Since Hellriegel's striking discovery farm crops have
been conveniently classified as nitrogen-accumulating and
nitrogen-consuming. To the former belong the ordinary leguminous
crops--the clovers, beans, peas, vetches or tares, sainfoin,
lucerne, for example--which obtain their nitrogen from the air,
and are independent of the application of nitrogenous manures,
whilst in their roots they accumulate a store of nitrogen which
will ultimately become available for future crops of other
kinds. It is, in fact, fully established that these leguminous
crops acquire a considerable amount of nitrogen by the fixation
of the free nitrogen of the atmosphere under the influence of
the symbiotic growth of their root-nodule-microbes and the higher
plant. The cereal crops (wheat, barley, oats, rye, maize);
the cruciferous crops (turnips, cabbage, kale, rape, mustard);
the solanaceous crops (potatoes); the chenopodiaceous crops
(mangels, sugar-beets), and other non-leguminous crops have,
so far as is known, no such power, and are therefore more
or less benefited by the direct application of nitrogenous
manures. The field experiments on leguminous plants at
Rothamsted have shown that land which is, so to speak,
exhausted so far as the growth of one leguminous crop is
concerned, may still grow very luxuriant crops of another
plant of the same natural order, but of different habits of
growth, and especially of different character and range of
roots. This result is doubtless largely dependent on the
existence, the distribution and the condition of the appropriate
microbes for the due infection of the different descriptions of
plant, for the micro-organism that dwells symbiotically with one
species is not identical with that which similarly dwells with
another. It seems certain that success in any system involving
a more extended growth of leguminous crops in rotations must
be dependent on a considerable variation in the description
grown. Other essential conditions of success will commonly
include the liberal application of potash and phosphatic
manures, and sometimes chalking or liming for the leguminous
crop. As to how long the leguminous crop should occupy the
land, the extent to which it should be consumed on the
land, or the manure from its consumption be returned, and
under what conditions the whole or part of it should be
ploughed in--these are points which must be decided as
they arise in practice. It seems obvious that the lighter
and poorer soils would benefit more than the heavier or
richer soils by the extended growth of leguminous crops.
Remarkable as Hellriegel's discovery was, it merely furnished
the explanation of a fact which had been empirically
established by the husbandman long before, and had received
most intelligent application when the old four-course (or
Norfolk) rotation was devised. But it gave some impetus to
the practice of green manuring with leguminous crops, which
are equally capable with such a crop as mustard of enriching
the soil in humus, whilst in addition they bring into the
soil from the atmosphere a quantity of nitrogen available
for the use of subsequent crops of any kind. In Canada and
the United States this rational employment of a leguminous
crop for ploughing in green is largely resorted to for the
amelioration of worn-out wheat lands and other soils, the
condition of which has been lowered to an unremunerative
level by the repeated growth year after year of a cereal
crop. The well-known paper of Lawes, Gilbert and Pugh (1861),
``On the Sources of the Nitrogen of Vegetation with special
reference to the Question whether Plants assimilate free or
uncombined Nitrogen,'' answered the question referred to in the
negative. The attitude taken up later on with regard to this
problem is set forth in the following words, which are quoted
from the Memoranda of the Rothamsted Experiments, 1900 (p. 7):--
``Experiments were commenced in 1857, and conducted for several
years in succession, to determine whether plants assimilate
free or uncombined nitrogen, and also various collateral
points. Plants of the gramincous, the leguminous and of
other families were operated upon. The late Dr Pugh took a
prominent part in this inquiry. The conclusion arrived at
was that our agricultural plants do not themselves directly
assimilate the free nitrogen of the air by their leaves.
``In recent years, however, the question has assumed quite
a new aspect. It now is--whether the free nitrogen of the
atmosphere is brought into combination under the influence of
micro-organisms, or other low forms, either within the soil or
in symbiosis with a higher plant, thus serving indirectly as
a source of nitrogen to plants of a higher order. Considering
that the results of Hellriegel and Wilfarth on this point were,
if confirmed, of great significance and importance, it was
decided to make experimenis at Rothamsted on somewhat similar
lines. Accordingly, a preliminary series was undertaken in
1888; more extended series were conducted in 1889 and in 1890;
and the investigation was continued up to the commencement
of the year 1893. Further experiments relating to certain
aspects of the subject were begun in 1898. The resuits have
shown that, when a soil growing leguminous plants is infected
with appropriate organisms, there is a development of the
so-called leguminous nodules on the roots of the plants,
and, coincidenrly, increased growth and gain of nitrogen.''
