Agriculture Reference
In-Depth Information
downside of quick release materials is that if too
much is applied, the plants can be harmed or even
killed; often referred to as 'root scorch' or 'burnt
plants' because water is drawn out of the root by
the high salt concentration (see osmosis p. 120
and disorders p. 271). Furthermore, care needs to
be taken to ensure ground water is not polluted by
applying more than can be taken up by plants or
watercourses contaminated by run-off following top
dressings.
Organic gardeners avoid the use of quick release
fertilizers because they identify the elevated level of
solubles in the soil water as being responsible for
reducing the effect of beneficial soil organisms.
Slow release fertilizers
do not dissolve immediately
in water but provide nutrients in soluble form over a
long period. Many are organic fertilizers such as bone
meal and hoof and horn that are decomposed by
micro-organisms, so the rate at which nutrients are
released depends on temperature. This fits well with
plant requirements which increase as they grow larger
over the late spring and early summer. However, a
downside is that there is little or no nutrient to take
up when plants start to grow after the winter when
temperatures have been too low for micro-organism
activity.
There are several fertilizers that dissolve
very slowly in the soil water. These include
Urea- Formaldehyde (UF) often used on turf because
it has the advantage of releasing nitrogen even in
cold conditions. Rock phosphate releases phosphate
in a form for plant uptake very slowly (over many
years).
Controlled release fertilizers
are slow release
fertilizers that are formulated to release nutrients in
a controlled way over a specified long period. One
group comprises quick release fertilizers held within a
permeable resin coating that lets water in and allows
nutrients to diffuse out. The other group is soluble
fertilizer coated with sulphur which is broken down
by micro-organisms so nutrients are released. In both
cases, the thickness of the coatings can be varied
to enable fertilizers to be designed so they release
nutrients in line with particular plant needs and for the
required period of time - for example, a three-month
formulation for bedding plants or a nine-month one for
container roses.
Base dressings
are the fertilizers that are applied to
the soil and worked into the seedbed, or incorporated
in composts, before sowing/planting.
Top dressings
are granular fertilizers applied to the
surface of soils because nutrients are needed after
plants have been established for ongoing nutrition
often in the form of a compound fertilizer - for
example, autumn treatment for lawns, spring dressing
for established borders.
In order to reach the root zone, they are usually
quick release fertilizers - for example, applications of
nitrogen, often with ferrous sulphate, to 'green up'
lawns. This means that care needs to be taken to
avoid harming plant leaves ('scorch'). An alternative
approach is to use liquid feed.
Liquid feeds
are fertilizers dissolved and watered on
to soils or composts to provide ongoing plant nutrient
requirements over growing season typically for pot
plants, hanging baskets, houseplants, bedding and in
greenhouse production. When used in conjunction
with irrigation systems ('fertigation'), nozzle blockages
can occur unless pure materials are used. As with top
dressing, care needs to be taken to avoid damaging
leaves ('scorch').
14
Fertigation
is supplying nutrients in the
irrigation water.
Foliar feed
is a liquid feed diluted sufficiently so
that it can be applied to leaves without causing
'scorch'. It can be used for routine feeding and has
the advantage of immediate uptake to treat deficiency
symptoms. Application is usually undertaken first
thing in the morning by spraying the leaves until there
is 'run-off'.
Fertilizers are provided mainly in the following forms
for use in the above situations:
X
Granules
which have the advantage of being easy
to deliver by hand or by fertilizer spreaders when
applying base or top dressings.
X
Powders
(or crystals) are provided for those
making up liquid or foliar feeds.
The importance of soil pH
Plants adapted to grow on more acid soils (low pH)
are seen on typical upland moorland, dry heaths and
wet peat bogs in Britain and Ireland. They feature
calcifuges
such as bilberry, ling and acid-tolerant
heathers on the moors, or gorse (furze), sheep sorrel,
broom, harebell and tormentil on the dry lowland
heaths. The peat bogs (see p. 7) are wet usually acid
areas often dominated by sphagnum moss with cotton
grass, cranberry, bog myrtle and sundews.
Calcifuge
plants commonly seen in the garden include many
Rhododendron
spp.,
Camillia
,
Pieris
,
Blueberries and
some heathers.
