Population Growth including farming methods, fertilisers and pesticides
The carrying capacity of an environment is the maximum size of population it can support.
This is when birth rate + death rate. The rapid growth rate (exponential) of the human
population started in mid 1800s. Factors contributing to this huge increases are;
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Food availability
Improved hygiene and sanitation
Improved medical treatment
Increase in female fertility.
The study of human population statistics is known as demography. A change in a country’s
population depends on four factors:
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Births
Deaths
Immigration
Emigration
It is suggested that birth control is the best way to control human population but there are
problems with this due to religious beliefs, medical care, cultural and social backgrounds.
Food and Population
In natural cycles plants die and decay. The mineralisation of the plants in the soil replaces
the nutrients as fast as they are used. This along with materials excreted by plants and
animals is a source of natural fertilisers. In ideal circumstances this is how nutrients would
be cycled.
However, in order to meet ever-increasing food demands large fields of monocultures (one
plant) are created. When these large fields are harvested this balance is disturbed and
synthetic fertilisers like nitrates have to be added. Synthetic fertilisers disturb the soil
structure and may be leached into surrounding water causing eutrophication. This in turn
leads to an algal bloom which causes a decrease in oxygen by:
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Blocking light and preventing photosynthesis
Providing nutrients for bacteria which multiply using up oxygen.
Indicator species
This is a species whose presence/absence shows the level of pollution in an area.
Species
mayfly
Rat-tailed maggot
Level of oxygen in
water
high
low
Level of pollution
low
high
Uses of minerals
Mineral
Use
nitrate
Building protein
and growth
Phosphate
Respiration and
growth
potassium
Respiration and
photosynthesis
magnesium
photosynthesis
Deficiency
symptoms
Poor growth and
yellow leaves
Poor root growth
and discoloured
leaves
Poor flower and
fruit growth,
discoloured leaves
Yellow leaves
Explanation
All amino acids
contain nitrogen.
Amino acids are the
building blocks of
protein
A component of
DNA molecules and
cell membranes
Must be present for
photosynthesis and
respiration
enzymes to work
Chlorophyll
molecules contain
magnesium ions. It
is the magnesium
that makes
chlorophyll green.
Vigorous weeds often compete with crops for light, space and nutrients. Farmers apply
herbicides to avoid this. Monocultures provide ideal conditions for pests and parasites.
Pesticides are used to avoid these. Fungicides are used against fungi and insecticides
combat insects. Some chemicals used in pesticides kill helpful insects e.g. bees. Some
pesticides take a long time to break down e.g. DDT which builds up in organisms as you go
along a food chain. This is known as bioaccumulation. It often reaches fatal level. DDT also
causes the thinning of birds’ eggs. The use of DDT has been banned despite it being an
effective pesticide. Some farmers use organic methods to get rid of pests:
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Spraying natural chemicals obtained from plants on to their crops
Picking pest off by hand
Placing natural predators of the pests onto crops
Growing plants that repel the insects amongst the crops.
Some farmers may use genetically modified crops, such crops are resistant to particular
pests. These methods may involve:
Bacterial carriers
Gene silencing
Gene splicing
Viral carriers
Many farmers are moving away from intensive farming using artificial fertilisers and
pesticides and developing organic farming methods.
Organic farming techniques
Technique
Manure
Replaces
Fertiliser
Crop rotation
Single crop
Weeding
Herbicides
Nitrogen-fixing
plants
Nitrogen fertilisers
Advantage
Recycles waste,
improves soil
structure
Reduces disease and
damage to soil
composition
Less environmental
damage or health
risk
Cheaper, longer
lasting
Disadvantage
Difficult to apply and
cannot control
mineral content
Less productivity.
Less efficient to grow
different crops
Labour intensive
Reduces area for
growing crops if part
of crop rotation