The Chalk Flora
Most of Porton Down is covered by a shallow, welldrained, chalk-based
soil containing low levels of nutrients. These conditions favour the
development of a rich variety of plants, up to 40 different species
occuring in one square metre of turf. The plants are small and slow
growing, having developed in response to a long history of grazing,
by sheep or rabbits. The variety is maintained so long as the grazing
pressure continues.
In some areas the mineral content of the chalk has been leached away,
these support acid loving plants such as heather. Yet other areas have
a thin layer of clay overlying the chalk where oak trees, gorse and
bracken will grow. Some very low nutrient areas support a lichen-rich
flora.
The grasslands are of various ages, from about 50 years to at least
200 years old. Some areas have never been ploughed. It is possible to
correlate the age of a grassland with the presence of particular flowering
species in it.
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