Agriculture Reference
In-Depth Information
The nutrient requirements of plants
Gardeners are familiar with ensuring that they need
to provide a
fertile soil
to grow most of their plants,
especially if they are intending to produce vigorous
productive vegetables and fruit. They have also
seen the consequences of having an impoverished
soil; a shortage of one or more nutrients has led
to disappointing plant growth. However, it can be
puzzling to find that in some circumstances gardeners
are advised to reduce the fertility of their soil to
get better flowering. Those who have considered
establishing a wild flower meadow might well have
wondered why it usually involves stripping the good
topsoil away (see Figure 14.1). In the feeding of
plants, so much depends on the soil available, the
plants involved and the intentions of the gardener.
Objectives can be as varied as high production from
vegetables and fruit, prolific flowering or attractive
wild flower meadows leading to very different
approaches to nutrition. Rather like the use of
medicines or herbicides, twice the recommended
dose does not produce better results; nor should it be
assumed that poor plant performance will be solved
by adding more nutrients.
Most of what is required to build the plant is taken
in as water or from the air as carbon dioxide (see
photosynthesis p. 112). However, a small but
significant amount of a plant's requirements is
made up of minerals commonly referred to as 'plant
nutrients'. These are normally taken up in soluble form
through the roots from the soil solution, but can also
be taken in through other parts of the plant, such as
foliar feeding.
Table 14.1
Major and minor nutrients in healthy plants
Element
Chemical
symbol
grams per kilogram
of plant dry matter
Major nutrients
Nitrogen
N
20-50
Phosphorus
P
2-5
Potassium
K
20-50
Magnesium
Mg
2-10
Calcium
Ca
5-2
Minor nutrients
Iron
Fe
0.1-0.3
short supply), the plant shows certain characteristic
symptoms usually related to their role in the plant.
Nitrogen (N)
The element nitrogen is needed by plants to form
chlorophyll and is associated with leafy (vegetative)
growth. Consequently, large dressings of nitrogen
are given to plants grown for their leaves, such as
cabbages, lawn grasses. A mobile nutrient.
Deficiency
causes slow, spindly growth in all plants.
There is usually a general yellowing (chlorosis) of the
leaves due to lack of chlorophyll (see p. 112), often
preceded by a bluing of the foliage appearing first on
the older, lower leaves. When nitrogen shortages
occur in the plant, the chlorophyll in the oldest leaves
is broken down to release nitrogen for use in the new
young efficient leaves (Figure 14.2); mobile in the
plant.
Excess
nitrogen produces soft, lush leafy growth,
making the plant vulnerable to pest attack and more
likely to be damaged by cold. Very large quantities
of nitrogen are undesirable because they can harm
the plant by producing high salt concentrations at the
roots (see osmosis, p. 120) and can easily be lost by
being leached.
Nitrogen fertilizers
commonly used by gardeners and
their nutrient content are given in Table 14.2.
Essential minerals
are inorganic substances
necessary for the plant to grow and develop and
are often referred to as nutrients.
Most gardeners who have used fertilizers will have
noted that the main nutrient contents listed on the
containers are nitrogen, phosphorus (also known as
phosphate) and potassium (also known as potash).
These are the major nutrients because they are
needed in relatively large quantities. Also needed in
quite large quantities are magnesium, calcium (usually
supplied in lime) and sulphur. Iron is an example of a
nutrient that is essential but needed in much smaller
quantities: a minor nutrient (micro- or trace element)
along with manganese, boron, zinc, copper and
molybdenum (see Table 14.1).
Essential minerals have very specific functions in
the plant cell processes. When they are deficient (in
Phosphorus (P)
This element is important in the production of the
major chemical required for energy transfer in the
plant (adenosine triphosphate, see ATP p. 116).
Consequently, large amounts are concentrated
in seeds and the meristems of roots and shoots.
Phosphorus supplies at the seedling stage are critical:
the growing root has a high requirement and the
plant's ability to establish itself depends on the roots
being able to tap into supplies in the soil before the
reserves in the seed are used up. The presence
