plex and difficult task.
Plant food consists of a dozen or more different substances. We have
talked about them in the pages of this book, and all I wish to say here
is that some of them are much more abundant, and more readily obtained,
than others. The three substances most difficult to get at are: nitric
acid, phosphoric acid and potash. All these substances are in the soil,
but some soils contain much more than others, and their relative
proportion varies considerably. The substance which is of the greatest
importance, is nitric acid. As a rule, the fertility of a soil is in
proportion to the amount of nitric acid which becomes available for the
use of plants during the growing season. Many of our soils contain large
quantities of nitrogen, united with carbon, but the plants do not take
it up in this form. It has to be converted into nitric acid. Nitric acid
consists of seven pounds of nitrogen and twenty pounds of oxygen. It is
produced by the combustion of nitrogen. Since these "Talks" were
published, several important facts have been discovered in regard to how
plants take up nitrogen, and especially in regard to how organic
nitrogen is converted into nitric acid. It is brought about through the
action of a minute fungoid plant. There are several things necessary for
the growth of this plant. We must have some nitrogenous substance,
a moderate degree of heat, say from seventy to one hundred and twenty
degrees, a moderate amount of moisture, and plenty of oxygen. Shade is
also favorable. If too hot or too cold, or too wet or too dry, the
growth of the plant is checked, and the formation of nitric acid
suspended. The presence of lime, or of some alkali, is also necessary
for the growth of this fungus and the production of nitric acid. The
nitric acid unites with the lime, and forms nitrate of lime, or with
soda to form nitrate of soda, or with potash to form nitrate of potash,
or salt-petre. A water-logged soil, by excluding the oxygen, destroys
this plant, hence one of the advantages of underdraining. I have said
that shade is favorable to the growth of this fungus, and this fact
explains and confirms the common idea that shade is manure.

The great object of agriculture is to convert the nitrogen of our soils,
or of green crops plowed under, or of manure, into nitric acid, and then
to convert this nitric acid into profitable products with as little loss
as possible. Nitrogen, or rather nitric acid, is the most cos