Lessons on Soil

by Edward J. Russell

Our supposition explains why, in the case of soils, the less the blackness, the less the loss on burning. If the brown or black combustible part is really mould formed by the decay of plant roots, etc., then we should expect that as the percentage of mould in the soil increased, so its blackness would increase and its loss on burning would become greater. This actually happens.

This, then, is our idea. We suppose that the plants that have lived in past years have decayed to form a black material like leaf mould which stops in the soil, giving it a darkish colour. The more mould there is, the darker the colour of the soil. We know that along with this decay there is a great deal of shrinkage. As the black material is formed from the plant, it only extends as far into the soil as the plant roots go, so that there is a sharp change in colour about 6 inches below the surface (see also p. 2). Like the plant the black material all burns away when the soil is heated sufficiently.

Thus we can explain all the facts we have observed, and in what seems a very likely way. This does not show that our supposition is correct, but only that it is useful. When you come to study science subjects you will find such suppositions, or hypotheses as they are called, are frequently used so long as they are found to be helpful. In our present case we could only get absolute proof that the black combustible part of the soil really arose from the decay of plants by watching the process of soil formation. We shall turn later to this subject.

The black material is known as humus. Farmers and gardeners like a black soil containing a good deal of humus because they find it very rich, and we shall see later on why this is so. Vast areas of such soils occurring in Manitoba, in Russia, and in Hungary are used for wheat growing, while there are also areas in the Fen districts of England.

There is something known as peat that looks rather like mould, but is really so different that you must be careful not to confuse the two. Peat is not good for plants, and does not make the soil fertile, but quite the reverse. You can see it being formed on a moor or bog, and you should at the first opportunity go and examine it. There was a peat bog near Wye that was examined with the following results. The peat was very fibrous and had evidently been formed from plants. It made a layer about 2 feet thick and underneath it was a bed of clay; this was discovered by examining the ditches, some of which cut right through the peat into the clay below. A sample of the clay put into a funnel, as on p. 14, did not allow water to pass through; this was also evident from the very wet nature of the ground. The peat bed was below the level of the surrounding land and was in a sort of basin; the water draining from the higher land could all collect there but could not run away, indeed it might very well have been a shallow lake. It was quite clear that the plants as they died would decay in very wet soil, and so the conditions are very different from those we have just been studying where the plants decay in soil that is only moist. This difference at once shows itself in the fact that peat generally forms a thick layer, while mould only rarely does so. In the north of England the moors lie high, but here again the peat bed is like a saucer or basin, and there is soil or rock below that does not let the rain water pass through. For a great part of the year the beds are very wet.

Look at a piece of peat and notice how very fibrous it is, quite unlike leaf mould. When it is dry peat easily burns and is much used as fuel in parts of Scotland, Wales and Ireland. It is cut in blocks during the spring, left to dry in heaps during summer, and then carried away in autumn. Fig. 19 shows a peat bog with cutting going on. Peat does not easily catch light and the fires are generally kept burning all night; there is no great flame such as you get with a coal fire, but still there is quite a nice heat.

Peat has a remarkable power of absorbing water. Fill an egg-cup with peat, packing it as tightly as you possibly can, and then put it under water and leave for some days. The peat becomes very wet and swells considerably, overflowing the cup just like the clay did on p. 12. After long and heavy rains peat in bogs swells up so much that it may become dangerous; if the bog is on the side of a hill, the peat may overflow and run down the hill like a river, carrying everything before it. Such overflows sometimes occur in Ireland and they used to occur in the north of England; you can read about one on Pendle Hill in Harrison Ainsworth's Lancashire Witches. But they do not take place when ditches are cut in the bog so that the water can flow away instead of soaking in; this has been done in England.

This great power of absorbing water and other liquids, so terrible when it leads to overflows, enables peat to be put to various uses, and a good deal of it is sold as peat-moss, for use in stables.