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of the agriculture of tropical countries, in tho cultivation of rice and similar crops."
(òoi/i/s Rural Chemistnj, 2nd edit., p. 196.)
1239. The chemical action o f irrigation is produced by the water which irrigates tho
land becoming decomposed and forming frcsli combinations with various elements which
It attracts, partly from the air and partly from the soil.
1240. The improvement produced on land hy irrigation is often most extraordinary
and It IS eff^ected not only by tlic water itself; but by the salts aud other mineral substances
winch It brings with it in solution and deposits on the land.
Subsect. 6. Changing the Condition o f Lands in respect to Atmospherical Influence.
1241. The infiuence o f the weather on soils may be afFected by changing the position
ot their surface, and by sheltering or shading.
_ 1242. Changing the condition o f lands, as to solar influence, is but a limited means of
improvement ; but is capable of being turned to some account in gardening. I t is
effected by altering the position of thcir surface, so that the surface may be mo?c or less
at a rigiit angle to tho plane of the sun’s rays, according as heat or cold is to be increased
or diminished. The influence of the sun’s rays upon any plane is demonstrated to be
as their nunibcr and pci-pcndiculai-ity to that plane, the effects of the atmosphere being
excepted. Hence one advantage of ridging lands, provided the ridges run north and
soutli ; for on such surfaces the rays of the morning sun will take effect sooner on the
cast side, and those of the afternoon will remain longer in operation on the west side •
wliilst at mid-day his elevation will compensate, in some degree, for the obliquity of his
rays to both sides of the ridge. In culture, on a small scale, ridges or sloping beds for
winter crops may be made south-east and north-west, with thefr slope to the south, at an
angle of forty degrees, and as steep on the north side as the mass can he got to stand ;
and on the south slope of such ridge, ccvteris paribus, it is evident much earlier crops
may be produced than on level ground. The north side, Iiowever, will be lost durino-
this early crojiping ; but as early crops are soon gathered, the whole can be laid level in
time for a main crop. Hence all the advantage of grounds sloping to the south, southeast,
or south-west, in point of precocity, and of those sloping to the north for lateness
and diminished evaporation. Anotlier advantage of such surfaces is, that they dry sooner
after rains, whether by the operation of natural or ai-tificial drainage : or, in the case of
sloping to the south, by evaporation.
1243. Shelter, whether by walls, hedges, strips of plantation, or trees scattered over
the surface, may be considered, generally, as increasing or preserving heat, and lessening
evaporation from the soil. But if the cuiTent of air should be of a higher temperature
tJian tlic cai-th, screens against wind will prevent the earth from being so soon heated •
and from the increased evaporation arising from so great a multiplication of vegetable
surfaces by the trees, more cold will be produced after rains, and the atmosphere kept
in a more moist state, than in grounds perfectly naked. When the temperature of a
current of air is lower than that of the earth, screens will prevent its canying off so
much heat ; but more especially scattered trees, the tops of which will be chiefly cooled
whilst the under surfaces of their lower branches reflect back the rays of heat as they
radiate from the surhice of the soil. Heat, in its transmission from one body to another
follows the same laws as light ; and, tliercfore, the temperature of the surface in a forest
will, m winter, be considerably higher than that of a similarly constituted soil exposed
to the full influence of the weather. The early flowering of plants, in woods and
hedges, is a proof of this : but as such soils cannot be so easily heated in summer and
are cooled like others after the sinking in of rains, or the melting of snows, the effect of
the reflection as to the whole yeai* is nearly neutralised, and the average temperature
of the ycai- of such soils and situations will probably be found not greater than that of
open lands. ®
_ 1244. Shading the ground, whether by umbrageous trees, spreading plants, or covering
it with tiles, slates, moss, litter, or other materials, has a tendency to exclude atmospherical
heat and retain moisture. Shading dry loose soils, by covering them with litter
slates, or tiles, laid round the roots of plants, is found very bcueiicial : tliis prevents the
cooling effects of evaporation, and radiation.
Subsect. 7. Rotation o f Crops.
1245. Growing different crops in succession is a practice wliich every cultivator knows
to be highly advantageous, though its beneficial influence has not yet been fully accounted
for by chemists. The most general thcoiy is, that though all plants will live on the same
food, as the chemical constituents of thcir roots and leaves are ncai-ly the same yet that
nuiny species require particulai- substances to bring thcir seeds or fmits to perfection, as
the analyses of these seeds or fniits often afford substances different from those which
constitute the body of the plant.
