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 such as sulphates 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