Geoscience Reference
In-Depth Information
acid, such as hydrochloric acid HCI, 'dissociates' in solution much more readily, giv-
ing as many as one free H ion to every ten that are still attached (a concentration of
10
-1
). On the other hand, caustic soda solution (1/10th 'normal'), produces very, very
few free hydrogen ions - only one in 10 million million (10
-13
). For convenience, this
wide range of values is represented on a logarithmic scale as the negative power value
of hydrogen ions. Thus the values given above for water, acid and alkali would be
quoted as pH 7, pH 1 and pH 13 respectively - the abbreviation pH being derived
from the original French expression 'puisance d'hydrogen'. (The feeble alkalinity of
blood is more neatly expressed on this scale as pH 7.35 rather than as 10
-7.35
or as 3.6
x 10
-8
).
Gardeners are usually only concerned with the more extreme cases of soil acid-
ity, typified by the ability or inability to grow heather and azaleas, or the need for lime
to be sprinkled around brassicas. Farmers are aware of more subtle differences which
are revealed in the growth of his crops or the productivity of his hill pastures. Such
plant indicators, however, are just the above-ground symptoms of pervasive influen-
ces acting within the soil environment. Many animals with hard external skeletons,
such as woodlice and snails, need calcium and therefore favour soils derived from ba-
sic rocks. Earthworms, too, need calcium for their digestive glands; most species react
unfavourably to acid conditions, whereas their smaller relatives, the enchytraeids, are
most numerous in slightly acid soils (
Fig. 5
). Soil microorganisms often flourish best
under neutral conditions but many kinds of fungi grow in acidic situations; in very
acid podzols (see
next chapter
)
, they are often the dominant members of the micro-
flora.
The typical range of pH values in soils runs from about 3.5 to a little over 7.0.
Soils in the range 3.5-4.5 (e.g. some heathlands) are considered extremely acid, soils
with pH 4.6-5.5 as very acid, 5.6-6.5 as moderately acid, around 7.0 as neutral, and
greater than this as calcareous or alkaline. Most arable soils are maintained by lim-
ing in the 5.5-7.0 range. At the extremes, colliery spoil containing the iron sulphide
mineral, pyrites, can have a pH less than 3, while some industrial waste tips, such as
those from the manufacture of sodium carbonate in Greater Manchester, can be ex-
tremely alkaline, as high as 12.7. The ecology and treatment of such chemically ex-
treme, man-made, 'soils' are considered in
chapter 10
.
Clay minerals tend to stabilize, or buffer, the pH of soils by resisting changes in
hydrogen ion concentration when small amounts of acid or alkali are added. If lime
is added, free H ions are neutralized but the clay then releases more H ions to restore
the original pH. Thus more lime is needed to produce the desired effect than would
be expected: a soil containing 50% clay will need more than twice as much lime to
raise the pH as a sandy soil with less than 10% clay. In natural conditions, sandy soils
