NATURAL HABITATS - The Soil - page 154

heath rush Juncus squarrosus and Sphagnum mosses become dominant. These soils
typically have extremely low pH values between 3.8 and 4.2.
The erosive power of glacier ice ground up and transported a mixture of these
rhyolite rocks with other rock types, depositing the material as glacial drifts in valley
floor situations within the Snowdon reserve. A complex of different soil classes now
occupies the crests and slopes of the hummocky drift terrain, and the hollows between
these hummocks. This association of Peaty Podzols on crests and upper slopes, Peaty
Gleys on lower slopes, and deep peat in the hollows, supports different vegetation
communities, controlled by variations in soil drainage. All these soils are slightly less
acid than the shallow peat soils, with pH values generally between 4.1 and 4.7, but all
have peaty surface horizons. At the drier end, the peaty podzols carry acid grassland
dominated by mat-grass Nardus stricta and sheep's fescue Festuca ovina. Rushes,
sedges and mosses are prominent in the peaty gley soils, and the deep hill peats are
dominated by cotton-grass Eriophorum and Sphagnum mosses.
In contrast to the virtually inert rhyolites, other outcropping rock types on Snow-
don weather chemically, or break down physically, very fast. Some slightly calcareous
soft volcanic ashes break up within tens of years when freshly exposed. These rocks
contain minerals which, on weathering, release calcium and magnesium. As a result,
Brown Earth soils with mull humus ( Fig. 13 ) occur on sloping sites side by side with
the peaty moorland soils on the more acid rocks and drifts. This is unusual as Brown
Earths are not the normal soil response to such cold wet climates; the rainfall here is
about 3,300 millimetres a year (130 inches). They support a relatively productive hill
grassland with sheep's fescue and bent grass Agrostis tenuis accompanied by clover
and a variety of herbaceous species. The rock type and sloping ground initially pro-
duce soils that are only moderately acid (pH 5.2-5.8) and are freely drained, but their
profile character is maintained by physical and biological cycling of material.
As with the woodland soils on limestone at Bedford Purlieus, moles and earth-
worms play a key role in this process by bringing mineral material to the soil surface.
Their activity counteracts the leaching and loss of soil calcium, and the consequent
soil acidification, which the high rainfall and free drainage would otherwise produce.
Additions of freshly weathered material from rock exposures at the head of the slope
continually replenish the upper part of the soil profile. The soil depth remains largely
unchanged, however, since fine particles are washed right down the slope to accu-
mulate ultimately as thick silty sediments on the lake floor at the foot. The sheep
that graze this area also help to maintain the level of plant nutrients in the soil by
their droppings. They can select where they feed on these open hill grazings, and
naturally favour the more palatable Agrostis-Festuca grassland to the less nutritious,