S16 SCIENCE OF AGRICULTURE. Part I L 



turnips has been produced on a soil containing 11 parts out of 19 saad, 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. 



2100. A certain degree of friahility 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 

 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 those two earths, in 

 suitable proportions, would furnish every thing wanted to form the most perfect soil as to 

 weter 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 minute division, so that every 

 part may be said to be moist, but not wet. [Grisenthiuaite.) 



2101. The water 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 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, at 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. 



2102. The power of the soil to absorb water by cohesive 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 diflerent 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 diflferences may, however, possibly depend upon the 

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



2103. The power of soil 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 m 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 gene- 

 rally burnt up almost as readily as on sands. The soils that are most efficient in supply- 

 ing 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 vege- 

 table 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 ; so that it affords one method of judging of the produo- 



tiveness of land. 



2104. As examples of the absorbent powers of soils : 1000 parts of a celebrated soil 

 from Ormiston, in East Lothian, which contained more than half its weight of finely 

 divided matter, of which 1 1 parts were carbonate of lime, and 9 parts vegetable matter, 

 when dried at 212^, gained in an hour by exposure to air saturated with moisture, at a 

 temperature of 62*^, 18 grains. 1000 parts of a very fertile soil from the banks of the 

 river Parret, in Somersetshire, under the same circumstances, gained 16 grains. 1000 



