when rubbed b/w fingers; lg. size and uneven surface = lg. pore
spaces & little contact b/w particles; highly permeable; fall apart easily when wet.
o Silt - feels powdery, like flour; do not hold together well when wet; small size =
particularly susceptible to erosion by water and wind.
o Clay - flat, platelike shape, small size - expose rel. lg. surface area so when joined tog.
they create an abundance of pore spaces. Clay has electrically charged surfaces. Neg.
charge on clay particles attract the positively charged ends of water molecules and
nutrients such as calcium, magnesium and potassium. Results in high water holding
capacity; important storage site for moisture and plant nutrients. Feels smooth and very
sticky when wet.
o Loam - soils exhibiting properties of sand, silt and clay; “ideal soil”; contains more silt or
sand than clay.
Structure - way in which soil particles are bound together into larger pieces of soil known as
“peds” or “aggregates”.
o Structure affects stability of soil, movement of water/ air, ease of cultivation, root
growth, susceptibility to degradation (erosion, compaction). OM can increase stability
and create good soil structure.
o Good structure = large pore spaces allowing room for soil organisms and root, air and
water penetration. Increases rate of water infiltration/ retention, aeration and plant
growth. Less prone to erosion b/c aggregates are held together.
o Poor structure = weak, compact soil that is prone to collapse and erosion due to
unstable soil aggregates. Reduces the movement of air, water, nutrients and soil biota.
o Formation of aggregates influenced by texture, composition and environment.
o Aggregates occur when charges soil particles remain close together due to interactive
forces that can be enhanced due to abiotic/ biotic that increase cementation ie. OM.
o Characterized by size, shape and strength into 6 major classes: platy, granular,
columnar, prismatic, blocky and structureless.
o Structureless - Some soils do not stick together to form aggregates. Sand remain as
single grains without structure. Some fine-textured soils form massive chunks without
structure that are hard to break apart.
o can be altered by use / management – ie. activities that affect level of organic matter
Bulk Density – indirect measure of pore space.
o Used to determine if soil is able to support a building foundation, plant growth and root
penetration and water infiltration. Lower bulk density is better than high.
Colour – can reveal important details about the properties of the soil and the processes
operating in the soil profile. Colour can is indicative of 3 important facts about the soil: state of
aeration and drainage, the organic matter content and the state of iron oxides. Ex. red/ brown
soil vs. blue-grey soil. Use Munsell Colour Chart to determine standard colour.
“Generally, moist soils and those with high organic matter appear darker in colour.
This is why the rich organic topsoil appears darker than subsoil. Red and brown soils are
usually well drained and aerated, allowing aerobic organisms in the soil to remain active.
Also, in well aerated soils, iron is oxidized more readily and develops a ‘rusty’ colour.
Gray and blue soils indicate an area of poor aeration due to poor drainage, prolonged
saturation or waterlogging. The lack of oxygen means that the iron is in the reduced
form, which gives soil its grey-blue colour.” From Chapter 3.
Temperature – indicator of energy needed to sustain the normal activity of plants and soil
organisms.
o All plants and microorganisms in soil have an optimal temperature range. w/in range
plants and organisms can carry out a complete life cycle.
o Rate of physical, chemical and biological processes is directly related to the changes in
soil temp. Cooler temperatures = less activity. Fluctuations can have a large impact on
nutrient cycling, decomposition, seed germination success, nutrient uptake, root
growth.
o Natural fluctuations occur daily and annually due to a number of factors – amount of
sunlight, slope of land, vegetative cover, air temp., soil moisture, soil colour.
