Agriculture Reference
In-Depth Information
The
pH scale
is a means of expressing the degree of
acidity or alkalinity (see Figure 14.8).
Very low pH values are associated with the strong
acids such as sulphuric, nitric and hydrochloric acids;
strong bases (alkalies) such as caustic soda have very
high values. As they are diluted with more and more
water, their pH moves closer and closer to pH 7 which
is neutral - for example, pure water. Weak acids and
bases (alkalies) have values nearer to pH 7 (neutral)
even when they are concentrated.
The soils of Britain and Ireland are usually between pH
4 and 8; the vast majority being between 5.5 and 7.5.
Although there does not appear to be much difference
across this range, the significance for organisms living
in the soil is considerable. The pH scale is logarithmic
(like measuring sound in decibels or earthquake
energy release on the Richter Scale): each 'unit' is ten
times larger or smaller than the one next to it, that is,
pH 5 is ten times more acid than pH 6 and pH 4 is one
hundred times more than pH 6.
The ideal growing condition for most plants is a soil
of pH 6.5 because at this point all the essential plant
nutrients are available for uptake by the roots of most
plants. Although the majority of plants grow well in
soils between pH 6 and 7, there are considerable
differences in the tolerance of plants to soil pH
conditions (see Table 14.3). Potatoes are considerably
more tolerant of soil acidity than most plants and are
still productive down to pH 4.9. In contrast, the yield
of celery falls significantly in soils below pH 6.3.
Calcifuge
plants are those adapted to grow on
acid soils below pH 5.5.
Calcicoles
such as Clematis and
Prunus padus
have
adapted to the calcium-rich soils (high pH). Cultivated
plants that thrive on these soils include
Agapanthus
,
Clematis,
Geranium, Echinacea, Jasminum
,
Lonicera
,
Parthenocissus, Rudbeckia
and
Verbascum
.
Calcicoles
are plants that are adapted to grow
on calcareous soils (calcium-rich, chalky).
Acids, bases and the pH scale
X
Acids
are a group of chemicals that have a
sour taste and are corrosive.
X
Bases
are those chemicals that neutralize
acids. Bases include lime (calcium carbonate).
X
Alkali
('lye') is a are soluble base. They have
a soapy feel and tend to be irritants; strong
bases/alkalies are corrosive.
When bases (alkalies) and acids are mixed
they neutralize each other and form 'salts' - for
example, common salt which is sodium chloride
made from sodium hydroxide and hydrochloric
acid. Many fertilizers are formed this way, such
as ammonium sulphate and potassium chloride.
The pH scale
The
pH scale
expresses the amount of acidity or
alkalinity in terms of hydrogen ion concentration.
X
pH 7 = neutral, e.g. pure water
X
below pH 7 = acids, e.g. rainwater, carbonic
acid, fi zzy drinks, lemon juice (contains citric
acid), vinegar (dilute ethanoic acid), sulphuric
acid
X
above pH 7 = bases, e.g. limewater, caustic
soda
NEUTRAL
Acid
Alkaline
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Pure water
Rain
Soils
Stomach
Acid
Caustic
soda
Vinegar
Seawater
Limewater
Lemon juice
Figure 14.8
The pH scale
