n that stored
dry, although the difference here is less marked. In an experiment in
which the soils were put up early in July and sown at the end of
September the weights of crops in grams obtained were:--

{48}

Green weight After drying

Plants grown in top soil stored in 16.9 2.6
moist condition (Pots 10 & 11) 18.9 2.8

Plants grown in top soil stored in 12.1 1.8
dry condition (Pots 8 & 9) 14.4 1.9

Plants grown in subsoil stored in
moist condition (Pot 13) 5.5 0.9

Plants grown in subsoil stored in
dry condition (Pot 12) 5.6 0.8

The crops on Pots 10 and 11 ought of course to weigh the same, and so
should the crops on Pots 8 and 9. The differences arise from the error
of the experiment. In all experimental work, however carefully carried
out or however skilful the operator, there is some error.

There is clearly an increase in crop as a result of storing the surface
soil in a moist condition, showing that additional plant food has been
_made_, since these pots were put up. On the other hand it does not
appear that much plant food has been made in the subsoil during this
time. Further evidence on this point is given by an experiment similar
to that in Fig. 22, but where mustard is grown in _subsoil_ kept moist,
but uncropped for some time, and in _subsoil_ previously cropped with
rye. The results in grams were:--

Green weight After drying

Mustard growing in subsoil
previously cropped with rye 12.6 2.27

Mustard growing in subsoil
previously uncropped 12.9 2.26

{49}

These should be compared with the figures on p. 45. Although the
subsoil lay fallow for a long time it produced no plant food but is
just as poor as the subsoil that has been previously cropped. These
observations give us a clue that must be followed up in answering our
next question.

What has the plant food been made from? Clearly it is not made from
the sand, the clay or the chalk since all these occur in the subsoil.
We have seen (Chap. I.) that the top soil differs from the subsoil in
containing a quantity of material that will burn away and is in part at
any rate made up of plant remains. It will be easy to find out whether
these remains fu