and phosphates. Of these, nitrates are generally regarded as most important because they are quantitatively the materials a normal plant requires from the soil in largest amounts. Thus the fertility of a soil is usually determined very largely by the rate at which it can produce nitrates, i.e. the rate at which the nitrogen locked up in the decaying organic materials can be released in a form available to plants. In actual fact, plants can also use ammonia, but the amounts of ammonia in a natural soil are normally small. This substance is the first simple substance to be formed by bacteria and fungi during the soil decompositions. It is converted by other soil organisms, the nitrifying bacteria, into nitrate. These processes of ammonia production and nitrate formation seem to be particularly sensitive to adverse soil conditions. So also are the parallel processes of nitrogen fixation by which a fertile soil is usually able to increase its nitrogen content (to an extent which balances leaching losses) by fixing the gaseous nitrogen present in the air. This process is brought about in most soils by bacteria, as well as by the nodule-forming organisms which are found living in the root-nodules of leguminous plants like peas, beans and clover. Adverse soil conditions almost always produce their effects by retarding these processes as well as the numerous other processes, e.g. of decomposition, going on in the soil. Special effects are produced by the different adverse factors.
AEROBIC AND ANAEROBIC—OXIDISING AND REDUCING SOILS
The principal result of a soil becoming saturated with water is that the amount of oxygen in the soil is reduced to vanishing point. Consequently as most of the soil organisms are aerobic, requiring oxygen, the soil population is reduced to the minimum, and there can remain active only a few anaerobic organisms with specialised methods of maintaining their existence without oxygen. The products of the decompositions going on in the soil also change in character. Instead of the formation of carbon dioxide, nitrates, sulphates and phosphates, all containing oxygen, there may be produced instead marsh-gas (or methane, CH4), ammonia (NH3), sulphuretted hydrogen (H2S) or other sulphides, and sometimes phosphine (PH3), a series of compounds devoid of oxygen. All of these products are associated with the activities of anaerobic types of moulds or bacteria, the latter usually being most abundant. The microflora of waterlogged soils is thus specialised in character as well as poor in numbers, while the products of anaerobic composition include substances, in addition to those mentioned above, which may be toxic to the larger rooted plants. Some of the products are also responsible for other manifestations peculiar to boggy soils, such as the “will o’ the wisp” and “corpse-light,” these being attributed to the burning of the highly inflammable marsh and phosphine gases.
In effect, in contrasting waterlogged and aerated soils in this way, we are contrasting two sorts of micro-biological activity—oxidising and reducing—depending on whether the organisms can form chemically oxidised products like carbon dioxide and nitrates or chemically reduced substances like marsh-gas and ammonia.
The particular value of being able to recognise these possibilities is because they give us information as to the effect of the soil conditions on the action of living organisms, and we may infer that the conditions which affect the soil flora will also affect its fauna (see here) as well as the larger plants. Moreover, the level of oxygen content which produces these biological effects is low and it is not one which can be detected in the field with any certainty, if at all, by measurements of soil oxygen. For our present purpose, therefore, it is useful to think of upland soils as belonging to the two types named above.
| oxidising soils contain | reducing soils contain |
|---|---|
| nitrate | ammonia |
| carbon dioxide | marsh-gas (methane) |
| sulphate | sulphides |
| phosphate | phosphine |
| ferric-iron | ferrous-iron |
It is useful also to realise that a large proportion of wet soils may be oxidising in drier periods and reducing in wet.
MULL AND MOR
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), 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
