ABSORPTIVE POWER OF SOIL FOR SILICIC ACID. 137 



It will be seen at once that these differences in 

 absorptive power are very considerable. One volume 

 of earth from Weihenstephan absorbs nearly twice as 

 much potash as an equal bulk of soil from Cuba ; the 

 Hungarian earth, here examined, absorbs 2-J times as 

 much. 



These figures show that a certain quantity of pot- 

 ash, say 2600 milligrammes, if supplied to the'Weihen- 

 stephan soil, will spread in a space of 1 cubic decimetre 

 of earth. If we were to pour the potash, in solution, 

 on a small plot of ground, 1 square decimetre in area, 

 the potash would penetrate to a depth of 1 decimetre 

 (= 3'94 inches), and no deeper ; every cubic centimetre 

 ( -061 cubic inch) would receive 2*6 milligrammes 

 ( -04 grain) of potash, but the layers beneath would 

 receive none, or at least no appreciable quantity of it. 



If the same solution were poured on an equal area 

 of Hungarian or Cuban soil, the potash filtering 

 through would penetrate, in the former, to a depth of 

 somewhat above 7 centimetres (= 2'7 inches) ; in the 

 latter, to a depth of 19 centimetres (= 7*5 inches). 



The diffusibility of potash in a soil is in an inverse 

 ratio to the absorptive power of that soil ; half the 

 absorptive power corresponds to double the diffusibility. 

 In a similar way potash will spread in a field during 

 the time of fallow. From the spot where the potash is 

 set free from a silicate by disintegration, it will diffuse 

 itself through a volume of earth so much the larger in 

 proportion as the absorptive power of the earth for 

 potash is smaller. 



The absorptive power of arable soil for silicic acid 

 differs just as much as for potash. 



Thus from a solution of silicate of potash, 1 cubic 

 decimetre (=61 cubic inches) of these different soils 

 absorbed the following quantities of silicic acid : 



Forest soil. Hungarian. Garden mould I. Bogenhausen. Garden mould II. 

 Milligr. Grains. Milligr. Grains. Milligr. Grains. Milligr. Grains. .Milligr. Grains. 

 15=0'23 2644=43-8 2425=37'3 2007=31 1085=16-7 



Whence to express the relative diffusibility of silicic 



