which, as we have already seen, is so
important an agent in the changes occurring in the soil. In fact, a pure
clay, that is to say, a clay unmixed with sand, even though it may
contain all the essential constituents of the plant, is for this reason
unfertile. Practically, of course, these extreme cases rarely occur; the
heaviest clay soils being mixtures of true clay with sand, and the most
sandy containing their proportion of clay; but frequently the
preponderance of the one over the other is so great, as to produce soils
greatly inferior to those in which the mixture is more uniform.

It is easy to understand how the proportions in which sand and clay are
mixed must affect the suitability of soils to particular crops, and that
an open soil must be favourable to the turnip, and a heavy clay, owing
to the resistance it offers to the expansion of the bulbs, unfavourable.
But these substances also exercise an important chemical action on the
soluble constituents of the food of plants, combining with them, and
converting them into an insoluble, or nearly insoluble state, so as to
prevent their being washed away by the rain or other water which
percolates through the soil. It has long been known to chemists that
clay has a tendency to absorb a small proportion of ammonia, and even
when brought up from a great depth frequently contains that substance.
It is to Mr. Thompson of Moat Hall, however, that we owe the important
observation, that arable soils rapidly remove ammonia from solution, and
Way, who pursued this investigation, showed that not only ammonia, but
potash, and several of the other important elements of the food of
plants, are thus absorbed. The removal of these substances from solution
is easily illustrated by a simple experiment. It suffices to take a tall
cylindrical vessel open at both ends, and filled with the soil to be
operated upon, which is retained by a piece of rag tied over its lower
end. A quantity of a dilute solution of ammonia being then poured upon
the surface of the soil, and allowed to percolate, the first quantity
which flows away is found to have entirely lost its peculiar smell and
taste; and in a similar manner the removal of potash may be illustrated.
This action is by no means confined to those substances when in the free
state, but is equally marked when they are combined with acids in the
form of salts, and in the latter case the absorption is attended with a
true chemical decompositio