Great Britain so as to obtain average values for the United
Kingdom, the Irish yields are calculated into bushels at
the rate of 60lb to the bushel of wheat, of beans and peas,
50lb to the bushel of barley and 39lb to the bushel of oats.
The figure denoting the general average yield per acre of
any class of crop need re-adjustment after every successive
harvest. If a decennial period be taken, then--for the
purpose of the new calculation--the earliest year is omitted
and the latest year added, the number of years continuing at
ten. Adopting this course in the case of the cereal crops
of Great Britain the decennial averages recorded in Table
X. are obtained, the period 1885-1894 being the earliest
decade for which the official figures are available.
It thus appears that the average yield of wheat in Great
Britain, as calculated upon the crops harvested during the
ten years (1896-1905), exceeded 31 bushels to the acre,
whereas, for the ten years ended 1895, it fell below 29
bushels. A large expansion in the acreage of the wheat crop
would probably be attended by a decline in the average yield
per acre, for when a crop is shrinking in area the tendency
is to withdraw from it first the land least suited to its
growth. The general average for the United Kingdom might then
recede to rather less than 28 bushels of 60 lb. per bushel,
which was for a long time the accepted average--unless, of
course, improved methods of cultivating and manuring the
soil were to increase its general wheat-yielding capacity.6
Crops and Cropping.
The greater freedom of cropping and the less close adherence
to the formal system of rotation of crops, which characterize
the early years of the 20th century, rest upon a scientific
basis. Experimental inquiry has done much to enlighten the
farmer as to the requirements of plant-life, and to enable
him to see how best to meet these requirements in the case
of field crops. He cannot afford to ignore the results
that have been gradually accumulated--the truths that have
been slowly established--at the agricultural experiment
stations in various parts of the world. Of these stations
the greatest, and the oldest now existing, is that at
Rothamsted, Harpenden, Herts, England, which was founded in
1843 by Sir John Bennet Lawes (q.v.). The results of more
than half a century of sustained experimental inquiry were
communicated to the world by Lawes and his collaborator,
Sir J. H. Gilbert, in about 130 separate papers or reports,
many of which were published, from 1847 onwards, in the
Journal of the Royal Agricultural Society of England.7
In the case of plants the method of procedure was to grow
some of the most important crops of rotation, each separately
year after year, for many years in succession on the same
land, (a) without manure, (b) with farmyard manure and
(c) with a great variety of chemical manures; the same
description of manure being, as a rule, applied year after
year on the same plot. Experiments on an actual course of
rotation, without manure, and with different manures, have
also been made. Wheat, barley, oats, beans, clover and other
leguminous plants, turnips, sugar beet, mangels, potatoes and
grass crops have thus been experimented upon. Incidentally
there have been extensive sampling and analysing of soils,
investigations into rainfall and the composition of drainage
waters, inquiries into the amount of water transpired by
plants, and experiments on the assimilation of free nitrogen.
Cereals--Amongst the field experiments there is, perhaps,
not one of more universal interest than that in which wheat
was grown for fifty-seven years in succession, (a) without
manure, (b) with farmyard manure and (c) with various
artificial manures. The results show that, unlike leguminous
crops such as beans or clover, wheat may be successfully
grown for many years in succession on ordinary arable land,
provided suitable manures be applied and the land be kept
clean. Even without manure the average produce over forty-six
years, 1852-1897, was nearly thirteen bushels per acre, or
about the average yield per acre of the wheat lands of the whole
world. Mineral manures alone give very little increase,
nitrogenous manures alone considerably more than mineral
manures alone, but the mixture of the two considerably more
than either separately. In one case, indeed, the average
produce by mixed minerals and nitrogenous manure was more
than that by the annual application of farmyard manure; and
in seven out of the ten cases in which such mixtures were
used the average yield per acre was from over two to over
eight bushels more than the average yield of the United
Kingdom (assuming this to be about twenty-eight bushels of
60 lb. per bushel) under ordinary rotation. It is estimated
that the reduction in yield of the unmanured plot over the
forty years, 1852-1891, after the growth of the crops without
manure during the eight preceding years, was, provided it had
been uniform throughout, equivalent to a decline of one-sixth
of a bushel from year to year due to exhaustion--that is,
irrespectively of fluctuations due to season. It is related
that a visitor from the United States, talking to Sir John
Lawes, said, ``Americans have learnt more from this field
than from any other agricultural experiment in the world.''
