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1. Growing Plants
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1A. Growing Plants
Key
ideas
☛
Plants require chemicals to live.
☛
When plants die the chemicals from which they were made will eventually be
recycled.
☛
Recycling maintains the chemical balance. If plant material is removed from
where it grows an imbalance is created.
☛
Natural cycles have evolved to recycle certain chemical elements – eg carbon,
nitrogen and phosphorus.
Soil
Text
fjda;fa fdaas]o0viv
au iosxp;zls asialk
tkl;a aajkdsk; fiuoafu
ipıkjae ot kjargkj
dsAikore gja tqlk ga
ifbz9obc iarglkat
ldbiobc9o[ag
arkjd sksks slslz90
azoz sajaja lfjkf'
SiAZi aksksi
aklqakl;akj;fdu xiox
dks a a aeuiofe a jfs
jlfdujipa uas ias isa
jfk dsjka kfdj fd jlvfp
zpzox aj aw ltaj; gd
ldsv lvDiovCI Ogda
ad lfd klaf0 j
Nearly all plants make their own food through the process called photosynthesis (see
chapter 2A). Carbon dioxide for the reaction comes from the air and water comes
from the soil. In addition, many other chemical substances are required for natural
growth and these are usually available in the soil. Soil is a solid and is composed of a
mixture of particles of coarse and fine sand, silt and clay which is interspersed with
liquids and gases. To a gardener a ‘good loam’ is soil where the sand or clay components are not present in excess and which contains some humus. Humus is decaying
plant and animal remains which helps to retain moisture, creates air spaces in the soil
and slowly breaks down to release chemical compounds including those of nitrogen,
phosphorus and potassium. These chemical compounds are normally found in the
soil as chemicals dissolved in water – ie as solutions. In waterlogged soil most of the
air spaces are filled with water; in compacted soil there are fewer air spaces.
Essential chemicals
As soon as a seed starts to germinate and grow into a seedling it uses the chemicals
required for growth from the supply within the seed. Once this supply is exhausted
the plant takes up the major chemicals required (ie those based on nitrogen, phosphorus and potassium) from the soil. Leaf production requires a great deal of nitrogen, root formation demands phosphorus while during the period of flowering and
fruiting potassium is in demand. In addition, plants require the chemical elements
magnesium, iron, manganese, sulphur, calcium, chlorine, boron, zinc and copper to
incorporate into new compounds as a requirement for healthy growth.
So far the names of the elements have only been used when referring to the
plant’s food requirements but while this is not strictly wrong it does not indicate what
happens in reality. Potassium metal for example will react with air and moisture and
has to be kept under oil. So potassium as the metal element is not the material to put
on your indoor plants! Yellow phosphorus has to be kept under water to prevent it
catching fire when exposed to the air so, again, it is not exactly a friendly fertilizer!
Nitrogen, the third main plant food is a gas and it is unreactive. So how are these
important elements made available to plants by the soil? The only way is for them to
be joined with other chemical elements to form chemical compounds.
For a compound to be used by the plant it must be soluble in water, non-toxic to
the plant and be acceptable to the metabolism of the plant. Nitric acid for example is
a compound containing nitrogen but it is not very acceptable to plants except in
dilute solution such as that produced when rain dissolves ‘nitrogen oxide’ gases
formed during a thunder and lightning storm (see chapter 1B). Fortunately both soil
and fertilizers contain compounds which plants can absorb eg nitrogen is usually
combined with oxygen to form nitrates; phosphorus is usually combined with oxygen
to form phosphates; and potassium is usually combined with oxygen to form potash.
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‘Potash fertilizer’ is an agricultural term and refers to a mixture of potassium
oxides and other compounds containing potassium. ‘Nitrate’ and ‘phosphate’ cannot
exist on their own; they have to be attached to another element (such as a metal) or
contained in a compound. In fertilizers this is quite useful because it makes it possible for other important chemical elements to be given to the plant. Calcium for
example can be given as calcium nitrate and chlorine can be given as potassium
chloride (see figure 1A.1). Some crops require particular chemical elements and most
of these are supplied as compounds - eg sodium chloride (table salt) which is a
requirement of sugar beet.
A seed can be considered to be an embryo with its own supply of food, so each
seed has its own basic supply of nutrients which are essential for the early life of its
seedling. After these reserves have been used up nutrients have to be available in the
soil for continued growth.
