" 6 6 6 6
------ ------
Difference 0 0 0 1

The experiment is not good enough to tell us exactly how much plant
food was present at the beginning. But we can say that the amount of
plant food in the soil is too small to be detected by such weighing as
we can do.

Here is an account of a similar experiment made 300 years ago by van
Helmont in Brussels, and it is interesting because it is one of the
first scientific experiments on plant growth:--

"I took an earthen vessel in which I put 200 pounds of soil dried in an
oven, then I moistened with rain water and pressed hard into it a shoot
of willow weighing 5 pounds. After exactly five years the tree that
had grown up weighed 169 pounds and about 3 ounces. But the vessel had
never received anything but rain water or distilled water to moisten
the soil (when this was necessary), and it remained full of soil which
was still tightly packed, and lest any dust from outside should have
got into the soil it was covered with a sheet {47} of iron coated with
tin but perforated with many holes. I did not take the weight of the
leaves that fell in the autumn. In the end I dried the soil once more,
and got the same 200 pounds that I started with, less about two ounces.
Therefore the 164 pounds of wood, bark and root arose from the water
alone." The experiment is wonderfully good and shows how very little
plant food there is in the soil. The conclusion is not quite right,
however, although it was for many years accepted as proof of an ancient
belief, which you will find mentioned in Kingsley's _Westward Ho!_,
that all things arose from water. It is now known that the last
sentence should read, "Therefore the 164 pounds of wood, bark and root
arose chiefly from the water _and air_, but a small part came from the
soil also."

But to return to our experiment with Pots 1 and 2. They had been kept
moist before the mustard was sown. Did this moisture have any effect
on the soil? Take two of the pots that have been kept dry and
uncropped, and two that have been kept moist and uncropped, also one of
dry uncropped subsoil and one of moist uncropped subsoil. Sow rye or
mustard in each pot and keep them all equally supplied with water.

It is soon evident that the top soil is richer in plant food than the
subsoil, and the soil stored moist is rather richer tha