266 NATURE AND PROPERTIES OF SOILS 



a very exhaustive study of the subject, the following colloidal 

 materials may function in the soil : 



1. Partially decayed remains of plant and animal tissue. 



2. Colloidal iron, aluminum, and silica. 



3. Colloidal silicates formed through weathering. 



Van Bemmelen also credits crystalline silicates with some 

 absorptive power, but he does not consider such action par- 

 ticularly important. 



The combinations produced by absorption are often weak, 

 it being possible to leach out the substances held in the water 

 of the colloidal gels. The following example of one kind of 

 absorption is given by Van Bemmelen x and shows how com- 

 plex the phenomenon may become: ten grams of a hydrogel 

 having the composition Si0 2 .4.2H 2 0, shaken with 100 cubic 

 centimenter solution of 20 molecular equivalent KC1, absorbed 

 0.8 to 1.1 molecular equivalent of the dissolved substance. 

 The absorption in this case was as if the solution had been 

 diluted with 4.2 to 5.8 centimeters of water. As the amount 

 of gel water in 10 grams of hydrogel of Si0 2 is about 5 cubic 

 centimeters, the assumption may be made that the dissolved 

 substance is taken up in equal concentration by the gel water. 

 Ten grams of hydrogel of Si0 2 shaken with 100 cubic centi- 

 meter solution of 50 molecular equivalent KC1 — that is, two 

 and a half times the concentration of the former solution — 

 absorbs two and a half times as much, or 2.1 to 2.5 molecular 

 equivalent. This applies also to concentrations five times 

 stronger than the first mentioned above, but beyond that the 

 relation is not so simple. It serves, however, to illustrate 

 the manner in which the absorption takes place from dilute 

 solutions. 



140. The absorptive capacity of soils. 2 — The absorptive 



1 Van Bemmelen, J. M., Die Absorptionsverbindungen und das Ab- 

 sorptionsvermogen der Acker erde; Landw. Vers. Stat., Band 35, Seite 

 75, 1888. 



2 A few important citations are as follows: 



Peters, E., tfeber die Absorption von Kali durch AcTcererde; Landw. 

 Ver. Stat., Bd. 2, Seite 113-151, 1860. 



