NATURE AND OFFICES OF EARTHS AND SOILS. 
II 
tions of finely-divided vegetable and animal decomposable matters in union with 
alumina. 
5. Friability, or looseness of texture, is chiefly occasioned by the admixture 
of sand, and in a certain degree this quality is of importance, " in order that the 
operations of culture may be readily conducted, that moisture may have free access 
to the fibres of the roots, that heat may readily be conveyed to them, and evapo- 
ration may proceed without obstruction." " As alumina possesses all the properties 
of adhesiveness in an eminent degree, and silex those of friability, it is obvious that 
a mixture of those two earths, in suitable proportions, would furnish every thing 
wanted to form the most perfect soil as to water and the operations of cultivation. 
In a soil so compounded, water will be presented to the roots by capillary attrac- 
tion : it will be suspended in it, as in a sponge, in a state of minute division, so that 
every part may be said to be moist, but not wet." 
6. " The power of soils to absorb water from the air is much connected with 
fertility. When this power is great, the plant is supplied with moisture in dry 
seasons ; and the effect of evaporation in the day is counteracted by the absorption 
of vapour from the atmosphere by the interior parts of the soil during the day, and 
by both the exterior and interior during the night." " The soils that are most 
efficient in the supplying the plant with water by absorption from the atmosphere 
are those in which there is a due admixture of sand, finely-divided clay, and car- 
bonate of lime," (or chalk, which mixture constitutes a loam,) " with some animal 
and vegetable matter ; and which are so light as to be freely penetrated by the 
atmosphere." 
The productiveness of soils is influenced by the nature of the subsoil on which 
they rest. When they are immediately situated upon a bed of rock, they are 
rendered dry by evaporation much sooner than when the subsoil is of clay or marl. 
" A clayey subsoil will sometimes be of material advantage to a sandy soil, and will 
retain moisture so as to be capable of supplying that lost by the earth above." " A 
sandy or gravelly subsoil often corrects the imperfection of a too great degree of 
absorbent power in the true soil. In calcarious countries, where the surface is a 
species of marl, the soil is often found only a few inches above the limestone, and 
its fertility is nevertheless unimpaired ; though on a less absorbent soil, this situa- 
tion would occasion barrenness ; and the sandstone and limestone hills in Derby- 
shire and North Wales may be easily distinguished at a distance, in summer, by 
the different tints of vegetation. The grass on the sandstone hills usually appears 
brown and burnt up, that on the limestone hills flourishing and green." In the 
Isle of Thanet, and other districts, where the subsoil is chalk to a considerable 
depth, the verdure of the grass, and of young trees and shrubs, is often retained 
during parching seasons, when in many other situations the grass is entirely 
scorched, and the trees lose their leaves, owing to the continuance of dry weather ; 
this was particularly observable in the hot summer of 1818. Chalk absorbs moisture 
readily, and retains it tenaciously ; hence, in hot, dry summers, it gradually affords 
moisture to the roots of plants at a time when more open and porous soils are 
comparatively deprived of moisture. 
