THE PHYSICAL PROPERTIES OF THE SOIL. 949 



THE PERMEABILITY OP THE SOIL P'OR AIR. 



This property is measured by the quantities of air which pass through 

 the soil, other conditions being the same. Except under special con- 

 ditions it does not depend upon the volume of the pores, but is influ- 

 enced primarily by the size of the pores, since the latter determines 

 almost exclusively the resistance ofiered to the circulation of the air. 

 The permeability necessarily increases as this resistance diminishes, 

 and is therefore smaller the smaller the soil particles, and vice versa. 



Of the different soil constituents in the dry state, clay is the least 

 permeable to air; and permeability increases generally with the quan- 

 tity of quartz present. Soils consisting of humus alone behave, when 

 dry, similarly to the coarse-grained sands. In soils made up of con- 

 stituents varying considerably from one another in the size of the par- 

 ticles, the permeability of the whole mass depends principally on the 

 finest-grained material. The remarkable permeability of most sands 

 can, therefoi'e, be reduced considerably by the admixture of compara- 

 tively small quantities of some fine-grained material, such as clay, loam, 

 or marl. 



In the case of stratified soils the permeability depends entirely on 

 the fiuCvSt grained layer, even if the layer is very thin. In a very 

 impermeable soil the i)ermeability is increased to a remarkable extent 

 by the formation of the crumbly structure. With an increase of 

 humidity the permeability of a soil for air diminishes, and more 

 strongly the greater the water capacity of the soil. All soils rich in 

 clay and humus become impermeable at a point below saturation. 

 Crumbly soils have their permeability affected to a very much less 

 degree by an increase in humidity than soils having a separate grain 

 structure. If a soil is wet only in the upper layers the entrance and 

 exit of air is hindered in proportion to the fineness of the grains. In 

 soils rich in clay and humus this may result in absolute impermeability, 

 even with slight precipitation. 



If air is forced by pressure through soil, the quantity of air passing 

 is proportional to the pressure, and inversely x)roi)ortional to the thick- 

 ness of the layer, so long as the velocity does not exceed 0.05 meter. 

 As soon as this limit is exceeded the volume of air does not diminish as 

 the pressure diminishes or as the thickness of the layer increases, but 

 to a less extent, and vice versa} 



THE DIFFUSION OF GASES FROM THE SOIL. 



The pores of the soil contain, along with atmosijheric air and water 

 vapor, different gases, ot which carbonic acid is the most important, as 



' H. Fleck, Ztschr. Biol., 16, p. 42. Erster Jahresbericht der chemischen Central- 

 Btelle fiir offentlicbe Ge8uiidlieit8i)flege in Dresden. F. Eenk, Ztschr. Biol., 15. 

 G. Amnion, Forsch. Geb. agr. Fhys., 3, p. 209. D. von Welitscbkowsky, Arch. Hyg., 

 11. R. Heiurich, Grundlagcn der Beurtbeilung der Ackerkruine, 1882, pp. 123, 221. 

 E- Wollny, Forsch. Geb. agr. Phys., 16, p. 193. 