Experiments upon the growth of barley for fifty years in
succession on rather heavy ordinary arable soil resulted in
showing that the produce by mineral manures alone is larger
than that without manure; that nitrogenous manures alone give
more produce than mineral manures alone; and that mixtures
of mineral and nitrogenous manure give much more than either
used alone--generally twice, or more than twice, as much as
mineral manures alone. Of mineral constituents, whether used
alone or in mixture with nitrogenous manures, phosphates are
much more effective than mixtures of salts of potash, soda and
magnesia. The average results show that, under all conditions
of manuring--excepting with farmyard manure--the produce
was less over the later than over the earlier periods of the
experiments, an effect partly due to the seasons. But the
average produce over forty years of continuous growth of
barley was, in all cases where nitrogenous and mineral manures
(containing phosphates) were used together, much higher than
the average produce of the crop grown in ordinary rotation
in the United Kingdom, and very much higher than the average
in most other countries when so grown. The requirements of
barley within the soil, and its susceptibility to the external
influences of season, are very similar to those of its near ally,
wheat. Nevertheless there are distinctions of result dependent
on differences in the habits of the two plants, and in the
conditions of their cultivation accordingly. In the British
Isles wheat is, as a rule, sown in the autumn on a heavier
soil, and has four or five months in which to distribute its
roots, and so it gets possession of a wide range of soil and
subsoil before barley is sown in the spring. Barley, on the
other hand, is sown in a lighter surface soil, and, with its
short period for root-development, relies in a much greater
degree on the stores of plant-food within the surface soil.
Accordingly it is more susceptible to exhaustion of surface
soil as to its nitrogenous, and especially as to its mineral
supplies; and in the common practice of agriculture it is found
to be more benefited by direct mineral manures, especially
phosphatic manures, than is wheat when sown under equal soil
conditions. The exhaustion of the soil induced by both barley
and wheat is, however, characteristically that of available
nitrogen; and when, under the ordinary conditions of manuring
and cropping, artificial manure is still required, nitrogenous
manures are, as a rule, necessary for both crops, and, for
the spring-sown barley, superphosphate also. Although barley
is appropriately grown on lighter soils than wheat, good
crops, of fair quality, may be grown on the heavier soils
after another grain crop by the aid of artificial manures,
provided that the land is sufficiently clean. Experiments
similar to the foregoing were carried on for many years in
succession at Rothamsted upon oats, and gave results which were
in general accordance with those on the other cereal crops.
Additional significance to the value of the above experiments
on wheat and barley is afforded by the fact that the same
series, with but slight modifications, has also been carried
out since 1876 at the Woburn (Bedfordshire) experimental
farm of the Royal Agricultural Society of England, the soil
here being of light sandy character, and thus very different
from the heavy soil of Rothamsted. The results for the
thirty years, 1877-1906, are in their general features
entirely confirmatory of those obtained at Rothamsted.
Root-Crops.--Experiments upon root-crops--chiefly white
turnips, Swedish turnips (swedes) and mangels--have resulted
in the establishment of the following conclusions. Both the
quantity and the quality of the produce, and consequently
its feeding value, must depend greatly upon the selection
of the best description of roots to be grown, and on the
character and the amount of the manures, and especially on
the amount of nitrogenous manure employed. At the same time,
no hard-and-fast rules can be laid down concerning these
points. Independently of the necessary consideration of the
general economy of the farm, the choice must be influenced
partly by the character of the soil, but very much more
by that of the climate. Judgment founded on knowledge and
aided by careful observation, both in the field and in the
feeding-shed, must be relied upon as the guide of the practical
farmer. Over and above the great advantage arising from the
opportunity which the growth of root-crops affords for the
cleaning of the land, the benefits of growing the root-crop
in rotation are due (1) to the large amount of manure applied
for its growth, (2) to the large residue of the manure left
in the soil for future crops, (3) to the large amount of
matter at once returned as manure again in the leaves, (4)
to the large amount of food produced, and (5) to the small
proportion of the most important manurial constituents of
the roots which is retained by store or fattening animals
consuming them, the rest returning as manure again; though,
when the roots are consumed for the production of milk, a much
larger proportion of the constituents is lost to the manure.
Leguminous Crops and the Acquisition of Nitrogen.--The fact
that the growth of a leguminous crop, such as red clover, leaves
the soil in a higher condition for the subsequent growth of
a grain crop--that, indeed, the growth of such a leguminous
crop is to a great extent equivalent to the application of a
nitrogenous manure for the cereal crop--was in effect known ages
ago. Nevertheless it was not till near the approach of the
closing decade of the 19th century that the explanation of
this long-established point of agricultural practice was
forthcoming. It was in the year 1886 that Hellriegel and
Wilfarth first published in Germany the results of investigations
in which they demonstrated that, through the agency of
micro-organisms dwelling in nodular outgrowths on the roots
of ordinary leguminous plants, the latter are enabled to
assimilate the free nitrogen of the air. The existence of
the root nodules had long been recognized, but hitherto no
adequate explanation had been afforded as to their function.