Chemical Nature of Soils – primary source of nutrients. Important to understand chemistry for
plant nutrition as well as fate of contaminants in the env’t
pH – measure of the concentration of hydrogen ions in the soil = measure of alkalinity or acidity
of a material.
o profound influence on plant growth – optimum pH is 6.0-7.5 for plant growth
o Influences solubility and plant-availability of nutrients – nutrients on surface of organic
matter and clay particles must first be dissolved before plants can uptake. Excessive
dissolution of elements can be toxic to plants or cause highly mobile elements to be
carried away by water and leave the soil devoid of a nutrient.
o below 6.0, essential elements such as N, P, K become less available. @ pH 5.5, toxic
elements such as Al, Zn, Fe, Mn become soluble and available for uptake by plants in
excessive amounts. At any pH above 7.5 essential nutrients are less available and
deficiency symptoms result (yellow leaves, stunted growth, etc.).
o soils of igneous and granitic origin produce acidic soils
o vegetation growing on surface like conifers can result in acidic soil
Cation Exchange Capacity – refers to soil’s ability to maintain reserves of positively charged
nutrients/ cations – Ca, Mg, Al, Na, K.
o Occurs when soil particles/ colloids exchange cations with solution.
o Property of soil texture. Related to clay and organic matter content in the soil which are
the most chemically active part of soil. Clay particles are negatively charged and cations
are positively charged.
o Once adsorbed, mineral elements are safely stored on soil particles to prevent any
losses. If not held, cations float loosely in water and are lost through water percolation.
o Low CEC means that the soil is unable to hold any nutrients that are applied through
fertilization and t/f limited availability of nutrients to plants and microorganisms. Low
CEC is reflection of low levels of organic and clay minerals.
o Soil pH can affect CEC. Increase in pH = increase in negative charges.
o Useful indicator of soil fertility, nutrient retention capacity, and capacity to protect
groundwater from cation contamination.
Essential Nutrients
One of the main reasons for the study of soil chemistry, soil pH, and cation exchange capacity is
to comprehend how nutrients are processed and made available to the plants and organisms
that need them.
Plants require a combination of adequate air, water, light, and temperature to grow. In addition
to these factors, plants also require a favourable concentration of 16 different elements or
nutrients for growth and survival. These 16 nutrients are divided into two groups: mineral
nutrients, and non-mineral nutrients. Nonmineral nutrients, namely hydrogen (H), oxygen (O),
and Carbon (C) are obtained from the air and water. The remaining 13 nutrients are mineral
nutrients that are obtained in different quantities from the soil solution and absorbed through
the plant’s roots.
There are two main groups of mineral nutrients, each distinguished based on the quantity of
the mineral that is required for healthy growth. Macronutrients are those nutrients that are
required in relatively large quantities whereas micronutrients are required in comparatively
small quantities.
The primary macronutrients are nitrogen (N), phosphorus (P), and potassium (K). These are the
first nutrients to be lacking from the soil, since they are used in the greatest quantity. The
secondary macronutrients are calcium (Ca), magnesium (Mg), and sulphur (S). These are usually
found in adequate quantities in the soil. The trace elements or micronutrients are boron (B),
zinc (Zn), chlorine (Cl), manganese (Mn), molybdenum (Mo), and iron (Fe) and are generally
obtained by recycling of organic nutrients.
Nutrients are recycled within the soil environment to support plant needs. Plants grow
normally until they run short of one or more nutrients. Growth and development become
limited by the least available plant nutrient, regardless of how much of all the other nutrients
are available to the plant. When one or more nutrients limit the plant’s ability to perform
normal tasks, we say that the plant is deficient in that particular nutrient. Deficiencies can cause
discolouration or deformations of plant structures.
Biological Nature of Soil
o The presence and functions of soil organisms are so vital that soil biodiversity is often used
as an indication of the soil’s quality. This is, of course, attributed to the fact that the routine
activities of soil organisms make it possible to have clean water, clean air, healthy plants,
and moderate water flow.
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Organisms are important for the breakdown of OM and cycling of nutrients into soil for
plant uptake. Main nutrients include carbon, nitrogen, sulphur, phosphorus. Plants need
the inorganic form of these nutrients in order uptake them. Microbial action in a process
called “mineralization” converts the minerals into the inorganic form.
