rnish any appreciable quantity of plant food.

Fill one pot with surface soil and another with the same weight of
surface soil well mixed up with 30 grams of plant remains--pieces of
grass, or stems and leaves of other plants cut up into fragments about
half an inch long. At the same time put up two pots of subsoil, one of
which, as before, is mixed with 30 grains of plant remains, and also
put up two pots of sand, one containing 30 grams of plant remains and
the other none. Sow all six pots with mustard and keep watered and
well tended. The result of one experiment is shown in Fig. 23 and the
weights of the crop in grams were:--

Green weight After drying

Top soil and pieces of plants (Pot 6) 42.0 5.0

Top soil alone (Pot 3) 17.7 2.6

Difference in top soil 24.3 2.4

{50}

Green weight After drying

Subsoil and pieces of plants (Pot 7) 10.5 1.9

Subsoil alone (Pot 4) 5.1 1.1

Difference in subsoil 5.4 0.8

[Illustration: Fig. 23. Pieces of grass, leaves, etc. change into
plant food in the surface but not to any great extent in the subsoil.
Mustard is growing in surface soil (Pot 3), in surface soil and pieces
of grass (Pot 6), in subsoil (Pot 4), and in subsoil and grass (Pot 7)]

Now let us look at these results carefully. The experiment with
surface soil shows that the pieces of stem and leaf have furnished a
good deal of food to the mustard and have caused a gain of 24.3 grams
in the crop. If we knew what the pieces were made of we {51} could
push the experiment still further and find out more about plant food,
but this involves chemical problems and must be left alone for the
present. We can, however, say that plant remains are an important
source of plant food, and since we suppose the black material of the
soil to be made of plant remains (see p. 36), it will be quite fair to
say also that this black material, the humus, is a source of plant
food. We have therefore answered the question we set, and we can
explain some at any rate of the differences between the surface soil
and the subsoil. The surface soil contains a great deal of the black
material, which forms plant food, while the subsoil does not. Thus
plants grow well on the surface soil and starve o