process is longer continued, ultimately to yield the small mineral particles which form the basal material of most soils. The weathered material may remain in situ, covering the original rock surface, or it may be removed by erosion and redeposited elsewhere by streams and rivers as banks of silt or alluvial plains, by solifluction or rain-wash, or formerly in Britain by widespread glacial movements.
The raw mineral material is, however, comparatively sterile. It is converted into what we call a soil partly by chemical modifications resulting from the presence of water, often charged with carbon dioxide or humic acid, and partly resulting from the gradual accumulation of organic materials derived from plant remains. This latter material is called humus, and is particularly important because it forms a medium upon which can grow various micro-organisms, mainly bacteria, moulds and protozoa. With the accumulation of humus and the gradual colonisation of the material by these organisms comes a final stage, when it is usual to imagine that the original particles of mineral substance have become covered by a jelly-like mass of colloidal material—in part gelatinised minerals but also including humus—on and in which the population of soil micro-organisms lives.
It will be evident from this brief summary that upland soils can usefully be considered as belonging to a developmental series. But it is true of any soil that one of its outstanding characteristics is its capacity for change. Soils are inherently dynamic systems even when they are developed in physically stable situations, and to a far greater extent is this true of mountain soils, most of which are of geologically recent origin, even if not physically unstable.
Five types of environmental factor control the development of a soil mantle. First comes the nature of the rock or other parent material, from which soil is formed by physical and chemical weathering. Climate also exerts a marked effect on the weathering process, affecting both its physical and chemical parts, and, in particular, determining the amount of rain-water percolating through the soil in any season, a process known as leaching, which is responsible for the removal of soluble substances, bases like lime as well as plant nutrients like nitrates. Relief influences the lateral movement of percolating water down a slope, the degree of drainage and the stability, and thus affects the degree of leaching. But none of these effects is instantaneous and so there is a time-factor to be considered. Lastly, there are the obvious biological factors, of which the action of vegetation is most significant. Vegetation derives part of its sustenance from the soil and so incorporates a portion of the soil material which is returned to the soil on the decay of the plant tissues. The fertility of a soil is the result of this cyclic exchange. An efficient type of plant which draws heavily on the soil nutrients keeps them in a form of biological circulation which mitigates the losses due to leaching. Thus there is a natural mechanism for maintaining soil fertility, which, by drawing on the deep layers of the soil, is capable even of increasing the fertility of the surface layers provided the leaching factor is not too intense. Further, in any environment where the climatic factors have remained reasonably stable for a long time, it is possible for the soil-vegetation system to achieve a measure of temporary stability. In upland Britain, however, the soils are generally in dynamic states moving along definite trends of soil development. The trends due to a severe climate are particularly marked, and they operate during the different stages of soil development in the following manner.
SKELETAL AND IMMATURE SOILS
The initial stages of soil development in which rock fragments predominate are what we can only call skeletal soils. They are found principally where the surface is unstable or where further development is retarded by hard rocks or low temperatures. Of these factors, the low temperatures have also distinct qualitative effects, because while they greatly retard chemical modifications, the associated physical disintegrations caused by frost and solifluction are especially vigorous. There may thus be much physical commination of rock fragments with little chemical change. Thus the soils, even if finely divided, are immature in the developmental sense because of the deficiencies in their chemical and biological equipment. Soils of this general type occur on mountain-tops, where they are found under the mountain-top detritus except, perhaps, where it is especially coarse and deep. Generally, however, the detritus seems to be a superficial layer of stones extruded from below during the frost-caused or solifluction movements of the materials. Beneath the stones there is commonly a sandy loam, generally brown in colour and little leached. When vegetation is present this merges at the top into the almost black humus that collects 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