The conclusions of Hellriegel and Wilfarth have thus been
confirmed by the later experiences of Rothamsted, and since that
time efforts have been directed energetically to the practical
application of the discovery. This has taken the form of
inoculating the soil with the particular organism required by the
particular kind of leguminous crop. To this end the endeavour
has been made to produce preparations which shall contain in
portable form the organisms required by the several plants, and
though, as yet, it can hardly be claimed that they have been
generally successful, the work done justifies hopes that the
problem will eventually be solved in a practical direction.
Grass.--Another field experiment of singular interest
is that relating to the mixed herbage of permanent meadow,
for which seven acres of old grass land were set apart in
Rothamsted Park in 1856. Of the twenty plots into which
this land is divided, two were left without manure from the
outset, two received ordinary farmyard manure for a series
of years, whilst the remainder each received a different
description of artificial or chemical manure, the same being,
except in special cases, applied year after year on the same
plot. During the growing season the field affords striking
evidence of the influence of different manurial dressings.
So much, indeed, does the character of the herbage vary
from plot to plot that the effect may fairly be described as
kaleidoscopic. Repeated analyses have shown how greatly both
the botanical constitution and the chemical composition of
the mixed herbage vary according to the description of manure
applied. They have further shown how dominant is the influence of
season. Such, moreover, is the effect of different manures
that the gross produce of the mixed herbage is totally different
on the respective plots according to the manure employed,
both as to the proportion of the various species composing
it and as to their condition of development and maturity.
The Rotation of Crops.
The growth, year after year, on the same soil of one kind of
plant unfits it for bearing further crops of the kind which has
exhausted it, and renders them less vigorous and more liable to
disease. The farmer therefore arranges his cropping in such a
way that roots, or leguminous crops, succeed the cereal crops.
It is not only the conditions of growth, but the uses to
which the different crops are put, that have to be considered
in the case of rotation. Thus the cereal crops, when grown
in rotation, yield more produce for sale in the season of
growth than when grown continuously. Moreover, the crops
alternated with the cereals accumulate very much more of
mineral constituents and of nitrogen in their produce than do
the cereals themselves. By far the greater proportion of those
constituents remains in circulation in the manure of the farm,
whilst the remainder yields highly valuable products for sale
in the forms of meat and milk. For this reason these crops
are known as ``restorative,'' cereals the produce of which
is sold off the farm being classed as ``exhaustive.'' With
a variety of crops, again, the mechanical operations of the
farm, involving horse and hand labour, are better distributed
over the year, and are therefore more economically performed.
The opportunities which rotation cropping affords for the
cleaning of the land from weeds is another distinct element of
advantage. Although many different rotations of crops are
practised, they may for the most part be considered as little
more than local adaptations of the system of alternating
root-crops and leguminous crops with cereal crops, as exemplified
in the old four-course rotation--roots, barley, clover, wheat.
Under this system the clover is ploughed up in the autumn, the
nitrogen stored up in its roots being left in the soil for the
nourishment of the cereal crop. The following summer the wheat
crop is harvested, and an opportunity is afforded for extirpating
weeds which in the three previous years have received little
check. Or, where the climate is warm and the soil light,
a ``catch-crop,'' i.e. rye, vetches, winter-oats or some
other rapidly-growing crop may be sown in autumn and fed off
or otherwise disposed of prior to the root-sowing. On heavy
soils, however, the farmer cannot afford to curtail the time
necessary for thorough cultivation of the land. The cleaning
process is carried on through.the next summer by means of
successive hoeings of the spring-sown root-crop. As turnips or
swedes may occupy the ground till after Christmas little time
is left for the preparation of a seed-bed for barley, but as
the latter is a shallow-rooted crop only surface-stirring is
required. Clover is sown at the same time or shortly
after the cereal and thus occupies the land for two years.
The rotations extending to five, six, seven or more years are,
in most cases, only adaptations of the principle to variations
of soil, altitude, aspect, climate, markets and other local
conditions. They are effected chiefly by some alteration in the
description of the root-crop, and perhaps by the introduction
of the potato crop; by growing a different cereal, or it may
be more than one cereal consecutively; by the growth of some
other leguminous crop than clover, since ``clover-sickness''
may result if that crop is grown at too short intervals, or
the intermixture of grass seeds with the clover, and perhaps
by the extension by one or more years of the period allotted
to this member of the rotation. Whatever the specific
rotation, there may in practice be deviations from the plan of
retaining on the farm the whole of the root-crops, the straw
of the grain crops and the leguminous fodder crops (clover,
vetches, sainfoin, &c;) for the production of meat or milk,
and, coincidently, for that of manure to be returned to the
land. It is equally true that, when under the influence
of special local or other demand--proximity to towns, easy
railway or other communication, for example--the products
which would otherwise be retained on the farm are exported
from it, the import of town or other manures is generally