124G. The necessity fo r a rotation o f crops was supposed by Dc Candolle and some
other foreign botanists to arise from every plant in the course of its growth throwing
off a quantity of excrcmentitious matter which was poisonous to plants of its own
nature, though it was not at all injurious to other plants, particularly if they chanced to be
of quite a dilfcrcnt kind. Thus ground in which turnips had grown was believed to be
poisonous for another crop of turnips the next year, though it was quite suitable for a
crop of peas ; and the ground which had produced peas was found the next year to do
best for potatoes, and so on.
1247. The rotation o f crops is now found to be necessary only bceausc certain crops
exhaust the soil of the particular salts or other mineral matters which arc requfrcd f'oi-
theii- nourishment. I t has already been stated that some plants requfre alkalies, others
acids, and others combinations of acids with an alkaline base, &c. When the substances
required by plants are found in the soil in which the plants grow, the plants are vigorous,
but when the soil becomes exliausted of them, the plants become weak and sickly,
luiebig illustrates tliis by that well-known case of fowls, which, when they can obtain no
carbonate of lime in thcir food, lay eggs without shells. lí e states also, that young
pigeons, when fed entirely on grains of wheat, in which phosphate of lime, the principal
constituent of bones, is deficient, have thcir bones so thin and weak that they may
be broken with the slightest touch ; and children ivho are fed entirely on biscuits
made of wheaten flour ivithout milk arc subject to what is called the rickets, a disease
occasioned by weakness of the bones. Milk contains a great quantity of phosphate of
lime ; and thus if a cow is fed entirely on potatoes and turnips, which, though tliey contain
phosphate of magnesia, have scai'cely any traces of lime, the cow wifr lose daily
a portion of the phospliate of lime deposited in her bones, and in time she wfil become
so weak as to be unable to stand.
1248. When the particular earths that a plant requires have heen exhausted in any soil
by a crop of that particular plant having been groivn in it, it is evident that another
crop of the same plant cannot be grown in the same soil till it has recovered a sufficient
quantity of that substance which had become exhausted; but it is equally evident that
another crop requiring a different substance may be grown in the same soil the foUowing
year. Thus plants that requiro potash, such as the beet, the mangold wurzcl, and the
turnip, may succeed plants that require lime, such as beans, peas, &c. ; and thus the
same result is obtained as was proposed by the fonner hypothesis, with this difference,
that the real cause why a rotation of crops is advisable being now known, the necessity
for it may be avoided by supplying the soil after each crop with the mineral substances
which had become exhausted, and thus the same crop may be grown on the same soil
for twenty years in succession.
1249. When no artificial means are used, nature gradually restores the mineral substances
which have been exhausted, and which the soU obtains from the atmosphere, the
rain, and other sources. The restoration when left entirely to nature is slow, and thus
it was the custom to let five or sometimes six years elapse before the same crop was
grown twice on the same land, unless the process of restoration was aided by iallowing
(see § 1217.) or by irrigation (see § 1237.).
1250. Influence o f rotations in destroying insects and weeds. Oliiicr, member of the Institute
of Eranco, has described all the insects, chiefly 2’ipulæand il/ùscoe, which live upon
the collai- or crown of the roots of the cereal grasses, and he has shoivn that they multiply
themselves without end, when tlie same soil presents the same crop for several years in
succession, or even crops of analogous species. But when a crop intervenes on which
these insects cannot live, the whole race perish for want of proper nourishment for their
larvæ. (Mém. de la Société Royale et Centrale d’Agr. de Paris, vol. vii.) Rotation is also
of gi-eat value in enabling the farmer more comjiletcly to eradicate and destroy those
weeds which constantly spring up in cultivated lands.
Chap. H.
O f Manures.
1251. Every species o f matter capable o f promoting the growth o f vegetables maybe considered
as manure. On examining the constituents of vegetables, wc shall find tliat
they arc composed of carbon, oxygen, hydrogen, and nitrogen, or azote, with a small
proportion of earthy and saline matters, which, however, arc as essential to the health
of the plants as all the other elements combined. It is obidous that when a soil is
deficient in any of the elements required, they should be supplied to it; and as the
substances thus supplied ai-c all included under the general name of manure, it is
obvious that manures must be of many different kinds, particulaily as none of tho