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a free siliceous soil will, if left untonchoa, become too compact for the proper admission of
air, rain, and heat, aud for the free growth of the fibres ; and strong upland clays, not
submitted to tho plough or the spade, will, in a few years, bo found in the possession ot
fibrous rooted poremiial grasses, which will form a clothing on their surface, or strong
tap-rooted trees, as the oak, which have forced thcir way through the interior of the
mass. Annuals and ramose-rooted herbaceous plants oaniiot penetrate into such soils.
'fo'oG Mechanical means o f dioiding the soil and separating its particles by the use of
the spaile, the fork, the hoe, and the rake, arc the most obvious, and conscqiicntly thoso
most generally employed to effect tliat loosening of tho earth which is called pulverisation.
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1207. T h e fr s t object o f pulverisation is to give scope to the roots oj vegetables, torwitn-
ont abundance of roots no plant wUl become vigorous, whatever may bo the richness of
tho soil in which it is placed. Tho fibres of the roots take up the extract of the soil hy
intro-snsccption 1 tho quantity taken up, therefore, will not depend aloim on the quantity
in the soil, but on the number of absorbing fibres. The more the soil is pulverised, tlie
more these libres are increased, tho more extract is absorbed, and the more vigorous docs
the plant become. Pulverisation, therefore, is not only advantageous previously to planting
or sowing, but also during the progress of vegetation, when applied in the spaces
between tlie plants. In the latter case it operates also iu the way of pruning, and Iiy
ciittiiiv off or shortoniiig the extending fibres, causes them to branch out into numerous
o th e i? by which the mouths or pores of the plants arc greatly increased, ami such food
as is in tho soil has the better chance of being soiiglit after, and taken up by them. ^
1208. Anothei' important use o f the pulverisation o fth e soil, is to admit atmosplicric air
to tho spongioles of the roots. I f the roots are exposed entirely to the air, they become
dry and withered, and lose thcir power of expansion and contraction ; but though it is
necessary to havo the roots covered with soil, the particles of that soil should bo as loose to
admit the passage of air as it was before stated to bo to admit finely divided particles of
water. I t must never be forgotten that a great portion of tho carbonic acid gas taken up
by the spongioles of plants, is obtained from the atmospheric .air ; and that if tho roots
of plants arc covered with soil caked together so as to be impervious to air, tlie^ plants
cannot obtain thcir proper supply of carbon, and consequently there will be a dcflciency
in their products. Fruit trees wdl not produce frnit, and timber trees wiU not produce
durable timber, if tho spongioles of thcir roots ai'C entirely deprived of access to atmospheric
air. 7- -7
1209. Pulverimtim increases the capillary attraction, or spongc-likc property, ol soils,
by which thcir humidity is rendered more uniform. It is evident this capillary attraction
must be greatest where the paiticles of the earth arc flnoly divided ; for gravels and
sands hardly retain water at all, whUe clays, not open by pulverisation or other means,
either do not absorb water, or when, by long action, it is absorbed, they retain too much.
Water is not only necessary as such to tho growth of plants, but it is cssontiffl as a kind
of food, and as a medium through whieh plants absorb other food. Maiiiiro is useless to
vegetation till it becomes soluble in wator, and it would remain ttscloss in a state of
solution, if it so abounded as wholly to exclude air; for then the iihrcs or months, unable
to perform thoir functions, would soon decay and rot oft'. Pulverisation, in a wann season,
is of great advantage in admitting tho nightly dews to tho roots of plants.
1210. The temperature o f a soil is increased by pulverisation. Earths, Grisenthwaite
observes, arc ¡imong the worst conductors of heat with which wc are acquainted, and
coiiscqimntly it wonld be a considerable time before tho gradually increasing tcmpcratnro
of spring could communicate its genial warmth to the roots of vegetables, if tho lower
parts of soils were not heated by some other moans. I t is therefore necessary to havo
the surface of tho soil open, that there may bo a free ingress of the warm air, and tepid
rain of spring,
1211. Pulverisation contributes to the increase o f vegetable food. Water is known to he
a condenser and solvent of osxrhonic acid gas, wliich, when tho land is open, can bo
immediately carried to the roots of vegetables, and contribute to their growth ; but if the
land bo close, and the water lie on or near its surface, then tho carbonic acid gas, which
always exists in the atmosphere and is carried down by rains, will soon bo dissipated.
An open soil is also most suitable for effecting those cliaiigos in the manure itself, whieh
are equally necessaiy to the preparation of such food. Animal and vcgetable.i)ubstaiiccs,
exposed to tho alternate action of heat, moisture, light, and air, undergo spontaneous
decompositions, which would not otherwise take place.
