Minor nutrients, also known as trace elements or micro-elements, are
present in plants in very small quantities, but are just as essential for
healthy growth as major elements. However, they can be toxic to plants
if too abundant. This means that rectifying deficiencies with soluble salts
has to be undertaken carefully.
are those in which too little of the nutrient is present
in the growing medium. Most soils have adequate reserves of trace
elements, so simple deficiencies in them are uncommon, especially if
replenished with bulky organic matter. Sandy soils tend to have low
reserves and so too have several organic soils from which trace elements
have been leached. In horticulture simple deficiencies of trace elements
are mainly associated with growing in soil-less composts which require
are those in which sufficient nutrients are present,
but other factors, such as soil pH or ion antagonism
, interfere with plant nutrient availability:
On mineral soils boron, copper, zinc, iron and
manganese become less available in alkaline soils, whereas molybdenum
availability is reduced severely in soils with pH levels below 5.5, as shown
in Figure 20.4. Trace element problems are aggravated in dry soils or
where waterlogging, root pathogens or poor soil structure reduces root
is induced by the presence of large quantities of calcium
and this ‘lime induced’ chlorosis
(yellowing) occurs on over-limed
soils and calcareous soils. The natural flora of chalk and limestone
areas is calcicoles. Other plants grown in such conditions usually have
a typically yellow appearance. Deficiencies can also be induced by high
levels of copper, manganese, zinc and phosphorus. Top fruit and soft
fruit are particularly susceptible, as well as crops grown in complete
nutrient solutions. The problem is overcome by using iron chelates.
tends to occur when pH is above 6.8. It is readily
leached from peat. Crops grown in peat are particularly susceptible
when pH levels rise (Figure 20.4). Boron can be applied to soils before
seed sowing in the form of borax or ‘Solubor’.
is more frequent on organic and sandy soils of
high pH. Plant uptake can be reduced by high potassium, iron, copper
and zinc levels. Manganese availability is greatly increased at low pH
and can reach toxic levels which most commonly occur after steam
sterilization of acid, manganese-rich soils. High phosphorus levels can
be used to reduce the uptake of manganese in these circumstances.
usually occurs on peat and sands, notably reclaimed
heath-land, and in thin organic soils over chalk. High rates of nitrogen
can accentuate the problem. Soils can be treated with copper sulphate or
plants can be sprayed with copper oxychloride.
are not common and are usually associated with
occurs in most soil types at a low pH (see
Figure 20.4). Availability becomes much reduced below pH 5.5,
especially in the presence of high manganese levels. Cauliflowers are
particularly susceptible and soils are limed to solve the problem. Sodium
or ammonium molybdate can be added to growing media or liquid feeds
where molybdenum supplies are inadequate.
Some knowledge of chemistry can give insights into biology and thus horticulture. Chemistry deals with the reactions that occur when
different chemical substances are brought into contact with each other.
All substances are made up of elements. There are 92 stable elements, such as oxygen and hydrogen. Each element occurs
in nature as atoms.
|Figure 21.6 Atoms.
(a) hydrogen (b) helium
The first step in chemical understanding involves the atom and its structure. All substances are made of atoms. Atoms are
about one ten millionth of a millimetre in size. Each one may be considered to have a central nucleus (containing protons and neutrons), and one or more outer shells (or orbitals) of circulating electrons (see Figure 21.6). Each proton
and each neutron contributes a unit of 1 to the atomic weight of the element. In the carbon atom,
there are 6 protons and 6 neutrons giving carbon an atomic weight of 12. The proton has a positive
charge whilst the neutron has no charge. The circulating electrons can be considered to have no
measurable weight, but have a negative charge equal but opposite to the proton. In each element,
the number of electrons equals the number of protons. In the carbon atom, there are 6 protons and
At its simplest, the chemistry of an element may be considered in terms of the group of electrons’
‘quest for stability’. The first (inner) orbital is described as stable when it has either zero or two
electrons. Outer orbitals are considered stable when they have zero or eight electrons. Four
examples are used to illustrate this principle.
|Figure 21.7 (a) Ionic bonding; sodium chloride (common salt). Sodium
electron and becomes a sodium cation; chlorine gains
electron and becomes an
anion; (b) covalent bonding, e.g. water;
oxygen and two hydrogen atoms share
electrons in their outer
(shells) to gain stability.
is the simplest of the elements. It has one proton and one electron in its one (inner)
orbital. Its atomic weight is 1. For stability, it can pair its electron with another hydrogen atom’s
electron. In this way the two atoms have a stable orbital of two electrons. Helium
is an inert
(non-reacting) gas. It has two protons and two electrons and therefore does not react with other
substances (see Figure 21.4).
Ionic compounds, such as salts, are made up of charged atoms (ions). In the
case of common salt (sodium chloride), the sodium
atoms react together (see
Figure 21.7). The metallic sodium atom gives away its negative electron, thus becoming a positively
charged ion called a cation:
At the same time, the chlorine atom receives the negative electron
from the sodium, thus becoming a negatively charged ion called an anion:
Ionic substances, such
as sodium chloride and many fertilizers, allow an electric current to pass through them when they
are dissolved in water. Horticulturists are able to assess the strength of dissolved fertilizers in soils
and composts by measuring this current (conductivity).
Some anions occur in a compound form. Carbonate (CO32-
), Nitrate (NO3-
), phosphate (PO43-
) and sulphate (SO42-
) are some examples. One cation, ammonium
) is commonly found in fertilizers. In contrast to ionic compounds, the element carbon shares
electrons with the element it reacts with to produce molecules, many of which are very large (see carbon chemistry
is a gas like hydrogen but differs from it in several ways. It is much heavier, having 8
protons and 8 neutrons (and 8 electrons) and an atomic weight of 16. Its inner electron orbital is stable, with 2 electrons, leaving
six electrons in the second orbital. Oxygen therefore needs to receive two electrons to become stable. It can be seen that oxygen’s
combining power is twice that of hydrogen. Oxygen is thus said to have a valency
|Table 21.1 Chemical information on horticulturally useful
(combining power) of two while hydrogen’s valency
is one. When oxygen and hydrogen react together, it becomes clear from the description above that the substance produced by the
reaction is H2
O, or water
, since the two hydrogens are needed to fill oxygen’s orbital.
Horticultural plants require 15 elements for their growth. Table 21.1 lists the atomic weights and valency for each of the elements.
Examples of simple molecules are given in the ‘common compounds’ section of Table 21.1 . The molecular weight
of a substance can be calculated by adding the individual atomic weights of the elements within it. For example, the molecular weight
of ammonium nitrate (NH4
) is 14 + 1 + 1 + 1 + 1 + 14 + 16 + 16 + 16 = 80. Note that there are two nitrogen atoms in the
compound, contributing 28 parts out of the total of 80 i.e. 35 per cent. Fertilizers of this type are not quite as pure and typically contain
33 per cent nitrogen.