224 SCIENCE OF GARDENING. Part II. 



fibrous radicles demand a firmer soil than such as have tap-roots or extensive lateral 

 roots. 



1055. The constituent parts of the soil which give tenacity and coherence are the finely 

 divided matters ; and they possess the power of giving those qualities in the highest 

 degree when they contain much alumina. A small quantity of finely divided matter 

 is sufficient to fit a soil for the production of turnips and barley ; and a tolerable 

 crop of turnips has been produced on a soil containing 1 1 parts out of 1 2 sand. A much 

 greater proportion of sand, however, always produces absolute sterility. The soil of 

 Bagshot heath, which is entirely devoid of vegetable covering, contains less than one twen- 

 tieth of finely divided matter : 400 parts of it, which had been heated red, afforded 380 

 parts of coarse siliceous sand ; 9 parts of fine siliceous sand, and 1 1 parts of impalpable 

 matter, -which was a mixture of ferruginous clay with carbonate of lime. Vegetable or 

 animal matters, when finely divided, not only give coherence, but likewise softness and 

 penetrability ; but neither they nor any other part of the soil must be in too great propor- 

 tion ; and a soil is unproductive if it consist entirely of impalpable matters. Pure alumina 

 or silica, pure carbonate of lime, or carbonate of magnesia, are incapable of supporting 

 healthy vegetation ; and no soil is fertile that contains as much as 19 parts out of 20 of 

 any of these constituents. 



1056. A certain degree of friability or looseness of texture is also required in soils, in order 

 that the operations of culture may be easily conducted ; that moisture may have free 

 access to the fibres of the roots, that heat may be readily conveyed to them, and that eva- 

 poration may proceed without obstruction. These are commonly attained by the presence 

 of sand. 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 culture. In a soil so compounded, water will be presented to the 

 roots by capillary attraction. It will be suspended in it, in the same manner as it is sus- 

 pended in a sponge, not in a state of aggregation, but minute division, so that every part 

 may be said to be moist, but not wet (Grisenthwaite.) 



1057. The water cliemically combined amongst the elements of soils, unless in the case of 

 the decomposition of animal or vegetable substances, cannot be absorbed by the roots of 

 plants ; but that adhering to the parts of the soil is in constant use in vegetation. Indeed 

 there are few mixtures of the earths found in soils .that contain any chemically combined 

 water ; water is expelled from the earth by most substances that combine with them. 

 Thus, if a combination of lime and water be exposed to carbonic acid, the carbonic acid 

 takes the place of water ; and compounds of alumina and silica, or other compounds of 

 the earths, do not chemically unite with water ; and soils, as it lias been stated, are formed 

 either by earthy carbonates, or compounds of the pure earths and metallic oxides. When 

 saline substances exist in soils, they may be united to water both chemically and me- 

 chanically ; but they are always in too small a quantity to influence materially the rela- 

 tions of the soil to water. 



1058. The power of the soil to absorb water by cohesive attraction depends in great measure 

 upon the state of division of its parts ; the more divided they are, the greater is their ab- 

 so:bent power. The different constituent parts of soils likewise appear to act, even by 

 cohesive attraction, with different degrees of energy. Thus vegetable substances seem to 

 be more absorbent than animal substances ; animal substances more so than compounds 

 of alumina and silica ; and compounds of alumina and silica more absorbent than car- 

 bonates of lime and magnesia : these differences may, however, possibly depend upon the 

 differences in their state of division, and upon the surface exposed. 



1059. The power of soils to absorb water from 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 aqueous vapor from the atmo- 

 sphere, by the interior parts of the soil during the day, and by both the exterior and in- 

 terior during the night. The stiff clays approaching to pipe-clays in their nature, which 

 take up the greatest quantity of water when it is poured upon them in a fluid form, are 

 not the soils which absorb most moisture from the atmosphere in dry weather. They cake, 

 and present only a small surface to the air ; and the vegetation on them is generally burnt 

 up almost as readily as on sands. The soils that are most efficient in supplying the plant 

 with water by atmospheric absorption, are those in which there is a due mixture of sand, 

 finely divided clay, and carbonate of lime, with some animal or vegetable matter, and 

 which are so loose and light as to be freely permeable to the atmosphere. With respect 

 to this quality, carbonate of lime, and animal and vegetable matter, are of great use in 

 soils ; they give absorbent power to the soil without giving it likewise tenacity ; sand, 

 which also destroys tenacity, on the contrary, gives little absorbent power. The absorbent 

 powers of soils, with respect to atmospheric moisture, is always greatest in the most fertile 

 soils j so that it affords one method of judging of the productiveness of land. 



