Lessons on Soil | Page 6

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 "
{4} 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.
[Illustration: Fig. 2. Columns showing what 100 parts of soil and subsoil were made of]
The dried lumps of soil are now to be broken up finely with a piece of wood, but nothing must be lost. It is easy to see shrivelled pieces of plant, but not easy to pick them out; the simplest plan is to burn them away. The soil must be carefully tipped on to a tin lid, or into a crucible, heated over a flame and stirred {5} with a long clean nail. First of all it chars, then there is a little sparkling, but not much, finally the soil turns red and does not change any further no matter how much it is heated. The shade of red will at once be recognised as brick red or terra cotta, indeed "terra cotta" means "baked earth." When the soil is cold it should be examined again; it has become very hard and the plant remains have either disappeared or have changed to ash and crumble away directly they are touched. On weighing a further loss is discovered, which was in our experiment:--
Weight of top soil after drying but before burning ... 83 decigrams " " " " " " after " ... 76 " -- The part that burnt away weighed ... 7 "
Weight of subsoil after drying but before burning ... 87 decigrams " " " " " after " ... 84 " -- The part that burnt away weighed ... 3 "
These results are entered on the column in Fig. 2.
The surface soil is seen to contain more material that will burn away than the subsoil does. When the burnt soil is moistened it does not become dark and sticky like it did before, it has completely changed and cannot be made into soil again. It is more like brick dust than soil.
For further experiments we shall want a fresh portion of the original soil.
On a wet afternoon something was noticed that enabled us to get a little further with our studies. The rain water ran down a sloping piece of ground in a tiny channel it had made; the streamlet was very muddy, and at first it was thought that all the soil was washed away. But we soon saw that the channel was lined {6} with grit, some of which was moving slowly down and some not at all. Grit can therefore be separated from the rest of the soil by water.
This separation can be shown very well by the following experiment. Rub ten grains of finely powdered soil with a little water (rain water is better than tap water), and carefully pour the muddy liquid into a large glass jar. Add more water to the rest of the soil, shake, and again pour the liquid into the jar; go on doing this till the jar is full. Then get some more jars and still keep on till the liquid is no longer muddy but nearly clear. The part of the soil that remains behind and will not float over into the jars is at once seen to be made up of small stones, grit, and sand. Set the jars aside and look at them after a day or so. The liquid remains muddy for some time, but then it clears and a thick black sediment gathers at the bottom. If now you very carefully pour the liquid off you can collect the sediments: they are soft and sticky, and can be moulded into patterns like clay. In order to see if they really contain clay we must do the experiment again, but use pure clay from a brick yard, or modelling clay, instead of