athering fertility in the subsoil and depositing it
so near the surface that it became easily accessible to the roots of
other plants sown after the clover and which had not the same power of
feeding so deeply. This theory was true in part. The three important
elements of plant food, nitrogen, phosphoric acid and potash, were and
are thus increased in the soil, but this does not account for the source
from which the greater portion of the nitrogen thus deposited in the
soil was drawn, as will be shown below.

It was also noticed that when the seed of any variety of clover was sown
on certain soils, the plants would grow with more or less vigor for a
time and then they would fail to make progress, and in some instances
would perish. It was further noticed that if farmyard manure was applied
freely to such land, the growth made was more vigorous. Yet, again, it
was noticed that by sowing clover at short intervals on such soils, the
improvement in the growth of the plants was constant. But it was not
understood why clover plants behaved thus under the conditions named. It
is now known that ill success at the first was owing to the lack of
certain micro-organisms, more commonly termed bacteria, in the soil,
the presence of which are essential to enable clover plants to secure
additional nitrogen to that found in the soil and subsoil on which to
feed. When manure was applied, as stated above, the clover plants
secured much or all of their nitrogen from the manure. Bacteria were
introduced in very limited numbers at first, it may be through the
medium of the seed or in some other way, and because of an inherent
power which they possess to increase rapidly in connection with
continued sowing of clover at short intervals, they came at length to be
so numerous in the soil as to make possible the growth of good crops of
clover where these could not be thus grown a few years previously.

Careful observers had noticed that certain warty-like substances were
found attached to the roots of clover plants, and that the more
vigorously the plants grew, the larger and more numerous were these
substances, as a rule. It was thought by many that these warty
substances, now spoken of as nodules, were caused by worms biting the
roots or because of some unfavorable climatic influence or abnormal
condition of soil. It is now known that they are owing to the presence
of bacteria, whose special function is the assimilation of free nitrogen
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