Lessons on Soil | Page 6

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 "

{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