Читать книгу Mountains and Moorlands - W. Pearsall H. - Страница 16

On the whole, the unstable surfaces are areas of enrichment, and as such show distinctive types of vegetation. When they become stable enough to permit a complete vegetation cover, they inevitably tend to become leached—a tendency that is in some slight measure accelerated or retarded according to the nature of the plant cover. Because heavy rainfall is the rule and because plant remains tend to accumulate on the soil surface, it is characteristic of upland soils, once soil-creep and the forces of erosion are sufficiently arrested, that they rapidly develop the stratification or “profile” indicative of the more advanced (or “mature”) stages of soil development.

The extreme form of stratification found is that of the soil type known as a podsol, in which, under a surface layer of peaty humus and of humus-stained soil, there is a grey soil-layer from which almost all of the available bases (especially lime and iron) have been removed by leaching. With them also have gone the finest particles of clay and the humus colloids. These accumulate at a lower level, commonly two or three feet below the surface, as a dark-brown layer of “humus pan,” while immediately below this can usually be seen a red- or orange-brown precipitate of iron compounds (“iron pan”). Still lower is the little altered parent material. Thus these highly leached soils show a characteristic soil profile, with the following layers:

A. a Surface peaty humus and “litter”

b peat-stained inorganic soil

c leached grey ” ”

B. d humus accumulation zone (“pan”)

e iron ” ” (“pan”)

C. f little altered parent material

In continental areas, podsols characterise the cold temperate climates on stable and porous substrata receiving moderate rainfall. They are there associated with the northern evergreen forests of coniferous trees (firs) and with an abundance of small shrubs like the heathers.

In the British uplands, podsols are most characteristic of the eastern regions where the annual rainfall is relatively low (say about 35–40 in.) and the parent material is often sandy or gravelly morainic material. They are often now associated with heather-moor and pine forest, and originally this association may have been still more widespread. In the wetter western areas, although podsolic features in the soils are frequent, good examples of podsols are infrequent. Often this is because of great irregularity or diversity in the composition of the parent material. In particular, the deposits of iron or of peaty colloidal matter which suggest the appearance of a podsol are often due to lateral seepage from higher up the slope and are not necessarily derived from the soil-layers immediately and vertically above them. Varying porosity, in particular, leads to very irregular local accumulations of the humus and iron colloidal layers, which may appear in blotches along the lines of seepage and at a varying distance below the surface. But it is probable that most of what were once free-draining forest soils have now been transformed to bog (see below and Chapter 10), and that those which are left are but transitional stages.

Between the extreme podsols and the young or slightly leached soils, there is a range of soil profiles usually classified together as “brown earths” from their ochreous brown colour, and in the lowlands characteristic of forests of deciduous trees, oak, beech and the like. The surface layers have been somewhat leached of bases but retain a brown colour along with a base-status sufficient to give a moderate fertility. This is the characteristic soil-type of lowland Britain. In the uplands, however, this condition can only be long maintained, where the original rock fragments are base-rich, where flushing of some sort maintains the base-supply, or where the vegetation is such as to renew the base-supply in the surface layers. The latter condition would be favoured, for example, by oak woodlands rather than by a covering of birch or of pine, for the lime contents of the leaves of these trees differ considerably: that of oak approaches 3 per cent, while those of birch and of pine are only about 1·5 and 1·0 per cent respectively. Thus by absorbing more lime from the lower layers of soil and returning it to the surface, oaks would maintain the base supply of the surface soil-layers for a longer time and so would tend to retard the effect of leaching.

The upland soils of brown earth type are usually recently stabilised “creep soils,” flushed glacial drifts, or soils derived from base-rich rocks. Almost always they were until recently under woodland, normally of oak though often with much ash. Now they are almost always cleared of trees and are covered by grasslands of various types. The removal of the original tree-cover was often followed by destruction of the surface humus through its oxidation and by the removal of the surface layers by rain-wash. Thus many upland soils of this general type, like the “frydd” soils in Wales, have been considered now to show “truncated profiles,” the top strata having been removed, often by erosion. In some cases, however, it is also probable that the profiles are immature and that the soils owe to this their comparatively high base-status.

It is quite clear that generally in the uplands the process of leaching can only be retarded and not completely stayed. The high summer rainfall in particular ensues that leaching will continue under the most favourable conditions of temperature. In lowland climates, in contrast, there is in summer an excess of evaporation and drying in the surface layers of soil so that base-rich water ascends from below by capillarity. This opportunity for replenishment is lacking even in a well-drained upland soil so that the soils as a whole must tend towards the leached condition.

Thus it seems inevitable that any porous soil, once stabilised, must ultimately develop towards a podsolic stage. In the majority of cases it seems that the process has not ceased at this stage. The downward movement of fine particles of clay and humus which characterises podsol development leads to the formation of impermeable pan-layers which impede drainage. Thus under conditions of high rainfall the upper layers of soil become waterlogged, seasonally if not permanently. This leads to peat accumulation, which in turn accentuates both leaching and poor drainage. Thus in mountainous Britain, podsols have almost always tended to become peat-covered bog soils, such as are described below, and it seems probable that the characteristic podsolic profile becomes modified when this change takes place, and ultimately disappears.

These trends of soil development favourable to bog formation are undoubtedly much accentuated by the topographic relations of the mountain soils. Once bog conditions have been established at any point on a hillside—a process which in most cases clearly must have taken place at an early stage in the physiographic history of the area—there must inevitably be a tendency for bog seepage to extend downward and to produce a general degeneration in the soils below it. The greater the area of peat accumulation in the upper bog zone, the more rapidly would this influence extend downwards. Moreover, because a mountain is a gathering ground with a considerable excess of rainfall over evaporation, it seems likely that the lowest slopes will often also show a marked trend towards the establishment of bog, the degree to which this happens being controlled by the geological structure of the mountain and the amount of mineral bases (lime, potash and so on) the mountain yields to the seeping water. So soon as the mountain-side becomes stable enough to show signs of leaching these trends are strongly in favour of it passing over to bog. Thus the upland soil problems are very complex, affected not only by the immediate characters of the soil but by lateral movements, by adjacent relief, rock and vegetation.