Читать книгу Mountains and Moorlands - W. Pearsall H. - Страница 17

The chemical stages which can be distinguished during the leaching process can now be outlined. In the first instance, most British soils are principally calcium soils, or, putting it in another way, they have lime as their principal base, and are lime-saturated. Agriculturally and ecologically, soils of this type possess high fertility. As leaching goes on, most of the lime and other bases are removed, being replaced by hydrogen, so that the soil finally becomes acid and sour (e.g. to the taste). For ecological purposes there are, however, two intermediate stages in this process which can usefully be distinguished: a state of partial lime-deficiency and one of higher lime-deficiency. Most British soils also contain a good deal of iron oxide—to which their colour is largely due—and under conditions of good aeration this is removed much more slowly than lime. The following four types of soil can conveniently be recognised as stages in the leaching process:

1 Lime-saturated;

2 Lime-deficient;

3 Base-deficient, iron oxide becoming mobile and relatively more important than lime;

4 Acid, with podsolic profiles in stable soils, often masked by peat accumulation.

Of these b and c represent the stages usually referred to as brown earths. The ecological value of this series lies in the fact that very decided transitions in vegetation occur at a point between b and c (which is also approximately half-way between a and d)—that is, a point of half-saturation with bases.

The technical methods of distinguishing these soil-types depend on the methods of measuring the percentage of base-saturation, which decreases from a to d, or of measuring the increasing acidity. The latter is perhaps a mode of expression more familiar to biologists. Estimates are made of the hydrogen-ion concentration and expressed in the following way. Concentrations, varying, for example, as 1/100, 1/1000 or 1/10,000 gm. per litre of hydrogen ions can be written either 1/10², 1/10³ or 1/10⁴, or 1 x 10^-2, 1 x 10^-3 or 1 x 10^-4 g/l, and for convenience these are termed pH 2, 3 or 4 respectively. The notation extends over a range of 1 to 14. In terms of this notation, pure water has a pH value of approximately pH 7, lime-saturated soils have a somewhat similar pH value, of above 6, while natural soils which are about half-saturated with bases have a pH value of approximately 5. The characteristic acid soils in the ecological sense lie below pH 3·8.

These soil types can, however, often be distinguished by their appearance and biological characters. The grey and leached zone in a well-developed podsol is likely to be mainly a hydrogen soil, as is the humus-stained layer on the surface. In many upland soils, the leached but still brown layer of inorganic materials below the surface humus has a characteristic orange-brown colour—not grey as in a proper podsol. This condition is associated with the removal of most of the lime and the mobilisation of iron, at first perhaps dissolved from the soil minerals near the surface by humus compounds, but then reoxidised on the surface of the soil particles in a state which accounts for the characteristic colour. This type of soil is probably very definitely associated with periods of waterlogging, such as are frequent in upland areas, and it normally has a lower base-status than the more typical soils of brown-earth type.

In these and in other ways, therefore, the characteristic appearances of soil profiles give a good deal of information about the base-status of the soils. It is perhaps worth emphasising also at this stage that two factors in particular are especially effective in removing iron and other bases during the final stages of the leaching process.

One of these is the increased acidity and especially the effects of acids derived from plants like oxalic and citric acids, in which iron salts are especially soluble. The second is the establishment or development of waterlogging, which, by eliminating the oxidising effects of air or oxygen, permits the reduction of iron to the ferrous state, in which it is very much more easily soluble, as well as more readily replaced by the process known as base-exchange. In aerated soils, however, waterlogging can only be temporary even though it must be frequent in winter in all upland soils. When it occurs permanently the soil commonly acquires a blue-grey appearance which we associate with the presence of ferrous iron compounds, and this contrasts very noticeably with the reds and browns of the ferric salts in air-containing soils.

Of course, where there are extensive areas of waterlogged soil and especially where peat is abundant, large amounts of ferrous salts may be present in solution in the soil-water. Wherever this becomes exposed to air it becomes oxidised either to metallic iron or to ferric salts and so considerable amounts of ferric substances may be precipitated. The orange-brown or metallic films due to this process are familiar objects round any peaty spring, and, long continued on a large scale, it has in the past been responsible for the production of deposits of bog iron ore. The same process continues in any peaty flush soil which receives drainage from waterlogged surfaces and into which air can, at times, penetrate. To the ecologist, the colours due to iron compounds are of great importance as visual evidence of the existence and progress of leaching or accumulation and also as useful clues to the presence or absence of aerated conditions in the soil. There are, of course, many chemical tests which can be used to confirm and extend these visible signs.