Ancient woodland or forest probably provides the only remaining examples of
natural recycling on land. Leaves, whole plants, branches and sometimes trees fall to
the ground. With the help of insects, fungi, bacteria, and their enzymes the materials
decay to release their chemicals into the soil for use by the next generation of plants.
‘This load of cattle went to market,
This load of grain went to the mill.
......................................and as result, a whole lot of mineral salts
were removed from the farm.’
If any part of a plant is taken from the spot where it has been growing a ‘packet’ of
chemicals is in effect removed. The harvesting of one tonne of wheat grain effectively
removes about 18.1 kg of nitrogen, 3.6 kg of phosphorus and 4.0 kg of potassium
from the soil. The despatch to market of a tonne of fat cattle (on average, two cows) is
equivalent to removing 24.5 kg of nitrogen, 6.8 kg of phosphorus, 1.4 kg of potassium and 11.8 kg of calcium from the land (see figure 4B.2).
Fertilizers
In intensive agriculture (and even in a heavily cropped gardens) a deficit in chemical
food can be created. An alternative scenario is that as a result of management
methods (eg the burning of vegetation), high concentrations of mineral salts accumulate in one area. After human intervention has taken place and the crops have been
harvested one of two things can be done to replace the nutrients. The grower can
either ‘move on’ and exploit another area, leaving nature to take its course as ancient
man did, or the grower can put some chemicals back into the soil by applying
fertilizer.
The idea of applying fertilizers is not new. The first British settlers in North
America found that the Indians improved their crop yield by burying a small fish with
every maize seed they planted. Medieval farmers recognised the benefits of planting
clover and other legumes in rotation to increase the level of nitrogen in the soil.
Legumes (eg peas and clover), increase ‘nitrogen’ in the soil through the action of
bacteria which are held in nodules on the roots of these plants (see figure 1B.2). The
bacteria are able to convert nitrogen gas in air into a form which plants can use. This
process is part of the nitrogen cycle and a simplified version of this is shown in figure
1A.2 (more details are given in chapter 1B). It is one of the recycling systems found
on earth, others include the carbon (see chapter 3) and phosphorus cycles.
Sulphate
Magnesium sulphate
As carbonate or
hydroxide or oxide
Sulphur
(S)
Magnesium
(Mg)
Calcium
(Ca)
Main area of
use in plant
Major plant nutrients
Throughout plant but
especially in areas of
active growth
Leaves and all green
areas
Throughout plant but
NOT associated
with particular function
Essential for flower
and fruit production
Root formation and
growth. Fruit ripening,
seed maturation and
germination
Leaf production
Figure 1A.1
Cell structure, plant
rigidity and as a
‘chemical carrier’
Production of green
pigment (chlorophyll)
Protein production
Not certain; possibly
in enzymes. Control
of water loss
and photosynthesis
Combined with
other chemicals eg
potassium sulphate
Potassium
(K)
Forming protein
Energy transfer
Nucleic acids and
enzyme systems
(i) Nitrates
(ii) Ammonium compounds
Nitrogen
(N)
Biochemical
function
Phosphorus (i) Phosphate
(P)
(ii) Oxides soluble in water
Main forms in which
chemical is supplied to plant
Chemical
element
Stunted growth,
especially of younger
leaves and growing
points
Older leaves become
chlorotic (yellowish)
between veins. Leaves
fall off
Younger leaves become
yellower especially
between veins
Slow growth, poor
fruit and flowers. Older
leaves turn yellow
with brown mottling
and withered edges
Slow stunted growth,
lower leaves can be
dark blue-green in
colour. Low yield of
leaves and fruit
Pale, yellowish
sometimes purplish
leaves
Effect of
shortage
Conditions for
nutrient uptake
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Slightly alkaline conditions are best for maximum nutrient uptake
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Lightning
Nitrogen in the atmosphere
Legumes eg
clover, peas and
beans take
nitrogen from the air
Rain during
thunderstorms contains
useable nitrogen
Fertilizer
manufacture
Plants
Soil
Some soil
bacteria
Dead plants
and animals
and animal urine
Decay
Figure 1A.2 Simplified nitrogen cycle
Challenge
1
Visit a garden centre. Look at a packet of fertilizer for maturing tomato plants and note
down the NPK ratio shown on the packet. Find a fertilizer for grass and one for
flowers. Again note the NPK ratio. What conclusions can you draw from these three
ratios?
