242 SOILS. 



650. A distinction may be properly made between (1) that 

 water which remains as a copious supply beneath the surface of 

 the ground, existing there plainly as a liquid, (2) that which ad- 

 heres to the particles of soil imparting to them a moist appear- 

 ance, (3) that which adheres to the particles of an air-dry soil 

 and which does not affect at all the appearance of the particles. 

 The first has been called hydrostatic, the second, capillary, the 

 third, hygroscopic water. It is from the two latter that the 

 roots of plants other than aquatics usually obtain their supply 

 of moisture. 1 



651. The relations which evaporation and drainage bear to 

 the total rain-fall upon the soil have been examined during a 

 series of nineteen years at Rothamsted, in England. The fol- 

 lowing figures are based on the results during ten years (Sep- 

 tember, 1870, to August, 1880). 



Rain-fall 30.68 inches. 



Drainage from soil 



at 20 inches depth 13.21 " 



at 40 " " 13.94 " 



at 60 " " 12.17 " 



Amount of water retained by soil, or evaporated 



at 20 inches depth 17.47 " 



at 40 " " 16.74 " 



at 60 " " 18.51 " 



Percentage of rain-fall lost by drainage 



at 20 inches depth 43.1 " 



at 40 " " 45.4 



at 60 " " 39.7 



Percentage of rain-fall retained by soil, or lost by evaporation 



at 20 inches depth 56.9 



at 40 " " 54.6 



at 60 " " 60.3 



652. Soils are not only acted upon b}- the solvent power of 

 water, as shown in 636, but many soils possess the remarkable 

 property of removing saline matters from aqueous solutions. 



The interesting fact that impure water can be freed from some 

 of its foreign matter by being filtered through earth has long- 

 been known, but its significance in the nutrition of plants does 

 not appear to have received attention until 1819. Gazzeri 2 at 



1 For a full discussion of this subject, which is most important in its bear- 

 ings upon the cultivation of plants, the student should study Johnson's "How 

 Crops Feed," p. 199. 



2 From a note by Orth: Versuchs-Stationen, xvi., 1873, p. 57. The discovery 

 is generally ascribed to Bronner, 1836. The fullest treatment was by Way : 

 Journal Royal Agricultural Society, 1850, and later. 



