among the surface detritus. The depth of the soil varies with the nature of the rock and the degree of erosion, but it is usually between one and three feet, and then comes disintegrating rock.
Parallel to these summit soils in general quality may be the scree-slopes of finely-divided material which occur lower down a mountain, often approaching stability but still subject to soil-wash and soil-creep, and so often distinguished as creep-soils. These show great variability in detail, but, like the mountain-top detritus, they often show coarse material at the surface and finer below. As they approach stability, they merge into the woodland soils described below, but in the earlier “gravel-slide” stages, a vertical section usually reveals a sequence of more or less alternating sandy or stony layers parallel to the slope (see Fig. 8). All soils of these types are alike in possessing a high base-status because they consist mainly of rock particles as yet not greatly modified by chemical change.
In all upland habitats there are in addition the overall trends caused by continual washing by rain, and as a result every exposed and porous surface will be more or less leached. Wherever leaching has taken place there must be corresponding areas that receive the products of leaching. The water that carries away lime or other bases from the higher upland surfaces must produce elsewhere lime-rich or base-rich habitats. Areas of this latter type may be distinguished as flushed or enriched habitats to distinguish them from the leached or impoverished ones.
FLUSHED SOILS
In general, of course, leaching will preponderate in upland regions and enriched soils will be commoner in lowland regions. Nevertheless enriched soils are always to be found occupying characteristic localities in mountain areas. Thus there is a flushed area around every springhead and around every rivulet. However, the water need not emerge as a separate spring but may perfuse the surface soil—a type of flush that can be recognised by a zone of greener vegetation. The various types of “damp flush” may be associated with a soil of almost any physical category. Enrichment by water from a higher level is greatest when such water has penetrated into the rock by means of structural fissures, and permeated the rock strata on its way down; a mere receiving area for surface run-off from acid upland soils is often as severely leached and as acid as the upland soil itself.
Parallel with enrichment by water (“damp flushes”), there is enrichment by presence of freshly-weathered rock particles, and areas of this type might be called “dry flushes.” The lower part of any steep slope is constantly enriched by such particles washed down from above. Screes and gravel-slides, in which the breakdown of new rock by weathering continually yields a supply of bases, could thus be considered among the enriched or flushed habitats. In this category also comes any unstable surface, crag, gullies and the like, where new rock surfaces are being exposed by erosion.
It will be observed that the flushed habitats are determined by a diversity of factors producing enrichment and have thus few physical characteristics in common. They tend to fall technically into four categories:
1 Bare rock or oversteepened slopes with soil particles washed away or present only in narrow fissures. Enrichment by continual weathering of freshly exposed rock surfaces.
2 Block scree in which leaching tends to preponderate over weathering, although the latter nevertheless does continually refurnish some of the bases lost, especially in the case of more rapidly weathering and base-rich rocks.
3 Unstable scree-slopes and solifluction areas with movement and accumulation of weathered soil particles, often below the surface layer of coarse detritus; enrichment both by weathering and particle accumulation.
4 Accumulation areas—nearly always showing fine and deep soils, with enrichment mainly by accumulation from above.
Upwelling of base-rich waters may occur in conjunction with any of the four categories above, although it is rarest in a and commonest in c and d. In both c and d the total “flush” effects normally counteract losses by leaching unless the soils are deriving from base-poor rocks.
LEACHED SOILS
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