In dealing with the composition of plants he says: "It is evident that
the most essential vegetable substances consist of hydrogen, carbon, and
oxygen, in different proportions, generally alone; but in some few cases
combined as carbon and nitrogen. The acids, alkalies, earths, metallic
oxides, and saline compounds, though necessary in the vegetable economy,
must be considered as of less importance, particularly in their relation
to agriculture, than the other principles."

Further on: "It will be asked, Are the pure earths in the soil merely
active as mechanical or indirect chemical agents, or do they actually
afford food to the plant?"

This question he answers by saying that "water, and the decomposing
animal and vegetable matter existing in the soil, constitute the true
nourishment of plants; and as the earthy parts of the soil are useful in
retaining water, so as to supply it in the proper proportion to the
roots of the vegetables, so they are likewise efficacious in producing
the proper distribution of the animal or vegetable matter. When equally
mixed with it, they prevent it from decomposing too rapidly; and by
their means the soluble parts are supplied in proper proportions."

_Value of Davy's Lectures._

The chief value of these lectures is due to the fact that they form the
first attempt to connect in a systematic manner the various scattered
facts, up to that time ascertained, and to interpret their bearing on
agricultural practice. We have in them, it is true, a strange mixture of
facts belonging rather to botany and physiology than to agricultural
chemistry; still they undoubtedly furnished a great impetus to inquiry,
and at the same time they did much to popularise the science.

But not merely did Davy summarise and systematise the various results
arrived at by others, he also made many valuable contributions to the
science himself. The conclusions he drew from the results he obtained
were, no doubt, in many cases false, and in other cases exaggerated;
still the results possess a permanent interest. He may be said to have
worked out many of the most important _physical_ or _mechanical_
properties of a soil, although exaggerating the importance of the
influence of these properties on the question of fertility.[10]

These experiments had to do with the heat- and water-absorbing powers of
a soil. He experimented on a brown fertile soil, and a cold barren clay,
and found at what rate they lost heat.