n the subsoil. We can
also explain why gardeners and farmers speak of black soils as rich
soils; they contain more than other soils of this black material that
makes plant food. Still further, we can explain why the farmer often
sows plants like mustard, tares or clover, and then ploughs them into
the ground. They are not wasted, but they make food for the next crop
that goes in.

Now let us turn to the results of the subsoil experiments. The leaves
and stems have increased the crop, but only by 5.4 grams: they have not
been nearly so effective as in the surface soil. It is evident that
the mustard did not feed directly on the leaves and stems put in; if it
had there should have been an equal gain in both cases. The leaves and
stems clearly have to undergo some change before they are made into
plant food and the soil has something to do with this change. After
the crops are cut the soils should be tipped out and examined. More of
the original pieces of leaf and stem are found in the subsoil than in
the surface {52} soil. That is to say, there has been more change in
Pot 6 containing surface soil than in Pot 7 containing subsoil. The
"something," whatever it may be, that changes plant remains like
leaves, stems, pieces of grass, roots, etc. into plant food therefore
acts better in the surface soil than in the subsoil. Here then we have
another difference between surface and subsoils.

SUMMARY. The experimental results obtained in this chapter may now be
summed up as follows:--

(1) Plant food is present in the top soil only and not to any extent in
the subsoil.

(2) There is not much present, so little indeed that we could not
detect it by weighing.

(3) It is, however, always being made in the top soil, if water is
present. Only little is made from the subsoil.

(4) The remains of leaves, stems, roots, etc. furnish an important
source of plant food.

(5) But they have first to undergo some change, and the agent producing
this change is more active in the top soil than in the subsoil.

(6) The top soil is much the most useful part of the soil and should
never be buried during digging or trenching, but always carefully kept
on top.

{53}

CHAPTER VII

THE DWELLERS IN THE SOIL

Apparatus required.

_Garden soil. Six bottles and corks [1]. Twelve Erlenmeyer flasks, 50
c.c. capacity [2]. Cotton wool. Milk (about half a pint). Leaf
gelatine. Soil baked in an oven. Six saucers [3]