Mountains and Moorlands

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In some respects the quantitative effects of leaching are similar to those produced by saturation with water—namely, a great reduction in the activity of the soil organisms. The qualitative effects of leaching on the soil micro-flora are, however, even more pronounced, and so much so that it is customary to give a special name, mor, to the peaty humus formed in leached soils, in order to distinguish it from the more fertile leaf-mould (or mull) typically associated with fertile forest soils. While mor is chemically different, as we shall see later, its most noticeable distinguishing features are biological and are easily recognised. There is a vegetation dominated by plants such as heathers, bilberry and wavy hair-grass (Deschampsia flexuosa), and normally an absence of earthworms. Usually, too, no tree seedlings are to be found except those of pine and birch. Moreover, leguminous plants such as clover are absent, while suitable tests show that the soil lacks nitrogen-fixing bacteria or, at least, effective strains of this type. Finally, the mor soil has a high and characteristic degree of acidity (see here (#litres_trial_promo)), normally marked by a pH value below 3·8. It yields low proportions of ammonia, while nitrates are absent. It is evident, indeed, that in mor the rate and extent of nitrogen transformation is greatly reduced, presumably by the reduction in the numbers of suitable bacteria. It is in fact usually considered that the micro-flora of this type of humus contains few bacteria and is mainly one of moulds and of other fungi like actino-mycetes and basidiomycetes, but the evidence is far from conclusive owing to the difficulties of identifying these microscopic organisms in a dark-coloured peaty soil. Moulds and other fungi are generally more tolerant of acidity than are many soil bacteria, which fail to develop either in acid or in lime-deficient maedia.

It may be useful to add here a note on the soil animals which are more characteristic of mor soils. They include certain mites, the larval stages of two-winged flies (Diptera), click-beetles (Elaterideae), and often centipedes and predatory beetles. In contrast, earthworms, snails and millipedes are particularly characteristic of good forest leaf-moulds.

Mor soils in the strict sense in Britain are confined to the acid types of soil (normally with a podsol profile) distinguished in an earlier section. Characteristic mull soils are found on lime-deficient as well as on lime-saturated soils. More strongly base-deficient soils generally have humus of this type which produces nitrates (for example) rather slowly and may, at times, be somewhat intermediate in other respects.

The third factor which must profoundly affect the properties of the soil organic matter is the low temperature of many upland habitats. There can be no doubt that the soil organisms, like other forms of life, have their activity greatly reduced by low temperature. Hence the rate of decomposition of soil organic matter declines very rapidly in cold climates and peat accumulates, for the rate at which higher and larger plants form organic matter is less affected by low temperature, depending rather on the carbon-dioxide content of the air and on light. So far as I am aware, little or no investigation of the characters of this peaty material has been attempted, and the matter would probably be most easily examined by studying the rather peaty humus that accumulates on and among the mountain-top detritus. This is usually a black and easily crumbling peat generally containing a good deal of sand. It differs very markedly from a typical mor, such as that from below heather, for example, which is generally red-brown in colour, closer in texture and more acid. On the other hand, both soil-types have certain features in common such as some resemblance in fauna, e.g. scarcity of worms and frequency of dipterous larvae. There is also a good deal of evidence, though it is mainly derived from studies in arctic regions, that the bacterial processes affecting the accumulation of nitrogen (by fixation) and its liberation in forms suitable for plant food are especially curtailed by low temperature. As a result, there is certainly a notable scarcity of soluble forms of nitrogen in mountain-top habitats, though the limited available evidence suggests that this scarcity is not as severe as is the case in either bog-peat or acid mor. In my own observations, about half the tested soils have given traces of nitrates in late summer, and higher proportions might possibly be observed at other times. The interesting feature of the samples of humus from among mountain-top detritus has been their high proportion of nitrogen, usually 4 per cent of the total humus content. This does in fact suggest that there has been a rather slow decomposition of nitrogenous materials, though this is not the only possible explanation. The following short table summaries a few values for the nitrogen proportions of characteristic humus types.

Table 6 NITROGEN CONTENT OF DIFFERENT SOIL TYPES AS PER CENT OF THEIR HUMUS CONTENTS

This table brings out the low nitrogen content of the upland bog-peats and mor when contrasted with the mull humus of an oak-wood. Still more evident would this contrast be if we included the humus of lowland woods or of agricultural soils. The well-decomposed humus of a fertile arable soil, such as a wheat field, has commonly a nitrogen content of between 4·5 and 5 per cent, a value which seems to be a characteristic of soils in the lowland north temperate climate.

SIGNIFICANCE OF NITROGEN CONTENT OF HUMUS

The lower nitrogen content which is characteristic of upland peats and humus is due partly to the slow breakdown of the original plant materials. These, as a general rule, are rich in carbon and poor in nitrogen, though these proportions vary with the plant species. Consequently undecayed plant remains reflect principally the low nitrogen content of the original material. On the other hand, the ultimate nature of the humus produced depends also on the relative rates of breakdown and removal of the two main components—which are the carbonaceous and nitrogenous compounds. In many peats the soil bacteria are unable to attack cellulose, one of the principal materials containing carbon—a substance which makes up the bulk of plant cell-walls and skeletal structures. Slow though the breakdown of nitrogenous matters may be, therefore, it still is often more rapid than that of the chief carbon compounds, so that the latter gradually become more abundant and the percentage of nitrogen remaining in the humus becomes less. This condition has a further harmful effect on the activity of the soil fungi and bacteria, for most of these organisms prefer a growth medium containing a low C/N ratio. Thus in order to facilitate the breakdown of dead leaves in a rubbish-heap, gardeners often mix with it nitrogenous manurial materials, a treatment that results in a greatly accelerated rate of bacterial decomposition of the plant remains.