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
| Soil | Vegetation | Percentage |
|---|---|---|
| Mountain-top detritus (2800–3000 ft.) | Moss-lichen grassland (see p. 85) | 3·6–4·0 |
| Mor over podsol (700–1000 ft.) | Heather-moor | 1·0–2·0 |
| Upland bog peat (1200–1500 ft.) | Mixed-bog with bog-moss (p. 147–152) | 1·7–2·4 |
| Flushed brown earth with mull | Oak-wood with soft-grass | 3·1–3·5* |
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.
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