soil
 Discuss salinization, nutrient depletion and soil
pollution as causes of soil degradation.
 Describe the relevance of the soil organic
matter (SOM) in preventing soil degradation,
and outline its physical and biological functions
 List common organic soil pollutants and their
sources.
: soil degradation
 Soil Degradation: Occurs where human activity (either directly
or indirectly) reduces the capacity of the soil to support life.
 Causes: intensive farming, desertification, erosion, and
pollution.
 Bad: reduces crop production
: soil degradation
Salinization
 Salinization is the result of irrigating soils i.e. watering them.
Water used for irrigation usually contains dissolved salts, which
are left behind in the soil after the water evaporates.
 In poorly drained soils, the salts left behind are not washed
away and begin to accumulate in the topsoil. Plants cannot
grow in soil that is too salty.
 Another problem caused by irrigation is that over
irrigation/water run off can remove vital nutrients such as Ca2+,
K+ , NH4+ and Mg2+ from the topsoil.
: soil degradation
Nutrient depletion
 Plants remove nutrients and minerals from the soil.
 If soil is not allowed to recover i.e. allowed time for the removed
nutrients to be replaced through natural processes, the soil becomes
depleted in these nutrients and this will effect future plant growth.
 Monoculture i.e. growing the same crop time after time increases
nutrient depletion.
 Allowing soils to remain fallow (no crops are grown and nutrients can
be replaced) for a while or use crop rotation i.e. grow different crops
which absorb different nutrients are some solutions.
: soil pollution
Soil pollution
 Use of chemicals such as pesticides and fertilizers.
 They can:
 disrupt the soil food web,
 reduce the soil’s biodiversity
 ultimately ruin the soil.
 These chemicals also run off the soil into surface waters and
move through the soil, polluting groundwater.
Soil organic matter (SOM)
 SOM includes:
 main-constituent: undecayed plant and animal matter
which can be:
 high-molecular-mass organic materials such as
polysaccharides and proteins and
 simpler substances such as sugars, amino acids and
other small molecules
 humus: the residue left of organic material after it has
been fully broken down by bacteria in the soil.
Soil
: SOM
 The functions of SOM can be broadly classified into two groups:
 biological: provides nutrients for the plants, in particular
nitrogen, as it provides amines and amino acids;
 physical:
 improves structural stability,
 influences water-retention properties: the OH- and NH2
groups on the SOM molecules allow hydrogen bonding
between these molecules and water molecules in the soil
 alters the soil thermal properties.
: organic pollutants
 Many organic compounds end up in soil through direct
spills, leaks (e.g. from landfills, underground storage
tanks), dumping or washed from the air through rain.
 Most organic pollutants are petroleum hydrocarbons
which are compounds which originally come from
crude oil, coal gas, tar, natural gas; either directly
through distillation (e.g. octane) or manufactured.
Soil
: organic pollutants
 These petroleum hydrocarbons are:
 volatile organic compounds (VOCs) such as shorter alkanes,
petrol, benzene, toluene, xylenes, …or
 semi-volatile organic compounds (SVOCs) which are organic
compounds with boiling temperatures higher than water
but which evaporate at temperatures above room
temperature; examples are phenol, naphthalene,
plasticizers, many PAHs and PCBs.
: pollutants
Pollutants
Sources
agrichemicals
pesticides (insecticides, fungicides, herbicides), fertilizers, growth
hormones through direct application which is fine if controlled;
problem is spills and overuse.
organic
solvents

polynuclear
aromatic
hydrocarbons
(PAHs)

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spills/leaks/dumping of dry cleaning solvents, paint thinners,
nail polish remover, detergents, …
many are also VOCs such as dichloromethane,
trichloroethane, and trichloroethylene
chemicals which do not degrade easily
synthetic PAHs banned in 1975
chemicals common in coal tar/creosote which have been used
to preserve wood in buildings, fences, ships.
PAHs are present in coal and crude oil and are released
during combustion e.g. industrial/power station emissions
: pollutants
Pollutants
polychlorinated
biphenyls (PCBs),
Sources
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organotin compounds 
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dioxins
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stable compounds, can function as fire retardants and
plasticizers
transformers, circuit breakers, electromagnets, hydraulic
oils, paints
bioaccumulate
compounds which contain at least one bond between tin
and a carbon part of a hydrocarbon chain
used in manufacture of:
o antifouling compounds (coatings or paints)
o fungicides
o pesticides / biocidal agents;
o catalysts
incineration plants, wood burning, traffic
spread trough the air and settle on soil