Book III. USES OF THE SOIL TO VEGETABLES. 319 



decomposed ; but the earths themselves cannot be supposed convertible into other sub- 

 stances, by any process taking place in the soil. In all cases the ashes of plants contain 

 some of the earths of the soil in which they grow ; but these earths, as has been ascer- 

 tained from the ashes afforded by different plants, never equal more than one fiftietli of 

 the weight of tlie plant consumed. If they be considered as necessary to the vegetable, 

 it is as giving hardness and firmness to its organisation. Thus, it has been mentioned 

 that wheat, oats, and many of the hollow-stalked grasses, have an epidermis principally 

 of silicious earth ; the use of which seems to be to strengthen them, and defend them 

 from the attacks of insects and parasitical plants. 



2147. The true nourishment of plants is water aud decomposing organic matter; 

 both these exist only in soils, not in pure earths : but the earthy parts of the soils are 

 useful in retaining water, so as to supply it in the proper proportions to the roots of 

 the vegetables, and they are likewise efficacious in producing the proper distribution of 

 the animal or vegetable matter. When equally mixed with it they prevent it from 

 decomposing too rapidly ; and by their means the soluble parts are supplied in proper 

 proportions. 



2148. The soil i.? necessary to the existence of plants, both as affording them nourishment, 

 and enabling them to fix themselves in such a manner as to obey those laws by which 

 their radicles are kept below the surface, and their leaves exposed to the free atmosphere. 

 As the systems of roots, branches, and leaves are very different in diflferent vegetables, so 

 they flourish most in different soils : plants which have bulbous roots require a looser 

 and a lighter soil than such as have fibrous roots ; plants possessing only short fibrous 

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



2H9. The constituent j>arts of the soil, wliich 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 lias been produced on a soil containing II parts out of 12 of 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 silicious sand ; 9 parts of fine silicious sand, and 1 1 parts of impalpable 

 matter, which was a mixture of fenugihous 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 consists 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. 



2150. A certain degree of friability or loosene.^s 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 

 evaporation 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 these 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 suspended in a sponge, not in a state of aggregation, but of minute division, so that 

 every part may be said to be moist, but not wet. (^Grisenlhwaite.) 



2151. The tvafer chemically 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 which contain any chemically combined 

 water ; water is expelled from the earth by most substances which 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 has 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 with 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. 



21 52. The power of the soil to absorb water by capillary attraction depends in great mea- 

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

 absorbent 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- 



