f certain micro-organic life is bound up with the problem of
fertility in a very direct and practical manner.

The importance of the conditions, other than those of a purely chemical
nature, have been thus far somewhat prominently emphasised, for the
reason that in what follows attention will be almost exclusively devoted
to the purely chemical conditions of fertility. It is well, then, to
realise that, while the latter conditions are by far the most important,
so far as the farmer is practically concerned, inasmuch as they are most
under his control, they are not the only conditions, and are not by
themselves able to control fertility.

FOOTNOTES:

[33] This statement perhaps needs qualification. While the important
_role_ played by the physical qualities of the soil were in the early
years of the science recognised, of more recent years the chemical
composition of the soil has been engaging almost exclusive
investigation. Physical properties of the soil have recently acquired a
further importance in the eyes of the agricultural chemist, from the
important influence they exert on what we have here called the
biological properties of a soil--viz., the development of those
fermentative processes whereby plant-food is prepared to a large extent.

[34] A good example of the absorptive capacity of a soil containing a
large quantity of vegetable matter is furnished by peat-bogs, which,
sponge-like, can absorb enormous quantities of water. (See Appendix,
Note I., p. 98.)

[35] Jethro Tull, an early well-known agricultural writer, who lived
about the middle of last century, propounded the theory, that as the
food of plants consisted of the minute earthy particles of the soil, all
that was required by the skilful farmer was to see that his soil was
properly tilled. He accordingly published a work entitled 'Horse-hoeing
Husbandry,' in which he advocated a system of thorough tillage. (See
Historical Introduction, p. 10.)

[36] See Introduction, p. 55.

[37] See Introductory Chapter, p. 55.

[38] It is not exactly known why excess of water should prevent normal
growth in the plant. Probably it is on account of the fact that free
access of oxygen is hindered in such a case. The roots are thus not
freely enough exposed to this necessary gas, and fermentative processes
of the nature of nitrification are not promoted. It may be also due to
the fact that the solution of plant-food is too dilute when such excess
of water pre