‘Organically grown - no chemicals used’
Although we think we know what this phrase means, if taken literally it is misleading.
What the producers should tell us is the form in which the chemicals are available.
There are three types of fertilizer: ‘organic’, ‘natural’ and ‘chemical’. ALL three release
essential chemicals into the soil for plants to use (see figure 1A.3).
Natural and chemical fertilizers do not provide humus. Although organic fertilizers
generally contain some humus, the amounts are often low and additional humus is
often required. Most general fertilizers are marked with the plant food ratio of the
major components in the order N (nitrogen), P (phosphorus as phosphate) and K
(potassium as potash). This is known as the NPK ratio or number.
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Organic fertilizers
Natural fertilizers
Chemical fertilizers
Processed dead
organic matter
Naturally occurring
minerals
Manufactured or
processed chemicals
Fish m
eal
Kainite
Saltpetre
Toma
fertiliz to
er
ss
Supergrar
fertilize
and
Hoof meal
horn
Rock
phosphate
Bone
meal
Dried
blood
Supplies nitrogen,
phosphate and potassium
in a general mix. Useful
chemicals released slowly
as microorganisms break
down organic matter.
These fertilizers are more
often used by gardeners
than farmers
Potassium
chloride
from
Canada,
Germany,
and “USSR”
Sodium
nitrate
from
Chile
Calcium
phosphate
from
N Africa
and US
Slow or quick acting
depending on how
quickly they dissolve
in the soil water. Separate
minerals supply individual
chemicals. This group is
often replaced with
chemical fertilizers
Figure 1.A3 Fertilizers
N PK
25/5/5
Flow
fertilizer
er
Designer fertilizers often
compounded for application
to a specific crop at a
specific time. Quick acting
because chemicals do not
need breaking down
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1B. Growing plants
☛
Chemical elements cannot normally be created or destroyed. They can exist
on their own (sometimes only for a short time), and they can be incorporated
into different compounds.
☛
In the nitrogen cycle, elemental nitrogen is transformed into different compounds.
☛
Some of the nitrogen compounds included in the cycle can be used by living
things; others cannot, and some can be toxic.
Key
ideas
The nitrogen system
Text
fjda;fa fdaas]o0viv
au iosxp;zls asialk
tkl;a aajkdsk; fiuoafu
ipıkjae ot kjargkj
dsAikore gja tqlk ga
ifbz9obc iarglkat
ldbiobc9o[ag
arkjd sksks slslz90
azoz sajaja lfjkf'
SiAZi aksksi
aklqakl;akj;fdu xiox
dks a a aeuiofe a jfs
jlfdujipa uas ias isa
jfk dsjka kfdj fd jlvfp
zpzox aj aw ltaj; gd
ldsv lvDiovCI Ogda
ad lfd klaf0 j
The chemical element nitrogen is found in many organic compounds (although it is
not as common as carbon). However, it is essential for protein production in both
animals and plants. Nitrogen is an unreactive gas, hence it is difficult to make
nitrogen react with almost anything else.
Converting the inert to the essential
The form in which nitrogen is most available is as a gas in the atmosphere where it
makes up about 80 per cent of the air we breathe. Most plants can only utilise
nitrogen if it is available as ‘nitrate’ (see chapter 1A). Nitrogen can be ‘fixed’ or
combined with other elements by the power of lightning or the equally ‘striking force’
of bacterial activity. Chemical reactions at high temperatures and pressures can also
make nitrogen combine with other chemical elements. In essence there are four ways
in which nitrogen gas can be converted to nitrate (see figure 1B.1).
By lightning
The electrical energy released when lightning flashes can cause nitrogen and oxygen
in the atmosphere to combine and produce oxides of nitrogen. These acidic oxides
are soluble in water and will dissolve in rain to form dilute nitric acid.
nitrogen + oxygen → nitrogen monoxide → nitrogen dioxide
nitrogen dioxide + water + oxygen → nitric acid
(rain)
On contact with the soil the nitric acid can react with (basic) compounds to produce
nitrates; eg
calcium carbonate + nitric acid → calcium nitrate + carbon dioxide + water
Fixation by this route removes about 7 million tonnes of nitrogen per year from the
atmosphere and transfers it into the soil.
