Nutrients are supplied naturally from the decomposition of organic
matter and are released as clay weathers in the soil. In horticulture,
additional supplies are made by the use of organic and inorganic
fertilizers, bulky organic matter and through green manuring.
are those that supply only one of the major
nutrients: nitrogen, phosphorus, potassium or magnesium (see
Table 21.2). The amount of nutrient in the fertilizer is expressed as a
percentage. Nitrogen fertilizers are described in terms of percentage
of the element nitrogen in the fertilizer, i.e. per cent N. Phosphate
fertilizers have been described in terms of the equivalent amount of
phosphoric oxide, i.e. per cent P2
, or increasingly as percentage
phosphorus, per cent P. Likewise potash fertilizers, i.e. per cent K2
percentage potassium, per cent K. Magnesium fertilizers are described
in terms of per cent of Mg. The percentage figures clearly show the
quantities of nutrient in each 100 kg of fertilizer.
are those that supply two or more of the nutrients
nitrogen, phosphorus and potassium. The nutrient content expressed as
for straight fertilizers is, by convention, written on the bag in the order
nitrogen, phosphorus and potassium. For example, 20–10–10 denotes 20
per cent N, 10 per cent P2
and 10 per cent K2
|Table 21.2 Nutrient analysis of fertilizers
|Table 21.3 Sources of nutrients for use in organic growing
Fertilizer regulations require that further details of trace elements,
pesticide content and phosphorus solubility should appear where
applicable on the invoice. Fertilizers and manures are available in many
different forms. Generally the term organic
implies that the fertilizer
is derived from living organisms, whereas inorganic
those derived from non-living material. However, in the context of
organic growing it is necessary to look at specific requirements of the
regulations ( Table 21.3).
Fertilizers are applied in several different ways.
- Base dressings are those that are incorporated in the growing
medium. Combine drilling with seeds and fertilizer running into the
same drill can achieve this. In horticulture, however, band placement of fertilizers is far more common, involving equipment that drills
the seeds in rows and places a band of fertilizer in parallel a few
centimetres below and to one side. The risk of retarded germination or scorch of young plants due to high soluble fertilizers placed near
seeds is thus avoided (see salt concentration). There is much less
risk if fertilizer is surface broadcast, i.e. scattered on prepared soil
surface, or broadcast on the surface to be cultivated-in during the final
stages of seedbed preparation.
- Top dressings are fertilizers added to the soil surface but not
incorporated. Such fertilizers must be soluble and not fixed by soil
because the nutrient is carried to the roots by soil water. Nitrogen is
the material most frequently applied by this method mainly because
the large applications to crops require a base dressing and one or more
top dressings to minimize the risk of scorch and loss by leaching.
- Liquid feeding is the application of fertilizer diluted in water to the
root zone; fertigation if incorporated in irrigation system.
- Foliar feeding is the application of a liquid fertilizer in suitably
diluted form to be taken up through leaves. This technique is usually
restricted to the application of trace elements.
fertilizers contain nutrients in a form which plant roots
can take up, and dissolve as soon as they come in contact with water,
e.g. ammonium nitrate and potassium chloride. Many are obtainable as
powders or crystals. These are difficult to spread or place evenly, but
several are formulated this way to help in the preparation of liquid feeds.
For this purpose they must be readily soluble and free of impurities
that might lead to blockages in the feed lines. Some of the less soluble
fertilizers, as well as lime, are spread in a finely divided form for
maximum effect on soil. One of the major problems with many of the fertilizers is their hygroscopic
nature, i.e. they pick up water from the
atmosphere and create storage and distribution problems. Powdered forms,
in particular, go sticky as they take up water then ‘ cake’ (form hard lumps)
as they dry. Fertilizers formulated as granules
are more satisfactory for
accurate placement or broadcasting. They flow better, can be metered, and
are thrown more accurately. Prilled
fertilizers represent an improvement
on granules because of their uniform spherical shape.
Slow release fertilizers
are those in which a large proportion of the
nutrient is released slowly. Several of these fertilizers, such as rock
phosphate, are insoluble or only slightly soluble and the nutrients are
released only after many months, even years. Micro-organisms break
down organic products and the rate at which nutrients become available
depends on their activity (see bacteria). Some slow release artificial
fertilizers, such as those based on urea formaldehyde, dissolve slowly
in the soil solution whilst others are formulated in such a way that the
soluble fertilizer they contain diffuses slowly through a resin coat, e.g.
Osmocote, or sulphur coating. Some of these slow-release fertilizers have
been formulated in such a way as to release nutrients at a rate that matches
a plant’s uptake and as such are sometimes referred to as controlledrelease
, made from fine glass powders containing
nutrient elements, are used either to release soluble materials slowly or to
overcome the trace element problem caused by the narrow limits between
deficiency and toxicity (see trace elements). Frits ease the difficulties
experienced in mixing tiny amounts evenly through large volumes
of compost. Ion-exchange resins
release their nutrients by exchange
with cations in the surrounding water. These resins help to overcome
the problems of high salt concentration and leaching of nutrients from
growing media based on inert materials (see aggregate culture).
Some plant nutrients are formed as chelates
to maintain availability in
extreme conditions where the mineral salt is ‘locked up’. There are many
different chelating or sequestrating agents selected, for each element
to be protected and for each unavailability problem. Iron is chelated
with EDDHA to form the product Chel 138 or Sequestrine 138 which
releases the element in all soils including those with a high pH (see iron
deficiency). EDTA, effective where there are high levels of copper, zinc
and manganese, is used to chelate iron to be applied in foliar sprays.
Bulky organic matter
Compost, straw, farmyard manure, bark and peat are important in
horticulture as a means of maintaining organic matter and humus levels. However, they tend to be low in nutrients and some,
such as straw and bark, lead to locking up of nutrients (see C:N ratio. They can be evaluated on the basis of their effect on the physical
properties of soil and their small, nutrient content.
Green manuring is the practice of growing plants primarily to develop
and maintain soil structure and fertility. The plants used are typically agricultural crops that cover the ground quickly and
yield a large amount of leaf to incorporate. The seeds are normally
broadcast sown in the autumn when there are no other overwintering
plants. The plants are then dug or ploughed in when the land is needed