Geoscience Reference
In-Depth Information
30 years before the study was made in the early 1970s. The longest established grass-
land had not been cultivated for about 150 years. The historical evidence showed that
cropping continued at different locations until the soil nutrient status and its organic
matter content had declined to levels at which, in the conditions of the time, further
cropping was uneconomic. The land was then left to revert to grass naturally. At most
sites, the abandoned crop land passed through a succession of grassland types during
which the soils regained their organic matter and nutrient content from the accumula-
tion and incorporation of grass litter. The annual production could be fully returned to
the soil since there was virtually no domestic livestock to remove it.
F
IG. 53
Ant-hills of
Lasius flavus
on chalk grassland at Porton Down in Wiltshire, covered with rock-rose. (See
Table 5
; Photograph B.N.K.D.)
The organic matter content of soil is readily measured, and so it was possible
to relate its gradual increase at Porton to the time since cultivation ceased.
Figure 52
shows this increase in the top ten centimetres of soil, from samples taken along sev-
enteen transects through different vegetation types of known age. From an extrapol-
ated starting point of about six percent, the organic matter reached about eight percent
after 30 years, and from 16 to 26 per cent on areas that had reverted to grassland 150
years ago. This gave an average annual increment of 0.08 percent organic matter over
the whole period.
As mentioned earlier, the yellow ant
Lasius flavus
is abundant in these uncul-
tivated grasslands. It makes an important contribution to the cycling of organic mat-
ter and nutrients, and also has significant effects upon the soil profile and vegetation