Industrial fixation
Due to the importance of nitrate in increasing the amount of food produced, chemical processes have been devised for converting atmospheric nitrogen to nitrate
fertilizer (see chapter 1C). In 1985 world production of one particular nitrate fertilizer
was 78 million tonnes!
Free-living nitrogen fixing bacteria
Some bacteria live an independent existence in the soil and can convert nitrogen gas
1. Growing Plants
Humus
Decay & death
Eaten by
animals
Plants
Fate of nitrogenous fertilizer when applied to cereal crops
10% 10%
lost
lost
by
by
leach- deniting to rificaground- tion
water
25%
becomes
part of
organic
matter of
soil
55% taken up by crop
Lightning
and death
Animals
(nitrogen as protein)
Industrial fixation
Ammonification
in legumes (Rhizobium)
Nitrogen fixing bacteria
bacteria (Azotobacter)
Free-living nitrogen fixing
Excretion
Ammonia & fertilizer manufacture
Nitrogen in the atmosphere
Nitrogen in the soil
Nitrosomonas
Ammonia
Nitrobacter
Nitrites
Nitrification by nitrifying bacteria
Nitrogen
N2
Nitrates
NO3–
Nitrous oxide
Nitrite ion
N2O
NO2–
Denitrifying bacteria, Pseudomonas, Bacillus
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Figure 1B.1 The nitrogen cycle
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to ammonia (eg the Azotobacter and Clostridium bacteria do this by using the
enzyme nitrogenase). Clearly the amount of nitrogen fixed in this way will depend on
the number of bacteria present and estimates indicate that these bacteria convert
about 170–270 million tonnes of nitrogen gas a year to ammonia which is then
converted to nitrate by different bacteria.
Nitrogen fixing bacteria in legumes
Legumes are a group of plants that include clover, peas, beans, soya beans and
lupins. Specialised bacteria (of the Rhizobium genera) resident in the soil colonise the
roots of legumes and induce the formation of small lumps or nodules (see figure
1B.2). In this relationship bacteria and plants coexist to their mutual benefit, the
bacteria in the nodules convert atmospheric nitrogen to ammonia, and about 35
million tonnes of nitrogen a year is converted in this way. This process can also take
place in some non-leguminous plants and a great deal of research is being directed to
engineer genetically this facility into other food plants. It is a farmer’s dream to have
all plants ‘fixing’ nitrogen from the air and this is certainly the biotechnologists goal
but it is probably the fertilizer manufacturers’ greatest nightmare!
Life depends on death
When waste material is produced by animals, and when plants and animals die, a
potential source of nitrogen is made available. However, this nitrogen is ‘locked’ up
in the form of large protein molecules in the waste material. Fortunately bacteria and
fungi which occur naturally are able to break down dead and excreted matter to
obtain the energy they require and, as a consequence, release nitrates into the soil. In
the first stage proteins are broken down to the basic units (amino acids) via a process
called microbial decomposition. The amino acids are then further broken down in a
process called ammonification (because it produces ammonia).
protein from
dead cells and
waste products
microbial
→
decomposition
amino
acids
microbial
→ ammonia
ammonification
Although ammonia plays a key role in the nitrogen cycle, plants can absorb very little
nitrogen in this form so it is left to bacteria to change the ammonia into nitrate which
plants can absorb. The bacteria change the ammonia by oxidising it (essentially
adding oxygen) to turn it first into nitrite and then into nitrate. This process is called
nitrification.
Nitrosomonas
ammonia →
bacteria
nitrite
Nitrobacter
→ nitrate
bacteria
The system isn’ t leak proof
It might appear that the recycling system for nitrogen is almost perfect. In the long
term it is, but there are three ‘leaks’ which give cause for concern in the short term.
i)
Nitrates (especially nitrogenous fertilizer) are very soluble in water and can
easily be washed out of the ground by heavy rain or flooding. This process is
called leaching and can account for nitrate finding its way into drinking water
(see chapter 1C). It is estimated that approximately 10 per cent of fertilizer
applied to soil can be lost in this way.
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ii)
Some ammonia gas can enter the atmosphere before and during the
ammonification process. This often comes from the slurry from animal rearing
units and not only is this a loss of nitrogen as ammonia, but it is difficult to
‘recapture’ the ammonia back into the nitrogen cycle.
iii)
The third ‘leak’ is very much part of the nitrogen cycle but from the point of
view of the farmer it is rather counter productive since nitrate can be converted back to nitrogen. This is brought about by denitrifying bacteria (eg
Pseudomonas and Bacillus) in the soil gradually removing oxygen from the
nitrate group to produce nitrogen as the end result.
nitrate → nitrite → nitrous oxide → nitrogen
Root nodules
Nodules are found on the roots
of mainly leguminous plants.
