Transcript soil degradation - chemistryatdulwich

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.
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.
Nutrient depletion (1)
 Plants remove nutrients and minerals from the soil.
 If the soil is not allowed to recover i.e. allowed time for the
removed nutrients to be replaced through natural nutrient
cycles the soil becomes depleted in nutrients and this will
reduce future productivity. Nutrients can also be replaced by
using artificial fertilizers or compost.
 Harvesting crops stops this natural nutrient cycle as the
nutrients are removed permanently.
 Monoculture, i.e. growing the same crop time after time,
accelerates nutrient depletion.
Nutrient depletion (2)
Nutrient depletion can be prevented or slowed by:
 allowing soils to remain fallow (no crops are grown
and nutrients can be replaced) for a while
 use crop rotation i.e. grow different crops which
absorb different nutrients are some solutions.
 using artificial fertilizers or compost.
soil pollution
 Excessive use of chemicals such as pesticides and
fertilizers.
 These chemicals 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) includes
 Plant and animal tissue such as leaves, twigs, animal parts e.g.
skin, hair, ... that have not been decomposed yet.
 Decomposition products of those animal and plant tissues such
as:
 high-molecular-mass organic materials such as
polysaccharides and proteins and
 simpler organic substances such as sugars, amino acids and
other small molecules
 Humus which is what is left after plant and animal tissues are
not being decomposed anymore.
SOM prevents soil degradation
 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.
pollutants
Pollutants
Sources
Agrichemicals
Pesticides (insecticides, fungicides, herbicides), fertilizers, growth
hormones through direct application which is fine if controlled;
problem is spills and overuse.
Hydrocarbons


Polynuclear
aromatic
hydrocarbons
(PAHs)


Transport, solvents, direct spills, leaks (e.g. from landfills or
underground storage tanks),
Spills/leaks/dumping of dry cleaning solvents, paint thinners,
nail polish remover, detergents, …
Chemicals common in coal, tar e.g. creosote which is 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 emission and
from incinerators.
pollutants
Pollutants
Sources
Polychlorinated
biphenyls (PCBs),

Leaks from electrical insulators, transformers, circuit
breakers, electromagnets, hydraulic oils, paints
Organotin
compounds


•
Compounds which contain at least one bond between tin
and a hydrocarbon chain
Used in manufacture of antifouling compounds (coatings
or paints) which prevent organisms (e.g. barnacles) from
growing on surfaces exposed to water
Fungicides
Pesticides / biocidal agents;


Incineration plants, wood burning, herbicides
Spread trough the air and settle on soil
Dioxins

Soil pollutants
Petroleum
hydrocarbons
(hydrocarbons from
crude oil)
Compounds such as hexane,
petrol, benzene, diesel oil, jet fuels,
lubricating oils.
Direct spills, leaks (e.g. from
landfills or underground storage
tanks), being dumped