shallow-rooted crops will therefore be towards
impoverishing the surface-soil; whereas the occasional growth of a
deep-rooted crop brings the plant-food in the subsoil into requisition.
In this connection it may be well to draw attention to the singular
capacity possessed by certain crops for absorbing nitrogen. Of these the
case of clover is the most striking, and has long puzzled
agriculturists. The discovery, which has been repeatedly referred to in
these pages, that the leguminous order of crops, to which clover
belongs, have the power of absorbing the free nitrogen of the air
through the agency of micro-organic life in the plant and in the soil,
has furnished an explanation of this long-debated problem.

_Period of Growth._

A further reason is the difference in the period of a crop's growth. A
crop which grows quickly, and consequently occupies the ground during a
comparatively short period, will naturally require a richer soil, and
therefore a more liberal treatment with manure, than one whose growth is
more gradual.

Another consideration is the season of the year during which active
growth of the crops takes place. For example, in the case of the wheat
crop, active growth takes place in spring and ceases early in the
summer. Since, however, nitrification goes on right through the summer,
and nitrates are most abundant in the soil in late summer and autumn,
such a crop as wheat is ill suited to obtain any benefit from this
bountiful provision of nature, and is consequently particularly
benefited by the application of nitrogenous manures. Root crops, on the
other hand, sown in summer, continue their active growth into autumn,
and are thus enabled to utilise the nitrates formed in the process of
nitrification. The custom of sowing a quickly growing green crop, such
as rye, mustard, rape, &c., after a wheat crop, is a practice which aims
at conserving the nitrates and preventing their loss by autumn and
winter rains. The name "catch crop" has been applied to such a crop. By
ploughing under the green crop, the nitrogen removed from the soil in
the form of easily soluble nitrates is restored in an insoluble organic
form, and the soil is at the same time enriched by the addition of much
valuable organic matter.[243]

It is chiefly the above facts that form the scientific basis of the
long-pursued practice of the rotation of crops.

_Variation in Composition of Crops._

A point of considerable interest is th