They contain bacteria which
convert atmospheric nitrogen
gas into a form which is useful
to the plant
Figure 1B.2 Root nodules
Challenge
2
This experiment illustrates soil microorganism activity.
Cut up about six 20 mm x 60 mm cotton material or paper strips and bury them in
the soil in a vertical position in a slot made by putting the blade of a spade in the
ground. Place them in six different spots throughout the garden. Pack the soil firmly
against each strip with the end of it just sticking out . Bury all six pieces on the same
day and after 4–6 weeks carefully retrieve the strips (they may be very fragile) and
examine them for signs of decomposition using a hand lens to help you.
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1C. Growing food
Key
ideas
☛
The manufacture of ammonia from nitrogen and hydrogen is one of the
world’s most important industrial chemical reactions.
☛
The ability to produce synthetic nitrogenous fertilizer has enabled food crop
production to be greatly increased.
☛
An excess of nitrate and phosphate can cause problems in water courses.
☛
Nitrates and nitrites can help preserve food in addition to helping produce it.
Making ammonia
Text
fjda;fa fdaas]o0viv
au iosxp;zls asialk
tkl;a aajkdsk; fiuoafu
ipıkjae ot kjargkj
dsAikore gja tqlk ga
ifbz9obc iarglkat
ldbiobc9o[ag
arkjd sksks slslz90
azoz sajaja lfjkf'
SiAZi aksksi
aklqakl;akj;fdu xiox
dks a a aeuiofe a jfs
jlfdujipa uas ias isa
jfk dsjka kfdj fd jlvfp
zpzox aj aw ltaj; gd
ldsv lvDiovCI Ogda
ad lfd klaf0 j
Chemical compounds containing nitrogen are required by plants in a greater quantity
than those with either potassium or phosphorus. At the turn of the century an increase in the amount of food available to satisfy an increasing population required
the supply of nitrogenous fertilizer in much higher quantities, and over a shorter
period of time, than the Chilean deposits (which were running out in 1900), and the
nitrogen cycle could deliver. In addition nitrates were required for the manufacture of
explosives for World War I. This supply problem was solved in about 1908 by the
chemist Fritz Haber and the engineer Carl Bosch who invented a way of making
nitrogen and hydrogen combine to form ammonia. By doing this, Haber in effect
found an artificial way of ‘fixing’ nitrogen from the earth’s atmosphere. The ammonia
produced in the reaction is a useful chemical in its own right and some farmers apply
ammonia (in solution) directly into the soil as a fertilizer.
In the Haber process nitrogen (N2 obtained from the atmosphere) and hydrogen
(H2 obtained from natural gas and steam) are combined together to make ammonia
(NH3). The reaction takes place at a high temperature (about 400 0C) and at a high
pressure (about 200 times atmospheric pressure). The reaction is speeded up by the
use of a catalyst (see chapter 2C).
high temperature, high
nitrogen + hydrogen → ammonia
pressure, catalyst
Since the reaction is reversible, ammonia is continually removed to prevent the
reaction going into reverse.
From base to acid
Ammonia from the Haber process is normally piped away to produce nitric acid
(HNO3). Ammonia and excess air are passed over red hot layers of a platinum–
rhodium alloy. Here the oxygen in the air combines with ammonia to form nitrogen
monoxide and steam.
ammonia + oxygen → nitrogen monoxide + water
(steam)
On cooling nitrogen monoxide (NO) and oxygen from the air combine to produce
nitrogen dioxide (NO2).
nitrogen monoxide + oxygen → nitrogen dioxide
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The nitrogen dioxide can then combine with more oxygen from the air and dissolve
in water to form nitric acid.
nitrogen dioxide + water + oxygen → nitric acid
Ammonia (NH3) is a base (see chapter 9C) and reacts with nitric acid to form ammonium nitrate, and with sulphuric acid to form ammonium sulphate, both of which are
important fertilizers.
ammonia + nitric acid → ammonium nitrate
NH3
HNO3
NH4NO3
ammonia + sulphuric acid → ammonium sulphate
Recent trends
The new Leading Concept Ammonia (LCA) process developed by ICI in 1990, uses
less energy and produces less waste than previous systems . In this process ammonia
and carbon dioxide are used to make a fertilizer called urea.
