distributed. By analysis of the soil we can
measure the total amount of fertility which it contains, but we are left
in ignorance in regard to the amount of the ingredients which are in
such a form that the crops we cultivate can make use of them.

At Rothamsted, among my experiments on the growth of continuous wheat,
at the end of forty years, the soil supplied with salts of ammonia has
yielded, during the whole time, and still continues to yield, a larger
produce than is obtained by a liberal supply of phosphates and alkaline
salts without ammonia.

When we consider that every one hundred pounds of wheat crop, as carted
to the stack, contains about five per cent. of mineral matter, and one
per cent. of nitrogen, it is impossible to avoid the conclusion that my
soil has a large available balance of mineral substances which the crop
could not make use of for want of nitrogen. The crop which has received
these mineral manures now amounts to from twelve to thirteen bushels per
acre, and removes from the land about sixteen pounds of nitrogen every
year.

Analyses of the soil show that, even after the removal of more than
thirty crops in succession, without any application of manure containing
ammonia, the soil still contains some thousands of pounds of nitrogen.
This nitrogen is in combination with carbon; it is very insoluble in
water, and until it becomes separated from the carbon, and enters into
combination with oxygen, does not appear to be of any use to the crop.

The combination of nitrogen with oxygen, is known as nitric acid. The
nitric acid enters into combination with the lime of the soil, and in
this form becomes the food of plants.

From its great importance in regard to the growth of plants, nitric acid
might be called the main spring of agriculture, but being perfectly
soluble in water, it is constantly liable to be washed out of the soil.
In the experiment to which I have referred above--where wheat is grown
by mineral manures alone--we estimate that, of the amount of nitric acid
liberated each year, not much more than one-half is taken up by the
crop.

The wheat is ripe in July, at which time the land is tolerably free from
weeds; several months, therefore, occur during which there is no
vegetation to take up the nitric acid; and even when the wheat is sown
at the end of October, much nitric acid is liable to be washed away, as
the power of the plant to take up food from the soil is very limited
un