1212. B y means o f pulverisation a portion o f atimspheric air is buried in die soil. Tliis
air, so conliiied, is dooomposcd by tho moistiiro retained in the earthy matters. Ammonia
is formed by the union of the hydrogen of the water with the nitrogen of the
atmosphere; and nitre, by the union of oxygen .and nitrogen; carbonie acid gas, and
carburottcd hydrogen are also obtained from the air. Heat is given out during these
processes, and “ hence,” as Dr. Darwin remarks {Phytologia, sect. An. 1.), “ the great
propriety of cropping lands immcdLately after they have been comminuted and turned
over ; and this tho more especially, if manure has been added at the same time, as the
process of fermentation will go on faster when the soil is loose, and the interstices filled
with air, than afterwards, when it becomes compressed with its own gravity, the relaxing
iiiHitcnco of rains, and the repletion of the partial vacuums formed by the decomposition
of the enclosed air. Tho advantage of tlio heat thus obtained iu exciting vegetation,
ivlicthcr in a seed or root, especially in spring, when the sod is cold, must be very con-
sidcrablc.”
1213. The depth o f pulverisation must depend upon the nature of the soil aud of the
subsoil, and also on tho nature of tbe plants grown in the soil. In orchards and kitelieu
gardens where- there arc fruit trees, deep pulverisation is exceedingly injurious, as it
I)revents the spongioles of tlie roots from rising to the surface of the soil, and consequently
places them beyond the reach of atmospheric air. Annual crops, on the contrary,
are generally better when the soil wliich is to receive them has been prepared by
tolerably deep pulverisation. Carc minst, liowevcr, always be taken not to dig below
the suriacc soil, as sometimes, by very deep trenching, the sm-facc soil may be buried,
and the subsoil brought to the surface, which, if the subsoil chances to be sand or gravel,
will be a change decidedly unfavourable. In some cases, however, a mixture of the
subsoil with the surface soil is decidedly bcueiicial, as it may supply the surface soil
with earthy or mineral substances wbicli may have become exhausted.
1214. Pulverisation should, in all cases, he accompanied hy the mixture o f the parts o f
the surface soil by turning them over. It is difficult, indeed, to pulverise witliout effecting
this end, at least by the implements in common use ; but, if it could be effected, it
would be injurious, because the difference of gravity between the organised matters and
the earths which compose soils has a constant tendency to separate them ; ancl stin-ing
a soil only with forks or pronged implements, would, iu a short time, leave its surface
too light and spongy, ancl the lower part too compact and earthy.
1215. Pulverisation o f the surface o f soils serves to retain moisture in the .substratum,
and to prevent it from being penetrated by heat from a wai-mcr, as well as from radiating
its heat to a colder, atmosphere than itself. The effect is produced by the porosity of
the pulverised stratum, which may be compared to a mulching of dry loose sand. The
operation is of great importance in horticultui’c.
Subsect. 2. O f ihe Improvement o f Soils hy Compression.
1216. Mechanical consolidation improve some soils, such as spongy peats and light
dusty sands. It is but a limited source of improvement, but still it deserves to be
noticed. Tlic proper dcgi'ce of acUicsivciicss is best given to loose soils by the addition
of earthy matters ; but mere rolling and treading are not to be altogether rejected. To
be benefited by rolling, a soil must be dry, and the operation must not be earned too
far. A peat-bog drained and rolled will sooner become covered with grasses than one
equally well drained and left to itself. Drifting sands may be well rolled when wet,
and by repeating the process after rains they ivill, in time, acquire a surface of grass or
herbage. Evciy cultivator knows the advantages of rolling light soils after sowing.
Gardeners tread in seeds ou certain soils, and roll grass lawns to improve the quality of
the tuif.
Subsect. 3. O f ihe Improvement o f Soils by Aeration or Fallowing.
1217. The uses o f aeration are partly mechanical and partly chemical; but though the
effects of aeration have been long known to [iractical men to be beneficial, it is only
lately that the maimer in whicli it acts has been understood.
1218. The mechanical effect o f aeration in winter is to obtain a minute division of the
particles of caitli by the freezing of the water in the soil ; for as water in a solid state
occupies more space than it did when fluid, the particles of earthy matters and decomposing
stones arc thus rent asunder aud crumbled down to a fine mould. In summer
the same effect is produced hy heat, as all substances expand by heat ; and in both
cases the parti<;lcs of earth arc rent asunder and a greater sur.iicc is exposed to the
action of the atmosphere. A secondary advantage which is derived from exposing soils
to the. heat of summer, and the cold of winter is, the destroying the seeds of weeds, aud
the eggs of insects ; but this advantage is too uncertain to bo counted upon.
1219. Soils are benefited chemically by the free admission of air to tlicir interior parts,
partly because it ciiabics them to extract from the air certain chemical agents of which
they arc in want ; and partly by means of the influence exercised by the constituents of
the atmosphere in disintegrating solid earthy particles.
1220. Carbon, nitrogen, and other gases, arc extracted from the ail* by humus aud
other component pai-ts of cultivated soil.
1221. The disintegration o f soils by ihe action o f Hie atmosphere has most effect when
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