RECLAMATION AND RESTORATION - The Soil - page 207

depth (cm) Profile description

Dark brown loam, large crumb structure, mully spongy, very porous
and friable … occasional worms … pH 4.8, fairly distinct from -

0-8

Brown loam, crumb structure deteriorating with depth … well rooted,
worms, merging into -

8-20

Grey silt loam, frequent large shale fragments and abundant small
fragments, structureless … merging into blue grey, scarcely
weathered shale.

20-31

A complex sequence of chemical changes is thought to occur in which the net effect
can be summarized as follows:-
iron sulphide + oxygen + water yields iron sulphate + sulphuric acid 6 FeS 2 + 21
O 2 + 6 H 2 O = 6 FeSO 4 + H 2 SO 4
An interesting feature of this process is the mediation of a particular bacterium,
Thiobacillus ferrooxidans (see chapter 6 ) , which appears to be necessary for two of
the intermediate stages. Without it, the full oxidation of pyrites does not occur or pro-
ceeds much more slowly. If there is much pyrites present, sites can actually deteriorate
with time because formation of the acid may continue for a hundred years, or possibly
several hundred years, before all of it will have been removed by natural leaching. On
the other hand, if there are substantial amounts of carbonate minerals present, such
as ankerite or siderite, these can act like lime in neutralizing the acid. High acidity is
directly toxic to plants, and it also initiates a series of other unfavourable conditions
in the spoil: by lowering the exchange capacity of the material, not only are beneficial
ions released and then more readily leached away, but harmful ions, such as alumini-
um, come into solution and exert phytotoxic effects. The rate of plant colonization,
and the kinds of grasses, herbs, shrubs and trees that become established, therefore
depend greatly on the net effect of these minerals on the pH of the spoil.
A large population of surface-living springtails can develop quite quickly on
some coal wastes, perhaps because of the scarcity of their natural predators. These
may perform a valuable role in the cycling of organic matter. The mite fauna tends
to be rather simple and is sometimes dominated by one or two oribatid species. Mix-
ochthonius latipes is particularly interesting because it occurs where the pH is as low
as 2.7, and has been found nowhere else in Britain. If the spoil is limed, numbers of
this mite decline in favour of another small oribatid, Oppiella nova. Colonies of M.
latipes presumably occur in some natural pockets of acid soil but these have yet to be
discovered. In Denmark and other parts of mainland Europe, this species is associated
with beech and oak forests, though even there it was only discovered in about 1950.