When OM broken down it provides structure and stability to soil as well as nutrients.
Results in aggregation of soil particles which decreases erosion and improves water
infiltration, aeration and movement of plant nutrients. Process aided by earthworms,
arthropods, hyphae (fungi)
pest control & disease suppression – soil biota can inhibit activity of pathogens
Nitrogen fixation - rhizobium bacteria. Live in root nodules of legumes- convert
atmospheric nitrogen into form usable by plants (symbiotic relationship) - in return, plant
provides bacteria with sugars/ nutrients they need.
Degradation of Pollutants – many soil org. have ability to break down hazardous substances
into less toxic or non-toxic substances. Bioremediation is a method of cleaning
contaminated soil using microorganisms like bacteria to break down spilled pollutants
within the soil.
Soil Organisms
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Organisms classified according to role in soil ecosystem: producers, consumers,
decomposers.
$ Producers = all green plants and algae - make own food through photosynthesis.
$Consumers - cannot make own food so eat other forms of life = herbivores (eat living
plants); detrivores (eat dead and decaying plants); omnivores (plant & animal =
earthworms); carnivores (flesh eaters = spider, robin).
$Decomposers - Bacteria, fungi digest food outside themselves by secreting enzymes into
dead organic matter and then absorbing the broken down component for their
nourishment. Break down organic matter small enough for absorption through plant
roots. Potassium, phosphorus would be locked in ground w/o decomposers.
 Primary producers obtain energy to fuel the soil food web from the sun through the fixation
of carbon dioxide and conversion to sugar (photosynthesis) (includes plants, lichens, moss,
photosynthetic bacteria, and algae); secondary & tertiary consumers.
Soil microbiota
o smallest form of life; single – cell colonies
o Concentrated in upper horizons, around roots, on humus and in dead/ decaying litter.
o Key decomposers of OM; carry out additional functions such as nitrogen fixation, aggregate
formation, and production of antibiotics.
o Include: Algae, Fungi, Bacteria, Actinomycetes, Protozoa, Nematodes.
Algae
 Photosynthetic organisms that capture energy of sun to convert inorganic substances
into OM t/f found near surface.
 Help fix nitrogen from atmosphere and provide N to nearby plants.
 Help increase soil structure with glue-like substance.
Fungi
 Most important soil fungi are molds, mushrooms and mycorrhizae.
 Not photosynthetic so must obtain energy from breaking down OM of the soil.
 Main source of food is dead plant material - many plant molecules would not break
down w/o fungi. First to attack esp. complex compounds.
 Able to produce plant hormones and antibiotics that encourage growth and destroy
disease and pests.
 Help increase soil structure with glue-like substance.
 Mycorrhizae – special group of fungi that live on plant roots - symbiotic relationship –
mycorrhizae absorb water and nutrients for plants as well as protect roots from pests;
plant gives fungi nutrients and carbohydrates
Bacteria
 Most numerous of soil organisms with capacity for rapid reproduction
 Responsible for the decomposition of dead plant and animal matter into molecules small
enough to be absorbed by plant roots.
 One-celled org. that are either autotrophic (energy from oxidation of inorganic
substances like ammonium, sulphur, carbon dioxide – increases solubility of nutrients
for plants to absorb) or heterotrophic (energy from breakdown of OM).
 Attack simple compounds.
 During decomposition, help release important elements for higher plants.
 Also make and release plant growth hormone to stimulate root growth.
 Some able to fix nitrogen.
Actinomycetes – thread-like organisms
 break down lignin – large complex molecule in tissues of plant stems.
 Produce antibiotics to fight disease of roots.
Protozoa – single celled organism
 secondary consumers – feed on OM, bacteria, fungi and other protozoa.
Nematodes – threadworms/ eelworms
 either beneficial or detrimental to soil depending on species.
