Mountains and Moorlands. W. Pearsall H.
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While the properties of the soil in wet flushes are determined largely by the inflowing water in bog soils, certain other properties commonly exist to which attention may now be directed. The development of a peat-covering not only marks a stage in the soil development but it also modifies subsequent development by acting as a blanket which insulates, as it were, the mineral soil from the plants growing on the peat surface. At first these plants are rooted and are drawing mineral matter from the soil, but they get less and less dependent on it as time goes on and the peat gets deeper, and the soil water becomes more and more that derived from rain. As a general rule, then, the vegetation might be expected to show a transition in its mineral salt requirements from eutrophic, with high demands, to oligotrophic, with low salt requirements. Two things result from this: first, a succession of vegetation types, and secondly, a resultant succession of peat types. We shall see later that these facts help us in the analysis both of moorland vegetation and of the history of moorland areas. For the moment, however, we are more simply concerned with its effect on the properties of the soils. There will clearly be a change in composition throughout the peat profile, and the amount of mineral matter present in the peat will decrease as the level rises above the mineral base. This is apparently a general rule in upland peats, though, locally, flush effects may disturb the normal sequence. It should be noted, however, that it does not usually apply to the actual surface peats. Moor-burning is an almost universal practice nowadays, and its effect is to destroy the existing vegetation, leaving the mineral matter it contains to enrich the surface peat. Similarly, any form of oxidation of the surface peat, due, for example, to drainage, must have a similar effect, for the oxidation products of the organic matter are mainly carbon-dioxide and ash—the former of which escapes to the air, leaving the ash to increase the amount of mineral matter in the residual peat. Thus the surface peat, where moor-burning is practised, commonly contains more ash than the layers below it. The table opposite gives illustrative figures from peat-profiles in different British areas.
There are, of course, other effects which appear to be associated with this distribution of ash. Thus the acidity of the peat almost always increases from the lowest levels upwards—showing a general correlation with the decreasing ash content.
It will be seen from Table 5, and it follows from the arguments used above, that typical upland peats are remarkable for the small amount of ash they contain, and when we seek to define the term bog, it is usual to regard it as referring to peat of this type supporting an extremely oligotrophic vegetation. In this use of the term a bog is mainly dependent on atmospheric water (i.e. rain) and uninfluenced by ground-water. The term bog contrasts in usage with the term fen—derived from the extensive peat deposits in East Anglia. In terms of this usage, fen-peat is characterised by its high mineral content and hence by its dependence on ground water. It usually shows signs of an abundance of lime and is always lime-saturated peat with a luxuriant and eutrophic vegetation of tall reeds and small trees, willows and alders. Peats of this type are almost non-existent in the British uplands to-day, although long ago they existed in some of the hollows where lime-rich waters accumulated in small ponds and lakes. These now often show their former character by an underlying bed of marl. Almost all these areas are now deeply buried beneath bog-peat, and only small areas of flush peat remain round the bog margins to illustrate the effects of this type of peat on the vegetation. Where the basal peats were originally calcareous the succession of peat types above usually shows a much more gradual decrease in mineral content, and the sequence of vegetation types was often different.
Table 5 ASH CONTENTS (AS PER CENT OF THE DRY WEIGHT) OF PEAT SAMPLES AT DIFFERENT DEPTHS
Finally, where a peat-profile generally shows signs of differences in botanical composition at different levels, it also usually shows differences in physical structure. The changes are due partly to alterations in the composition of the vegetation from which the peat was formed, and sometimes they may be due to changes in the conditions (of humidity or temperature or drainage perhaps) under which the peat was formed. As a general rule, however, the most important progressive change in the peat must be associated with its decomposition as it gets older. Two sorts of change are possible: partial oxidation, which occurs particularly in the surface layers, and the slower changes which can ensue in water-saturated peat from which oxygen is absent. We assume that these are mainly hydrolytic—that is, caused by the slow action of water on the organic materials present. These are the changes which are generally implied when we say that a peat is humified or that it is undergoing humification. They are thought to result in the plant-remains gradually becoming gelatinous, so that, although the peat appears to retain visible structure, it escapes as a jelly through the fingers when squeezed in the hand. In contrast, the more recent peats usually retain a firm and fibrous structure. Even when a peat bed appears, on first opening it up, to consist of more or less uniform material, the bottom layers will normally differ in the degree of humification from those at the top. One result of this is that if such a bed is cut and a profile exposed to the air, it will soon appear to consist of two different types of peat. The fibres quickly become prominent and some are exposed by weathering, while the upper layers which contain them are often more quickly oxidised and so may become darker in colour. The lower layers, however, remain as a damp gelatinous mass and show less alteration on exposure. After a period of exposure they may thus appear to be of quite different composition, although the apparent difference is really one mainly of different physical and chemical condition.
THE BIOLOGICAL CHARACTERS OF SOILS
The peaty nature of upland soils is one of their most easily observed attributes. It is by no means confined to waterlogged areas, but it is equally noticeable on leached podsolic soils and even in the early stages of soil-formation on mountain-top detritus. This clearly indicates that the climatic effects characteristic of these three extreme types of habitat—waterlogging, leaching and low temperatures—are alike in leading to humus or peat accumulation in the soils affected. They do so in a generally similar way by reducing the activity of the soil micro-organisms.
Their effects differ in some respects and it will therefore be convenient to consider them separately, although actually they overlap in nature to a considerable degree. The effects produced on and by the soil organisms in their turn affect the vegetation of larger plants, and hence, as later chapters will show, have much effect on the larger animals.
Numerically, the soil flora mainly consists of vast numbers of bacteria and fungi. These are colourless plants, mostly of microscopic size in the soil, that obtain their nutriment by chemically changing the remains of dead plants and animals. They are responsible for what we usually call decay, although the chemical processes involved are analogous to and, indeed, often identical with, the processes of food digestion and utilisation in animals. The breakdown of plant organic matter in soil is, however, very generally initiated by small invertebrate animals, sometimes by the larvae of flies but particularly, as Charles Darwin showed, by earthworms. These break down the cellular structure of plant-remains and partly transform it, making it suitable for further transformation by fungi and bacteria. Most soils also contain single-celled animals, protozoa, which browse on the fungi and probably serve to keep them in check. There are also insects, such as springtails and fly larvae, whose role in soil economy is usually less well known.
This large soil-population requires air, or rather oxygen from the air, for breathing and it is consequently said to be aerobic in character.
In a normal soil the chemical materials produced during these transformations by the soil organisms are substances containing oxygen, carbon dioxide, which escapes to the air, nitrates, which contain the nitrogen which was present in the animal and plant proteins, and other substances