Geoscience Reference
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Soil microbial populations tend to decline in stored heaps but quickly reassert
themselves when the topsoil is respread. Aerobic heterotrophic bacteria, Streptomy-
ces , ammonium oxidizers and fungi are all active within a few months, and build
up to high numbers as they consume dead organic matter. Comparisons in America
between a site restored with topsoil and an undisturbed site nearby suggested that the
microbial community had begun to stabilize after about four years, as judged by the
diversity and density of their soil fungi. Sites without topsoil had a less varied fungal
micro-flora, even after eight years, with a great predominance of Aspergillus and Pen-
icillium , suggesting an immature soil environment.
In the early years of agricultural restoration, the emphasis is on the need to estab-
lish good soil structure and to build up the nutrient status. These objectives are usu-
ally achieved by sowing a mixture of grasses and clovers with generous applications
of fertilizers for several years. A common programme for the first five years might
use about 2000 kg/ha of compound fertilizer - about twice the amount normally used
on farmland - with 22 tonnes/ha of ground limestone and 500 kg/ha of nitro chalk to
counteract any tendency to acidity.
The extraction of alluvial gravel, for example in the Thames valley, creates
somewhat different problems. Often the amounts of gravel removed are so large that
it is not feasible to fill in the holes that are dug, and so they have been developed
as reservoirs and attractive lakes. (The restoration of flooded gravel pits for amenity
and wildlife is well developed but does not concern us here.) However, there is also
a great need around urban centres for holes in which to deposit the vast quantities of
wastes which modern society generates. Furthermore, the gravels sometimes under-
lie prime agricultural or horticultural land. Increasingly, therefore, there has been an
effort to combine gravel extraction, waste disposal and agricultural restoration, using
the land three times over, through a rolling programme of extraction, infilling and soil
replacement.
If the waste consists of inert materials, such as builders' rubble, the problem is
mainly one of careful soil handling with the proper equipment and under good weath-
er conditions. A procedure has been developed by Green-ham Sand and Ballast and
used with great success for horticultural soils at Shepperton in Middlesex. Essentially,
this involves the use of two tracked excavators, for soil removal and reinstatement,
and a dumper truck for moving the soil from one site to the other. The topsoil and
subsoil are removed and replaced separately in bands a few metres wide. The dumper
truck always runs on the exposed gravel or filled surface to prevent compacting the
soil, and the excavator at the receptor site also runs only on the filled surface ( Fig.
68 ) .
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