 Soil Mesobiota – Earthworms, Ants, Arthropods
o macro-organisms = centipedes, earthworms, pillbugs, springtails - help decompose
dead plant and animal material, mix soil and promote air and water circulation
Earthworms
 Burrow through upper soil layers enhances aeration, nutrient cycling, oxidative rxns and
increased pathways for water.
 Ingest dead OM, minerals and bacteria
 Excreted fecal casts are rich in nutrient and organic content.
 Prefer neutral pH, moist habitats with high organic matter.
Ants
 bring about extensive turnover in the soil when deeper soil is brought to the surface.
 Efficient at breaking down woody debris.
Arthropods
 millipedes, sowbugs, termites, mites, roaches, centipedes, springtails –
 primary decomposers – eating and shredding large particles / mixing residue with soil.
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Waste produced by arthropods is extremely rich in plant nutrients that are released after it is
further worked on by bacteria and fungi.
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Soil Macrobiota
o Macrofauna – moles, rabbits, snakes, prairie dogs, badgers – burrow in the soil and
spend part of life below.
o Macroflora – roots of plants, trees, shrubs
Soil Degradation causes
 occurs as a result of economic, social, and political pressures.
 primarily a result of overgrazing, deforestation, agricultural activities, overexploitation of veg.,
industrial activities and degradation.
 results include: erosion, loss of organic matter, salinization and acidification
Loss of Organic Matter - reduced productivity; deterioration of soil structure, more prone to
erosion, compaction, runoff, poor water infiltration drainage, decreased water holding
capabilities; decrease in nutrients; surface crusting.
Salinization
 When water soluble salts – sodium, potassium, calcium, magnesium, and chlorine –
accumulate in excess concentrations in the root zone of plants to such an extent that they
lead to degradation of soil and vegetation.
 When salinization is due mainly to the concentration of excess sodium, it is referred to as
sodicity.
 Sodium causes clay and organic matter of the soil to disperse. The soil becomes compacted
and hampers the growth of roots. When sodic soils become dry, the sodium-clay forms a
hard crust that is characterized by a white surface coating. Crusted soils are impenetrable to
plant roots and may even limit the emergence of seedlings.
 Soil salinization may occur naturally or due to conditions resulting from mismanagement of
the land. Salinization occurs when conditions of low rainfall, high evaporation, high water
table, and the presence of soluble salts in the soil co-occur and work hand in hand to
augment salt build-up.
 In poorly drained soils, where the groundwater table is 3 m or less from the surface of the
soil, water is unable to leach down, and instead rises to the surface by capillary action.
Capillary action is the natural upward movement of water between soil particles. In hot and
dry region, this water leaves the surface of the soil through evaporation. Since groundwater
contains naturally dissolved salts, the water evaporates leaving salts behind. The
phenomenon repeats constantly, and over time salts concentrate until they reach levels in
the root zone that are detrimental to plants.
 A similar process occurs in semi-arid drylands where the use of irrigation water is
unavoidable. The evaporation of irrigated water leaves natural salts to accumulate on the
surface of the soil. Due to the lack of adequate precipitation in arid regions of the world, the
accumulated salts are never able to leave the soil through leaching. The build-up of salts can
lead to plant toxicity in extreme situation.
– occurs when conditions of low rainfall, high evaporation, high water table and the presence of
soluble salts in the soil co-occur. Poorly drained soil in a hot dry region= capillary action (pulling
of water to surface) – evaporates leaving salt behind on surface. Also occurs in areas where
lands are irrigated. Irrigated water evaporates leaving salt behind.
Desertification – degradation and deterioration of arable land, caused chiefly by overgrazing,
overcultivation, deforestation and poor irrigation. Usually happens in arid, semi-arid and dry
sub-humid regions where evapotranspiration exceeds precip. and veg. cover already sparse.
 World’s most threatening form of soil degradation
Acidification – natural process that is aggravated by mismanaged agricultural practices.
 decrease in the pH of the soil beyond those ranges tolerable by plants and soil organisms.