In 1985 world production of one particular nitrogenous fertilizer was 78,000,000
tonnes but production in the Western world has now eased back. Even so we are still
seeing the effect of too much nitrate and phosphate entering water courses.
Richer in food?
Eutrophication, or perhaps it should be called death by enforced gluttony, is a term
which comes from the Greek word eutrophos, meaning ‘well fed’. Streams suffering
from eutrophication contain so much plant food (including nitrates and phosphates),
that the algae reproduce rapidly. Growth of unicellular algae can cause the water to
look like pea soup and filamentous algae causes ‘blanket’ weed growth on or near
the surface. Both types of growth are called ‘algal bloom’ and prevent light reaching
submerged plants. In addition blanket weed also reduces water flow. Many farm
streams have become eutrophic and some larger areas in the English Lake District
and East Anglian Broads have also suffered.
It is generally considered that fertilizer run off has contributed to the nitrogen
levels in waterways, and that human waste products – eg some washing powders –
have mainly contributed to the excess of phosphate. (In both East Anglia and the Lake
District phosphate stripping equipment has been installed at some sewage works. In
the stripping process iron (ferric) sulphate is added to the effluent and this produces
insoluble iron phosphate which drops out of the water leaving the liquid virtually free
of phosphate.)
When the algae die, bacteria which live off dead matter increase in number and
demand high levels of oxygen to live so there is a high biological oxygen demand
(called BOD for short). As a result, most of the animals die because there is no
oxygen left in the water, so in time the water becomes almost devoid of live animals
and plants with the bacteria having a great feast until they too die. However, if
enrichment stops the water will regain oxygen and life, given time (see figure 1C.1).
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Eutrophication is caused
by excess nitrate and
phosphate entering water
courses
1. Healthy stream.
Mineral salts in
balance. Water
oxygenated
Surface runoff
Outfall from
sewage works
2. With excess nitrate and
phosphate in water, algae grow
rapidly. Water can look like
pea soup. Some algae can form
a ‘blanket’ on the surface. Dense
algal growth prevents some
animal movement. Water flow
reduced
Water health
can be restored
by reducing inflow
of nitrogen and
phosphate and
possibly
oxygenating
the water
Seepage
from
septic
tanks
and
livestock
units
Fertilizer leaching
through
soil and land
drains
3. Algae die. With extra dead
organic matter to feed on,
bacteria reproduce in large
numbers. Bacteria require
oxygen to live and remove it
from the water. Fish and other
animals die due to lack of
available oxygen. Fish
sometimes float to
the surface
Figure 1C.1 Eutrophication
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Fertilizers, explosives, bacon and sausages
A strange grouping perhaps, but all of these products are associated with nitrogen
containing nitrates and nitrites.
Nitrates
These are the salts of nitric acid. The nitrates of metals (eg potassium nitrate) are all
soluble in water, and the capacity of some nitrates to ‘draw’ water out of meat by
osmosis makes them useful for curing bacon and ham joints (eg saltpetre – potassium
nitrate – was used extensively for ‘home curing’). Solubility in water is also important
when a nitrate salt is used as a fertilizer because in solution it is a readily used plant
food. Nitrates contain quite a lot of oxygen linked to nitrogen (and other elements).
This available oxygen is used when potassium nitrate is used to make gunpowder
(which is a mixture of carbon, sulphur and potassium nitrate). On ignition, large
volumes of gases are produced over a short period of time which, if confined, can
lead to an explosion.
heat
potassium nitrate → potassium nitrite + oxygen
Nitrites
Look at the ingredients label on a packet of cooked meat or sausages and there is a
good chance that sodium nitrite will be listed as an antioxidant. When used in red
meat, sodium nitrite prevents the meat turning grey or brown by forming a red
compound, nitrosomyoglobin, which keeps the meat looking attractive. It also has
the effect of inhibiting the growth of some microorganisms and gives rise to the
production of aromatic substances reminiscent of roast meat!
Challenge
3
Try and find out how foods which are cured by ‘smoking’ are preserved.
Challenge
4
Look at the labels on cooked meat and sausages and compile a list of those products
which contain sodium nitrite. Make a note of whether the product is naturally red or
is coloured.