Mineral ions can only be absorbed
by plants through the hairs of the root cells.
The concentration of minerals
is quite low in soil compared to the concentration in the
fluids of root cell hairs - hence the need for active
transport for the plant to absorb them - and why there is a
high concentration of mitochondria in root hair cells to
provide the energy needed.
Several ions found in soil are
particularly important.
The health of any plant is
harmed if it is deficient in access to these mineral ions.
Just look at their
life-supporting roles in the biochemistry of plant cells listed
below.
Nitrogen for amino acids
and protein synthesis (including enzymes) is obtained from the
nitrate ion (NO3-).
Lack of nitrogen gives
stunted growth because important and proteins cannot be
synthesised.
The magnesium ion (Mg2+)
is required by the chlorophyll molecule to function in
photosynthesis and is also essential for the metabolism of
carbohydrates.
The magnesium ion forms a
complex with the chlorophyll molecule.
If a plant is deficient
in magnesium not enough chlorophyll can be made and the
plant suffers from chlorosis - a yellowing of the leaves,
and photosynthesis is much reduced - as is the supply of food
and energy for the plant.
The potassium ion (K+)
is involved in the mechanism for opening and closing stomata (water
in/out of guard cells), the
activation of some enzymes, involved in photosynthesis, and the production of ATP
in respiration.
Phosphorus, from
phosphate ions. A phosphate structure is part of the ATP
molecule from respiration - energy source, and part of the structure of
cellular DNA and RNA.
Iron ions (e.g. Fe2+
or Fe3+) are important in the synthesis of
chlorophyll and some enzyme functions.
You can also get chlorosis in
plants from iron deficiency. (see magnesium above)
Borate ions supply the element boron
which plays a key role in many plant functions including cell wall
formation and stability, maintenance of structural and functional
integrity of biological membranes, the movement of sugar for energy
into growing parts of plants.
Soil that is deficient in any of
these minerals can be improved by using synthetic artificial
fertilisers (NPK types for nitrogen, phosphorus and potassium) often
supplemented with lots of additives to supply other trace elements
like iron or magnesium that plants need. You must compost or manure
if you are a true organic gardener!
For more see
Plant diseases and defences against pathogens
and pests
All plants need a variety of
minerals to grow and maintain themselves in a healthy state.
In the previous section, I've
described some of the uses plants make of some nutrients.
If plants do not receive any of
their essential mineral nutrients ('mineral deficiency'), symptoms will be observed as the
plant declines in health e.g.
lack of nitrates - poor
growth and yellowing of leaves,
lack of potassium - poor
fruits and flowers and discolouring of leaves,
lack of phosphorus - poor
root growth and discolouring of leaves
lack of magnesium - yellowing
of leaves,
lack of iron - loss of green
colour in stems and leaves - known as chlorosis,
lack of boron - the growing tips of the
root or shoot show stunting and distortion of the growing tip
that can lead to tip death, brittle foliage, and yellowing of
lower leaf tips.
How do mineral deficiencies
arise?
(i) Naturally poor soil
Some soils are naturally
very low in, or devoid of, some essential minerals.
Lime-rich soils are
deficient in iron.
This land is not
productive unless it is treated with a synthetic inorganic
fertiliser or an organic fertiliser to increase the mineral
content of the soil.
(ii) Farming methods
From the prehistoric
times, throughout the world, woodland has been cut down and
burnt to provide land to grow crops - known as 'slash and
burn'.
Repeated planting of the
same annual crops on the cleared land uses up the minerals
in the soil and so the soil becomes deficient in essential
minerals.
More woodland is then cut
down, a process that continues to this day in many forested
areas e.g. the rainforests of Brazil.
Again, use of
fertilisers, enables repeated growing of crops.
Modern synthetic
inorganic fertilisers have become more complex in their
formulation and may contain a dozen different elements that
plants need for healthy growth and development.
How can we investigate which
minerals are essential for healthy plant growth
You can grow plants without soil
if you suspend them in a solution of the minerals they need - the
culture solution of a
hydroponic technique - no soil used - the experiment is illustrated in the diagram above
(I guess I'm no artist, but you should get the 'picture').
The plant is suspended in a culture solution of nutrients,
so that the roots can absorb what they need for healthy
growth and development - water for transpiration, mineral
ions (for reasons discussed above) and oxygen (for
respiration).
The culture vessel is covered to
simulate the darkness that roots would experience in soil.
A tube supplies air, containing
oxygen, to keep the culture solution aerated.
You grow control plants
with a culture solution containing all the nutrient minerals they
need.
You can then grow the same plants
in culture solutions that do NOT contain any of a specific mineral -
to artificially create a deficiency of a plant nutrient.
The growth of the plant can be
monitored to see the effect of a particularly deficiency -
observed symptoms resulting from the specific deficiency
You must only omit one mineral
at a time in the investigation, otherwise you could not be sure
which one caused the symptoms you are observing.
Using this relatively simple
technique, you can investigate the effects
of deficiencies in nitrate ions (for nitrogen), phosphate ions (for
phosphorus), magnesium ions (Mg2+), potassium ions (K+),
iron ions (Fe2+/Fe3+) and borate ions (for
boron).
The relevance
of nutrient knowledge to food production
If you know what minerals are
soil is deficient in, you can add a fertiliser to provide the
missing nutrients.
Farming, agriculture in general,
uses two types of fertiliser.
'Natural' organic
fertilisers
Organic fertilisers come from
animal ('smelly muck') or plant ('compost') waste material and
are considered environmentally superior to inorganic
fertilisers.
They produce better quality
soil that is more resistant to erosion and better at retaining
water.
Organic fertilisers only
break down slowly in the soil to release their nutrient minerals
- 'intensive farming' farmers would see this as an economic
disadvantage.
Good quality organic
fertiliser should supply all the essential minerals a plant
needs to grow and develop.
'Man-made' synthetic
inorganic fertilisers
These are concentrated
formulations of particular essential elements plants need for
healthy growth.
They can be made to any
composition to suit particular soil deficiencies in any soil.
The inorganic fertiliser
formulation can also be made to promote the growth of a
particular crop to increase the yield.
They do not smell, easily
stored (chemically stable), easily and evenly spread on the soil
as small pellets and the chemicals are quickly absorbed by plant
roots.
The solid pellets can be
designed to break down slowly to minimise the polluting
effects from run-off into water courses.
These fertilisers do not
improve the quality of the soil, but must be regularly
applied-replaced, especially if fields are used to grow several
yields of crops each year (same or different).