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Common causes attributed to high rainfall and overuse of ammonium-based fertilizers.
Can cause uptake of heavy metals and may cause toxicity in plants also a decrease in nutrients;
negative impact on soil organisms
Soil Compaction – occurs from machinery or animals mostly when soil is wet;
 increase in bulk density of soil that occurs when soil particles are packed closer tog.
reducing pore space between them;
 decreased pore space reduces water infiltration, higher runoff, increased erosion; decreases
soil organism activity due to lack of air and water which decreases OM decomposition and
available nutrients;
Soil pollution – occurs when hazardous chemicals are spilled or buried directly into soil or when
they migrate from elsewhere. Common causes - agriculture, sewage sludge, mining
overburden, leaking fuel tanks, disposal of chemical wastes, drift of pesticides when windy;
toxins can transfer to terrestrial and aquatic organisms; sewage sludge contains heavy metals;
atmospheric fallout of chemicals and metals: smelters, oil and gas facilities.
 soil disruption: surface mining, oil and gas pipelines
 Subsidence lowering of surface elevation of organic muck soil Holland Marsh lost 73 cm soil
between 1945 sand 1983 (less than 40 years)
 Urbanization
EROSION
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natural and continuous process that occurs in all environments
loosening, transport & relocation of soil particles by the forces of wind, water, ice or gravity
rate and extent of erosion determined by soil texture, permeability, slope steepness &
length, plant cover, land use and climate
Water Erosion
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detachment and transport of soil particles either directly by the impact of raindrops or
indirectly by rapidly running water on the surface of soil.
 Snowmelt and rainfall are the primary driving forces behind water erosion.
 Detachment occurs when the impact of raindrops causes soil particles to loosen and
become dislodged from their original aggregates. The force of impact is determined by a
combination of the weight of the rain droplet and its velocity.
 Types –
1. Splash (raindrop breaks soil particles apart and splashes them into air - similar to a bomb
on land)
2. Sheet erosion (thin surface layer of soil washed away),
3. Rill erosion (water forms small, well-defined channels and carries soil away - also called
channel),
4. Gully erosion (when rills become large called gully)
5. Slumps/ slips =types of mass erosion/ wasting ie. Landslide, streambank, coastal
5 factors that determine intensity of water erosion
1. Slope of land ie. Steeper the slope the greater the erosion potential
2. Soil texture: silt and very fine sand – highly erodible compared to sand, clay with OM
3. Soil Permeability: rate of infiltration and runoff: low infiltration rates have increased runoff
= greater potential for erosion; these soils usually have low organic matter & poor structure
4. Vegetative cover - protect soil from rain, increase infiltration
5. Precipitation - intensity of rain storms & total annual precip. affect the erosion potential
Wind erosion
 greatest on dry soils bare of vegetation
 wind turbulence and velocity initiate movement of soil
 organic soils and sandy soils are prone
 soils in areas that experience high wind are prone
 on exposed areas wind dislodges easily erodible particles of soil - particles carried upward
and forward and strike the ground further along = saltation or particle bounce - most
important mechanism of wind erosion - particles = 0.1-0.5 mm in diameter
 suspension - another process of wind erosion - wind picks up v. small particles and carries
them for hundreds of kilometres in upper atmosphere = less than 0.1mm
 surface creep = particles that are too heavy to be carried by wind roll or creep along surface
colliding with other particles.
5 factors that determine intensity of wind erosion
1. Climate - dry soils are highly susceptible to wind erosion
2. Inherent erodibility of soil - how easily ind. particles can be detached from the main
body of soil and moved. Coarser grains, particularly sand are hard to move due to
weight.
3. Soil surface - rough surface traps moving particles and prevents wind erosion
4. Vegetative cover - plants help slow down wind and provide sheltered areas for the
collection of loose grains.
5. Width of exposed area - quantity of soil the wind will move depends on the distance
over which particles can travel w/o coming into contact with obstacles.