as
clover, that not only do they contain a large amount of nitrogen, but
that by growing them on a soil the soil is largely enriched in this
valuable fertilising constituent, has long waited for a satisfactory
explanation, which at last has been forthcoming. The discovery that
leguminous crops can draw on the boundless store of nitrogen present in
the air has done much to clear up the mystery. There are, however, other
problems with regard to the growth of leguminous plants which still
await solution.

_Clover-sickness._

One of these is the fact that land on which a leguminous crop like
clover has been growing for a number of years becomes unfit to support
its growth any longer. Such a soil is termed "clover-sick"; and many
have been the theories put forward to explain the phenomenon, but none
of them can be regarded as satisfactory.

The knowledge that leguminous plants have the power of deriving their
nitrogen from the air, furnishes us with an economical means of
enriching our soils in nitrogen. By growing leguminous crops alternately
with cereals, for example, the air should be made to furnish the
necessary nitrogenous manure. As a matter of fact, modified forms of
such a practice have long been in use--indeed the ordinary rotations of
crops are, to a certain extent, adaptations of this practice.

_Alternate Wheat and Beans Rotation._

An interesting experiment carried out at Rothamsted may be here cited
which illustrates in a striking manner the truth of the above statement.
Wheat and the leguminous crop beans were grown alternately. It was
found that almost as much wheat (containing nearly as much nitrogen) was
yielded in eight crops of wheat so grown as was yielded by sixteen crops
of wheat grown consecutively in an adjoining field.

The most commonly cultivated leguminous crops are clover, beans, and
peas. Clover having been already discussed, we need only say a word or
two on the manuring of beans and peas.

BEANS.

Beans do best on strong land, and, unlike some of the crops considered,
do not require a particularly fine tilth. They are generally grown after
cereals, and as a rule are sown in spring. More rarely, however, they
are sown in autumn. Spring-sown beans take about seven months to come to
maturity. They are much affected, like other crops, but to a greater
extent, by the nature of the season--a wet season inducing an undue
development of straw.

_Manure for Beans._

In common p