Fig. 1. Soil and subsoil in St George's school garden
We want to learn something of the soil round about us, and we will begin by digging a hole about three feet deep to see what we can discover. At Harpenden this is what the scholars saw:—the top eight inches of soil was dark in colour and easy to dig; the soil below was reddish brown in colour and very hard to dig; one changed into the other so quickly that it was easy to see where the top soil ended and the bottom soil began; no further change could, however, be seen below the eight inch line. A drawing was made to show these things, and is given in Fig. 1. You may find something quite different: sand, chalk, or solid rock may occur below the soil, but you should enter whatever you see into your notebooks and make a drawing, like Fig. 1, to be kept for future use. Before filling in the hole some of the dark coloured top soil, and some of the lighter coloured soil lying below (which is called the subsoil), should be taken for further examination; the two samples should be kept separate and not mixed.
First look carefully at the top soil and rub some of it between your fingers. We found that our sample was wet and therefore contained water; it was very sticky like clay and therefore contained clay; there were a few stones and some grit present and also some tiny pieces of dead plants—roots, stems or leaves, but some so decayed that we could not quite tell what they were. A few pieces of a soft white stone were found that marked on the blackboard like chalk. Lastly, there were a few fragments of coal and cinders, but as these were not a real part of the soil we supposed they had got in by accident. The subsoil was also wet and even more sticky than the top soil, it contained stones and grit, but seemed almost free from plant remains and from the white chalky fragments.
A few experiments will show how much of some of these things are present. The amount of water may be discovered by weighing out ten grams of soil, leaving it to dry in a warm place near the fire or in the sun, and then weighing it again. In one experiment the results were:—
Weight of top soil before drying ... 10 grams = 100 decigrams
" " " after " ... 8.3 " = 83 "
---- ---
Water lost ... 1.7 " = 17 "
A column 100 millimetres long was drawn to represent the 100 decigrams of soil, and a mark was drawn 17 millimetres up to show the amount of water (see Fig. 2).
Weight of bottom soil before drying ... 10 grams = 100 decigrams
" " " after " ... 8.7 " = 87 "
Water lost ... 1.3 " = 13 "
Another column should be drawn for the subsoil. On drying the soil it becomes lighter in colour and loses its stickiness, but it has not permanently changed because as soon as water is added it comes back to what it was before.