ome idea of the
enormous amount of water transpired by plant-leaves may be gained by the
statement that from 233 lb. to 912 lb. of water are transpired for every
pound of plant-tissue formed.[32]

_Agronomy._

When we come to deal with questions relating to the chemistry of the
soil, we find that so much investigation has been devoted to this one
branch of agricultural chemistry as to constitute it a special branch by
itself--known in France under the name of _agronomie_--and being taught
in the large agricultural colleges by special professors of the subject.
The value of studying the properties of soils was recognised at an early
period. This study was for long largely confined to their _physical_,
or, what are popularly known as their _mechanical_ properties. Thus Sir
Humphry Davy ascertained many important facts with regard to the heat
and water absorbing and retaining properties of soils.

_Retention by Soil of Plant-food._

It was not till a later period that the power soils possess of fixing
from their watery solutions various plant-foods, both organic and
inorganic, was discovered. The earliest recognition of this most
important property of soils was made by Gazzeri, who, in 1819, called
attention to the fact that the dark fluid portion of farmyard manure was
purified on passing through clay. He concluded that soils, more
especially clayey soils, possessed the property of being able to fix
from their watery solutions the necessary plant-food constituents, and
fix them beyond risk of loss, only affording a gradual supply to the
plant as required.

The first experiments carried out on this subject were those by Huxtable
and Thompson in 1850. The liquid portion of farmyard manure was filtered
through soil and subsequently examined, when it was found to have not
only lost its colour, but also to have lost its smell. Ammonia and
ammonia salts were also experimented with, and it was found that soils
possessed the power of fixing ammonia.

To Thomas Way, however, we are indebted for the most valuable
contribution on this important subject made by any one single
investigator. His experiments were not merely carried out with regard
to ammonia, but also with regard to other bases--such as potash, lime,
magnesia, soda, &c. Since Way's experiments much work has been done by
Liebig, Stohmann, Henneberg, and Heiden, as also by Voelcker, Eichhorn,
Knop, Rautenberg, Pochwissnew, Warington, Beyer, Bretschneider, Sesti