265 



case, it is difficult to draw a line between physical combination (absorption) 

 and chemical combination. 



Absorption becomes of importance, only where large absorptive sur- 

 faces are offered, as in organic substances and certain inorganic ones, to 

 which belong the colloidal silicic acid and the colloidal ferric oxid of the 

 tropical red soils. Those humus substances, capable of being swollen, seem 

 of the greatest significance which are precipitated in soils rich in nutritive 

 substances, such as salt-like compounds, but remain to a great part in solution 

 in impoverished soils. In the absorption of humus substances the first role is 

 played by their capacity to take up free bases and their carbonates. The 

 acid humus substances are especially effective for the ammonia found in the 

 soil and for ammonium carbonate and we take advantage of this fact 

 especially when using a peat mulch. 



Besides colloidal substances, the finely distributed mineral elements 

 should be kept in view as a means of absorption. Of the minerals, how- 

 ever, quartz always and kaolin, when not combined with alkali silicates to 

 form the absorptive double silicate, have no capacity for absorption. The 

 chief bearers are the hydrated silicates, especially the double silicates of 

 aluminum, which, crystallized as zolites, are found in rocks, and also those 

 of ferric oxid. They make possible the exchange of bases observable in the 

 soil. 



This becomes effective with the exhaustion of the soluble nutritive sub- 

 stances in the soil as is made clear by the following experim.ent carried out 

 by Lomberg\ A hydrated silicate was kept for three weeks in contact with 

 water containing carbon dioxid, and, after some time, the following compo- 

 sition was found : — 



I. 11. 



Original silicate. After treatment with water 



containing carbon dioxid. 



Silicic acid 46.64 per cent. 54-03 per cent. 



Aluminum oxid 29.38 " " 39-65 " " 



Potassium 22.75 " " 5-34 " " 



Sodium 1.83 " " 0.00 " " 



If this leached silicate II. was agaixi treated with a solution of caustic 

 potash, the following composition was found, — silicic acid, 46.60 per cent. ; 

 aluminum oxid, 35.67 per cent. ; potassium, 17.73 per cent. Therefore, in the 

 silicate skelton, the greatest part of the potassium had been taken up again, 

 so that a new condition of chemical equilibrium had been set up. 



If ammonium chloride was added to the original silicate I, the reaction 

 resuhed in, — siUcic acid, 56.17 per cent.; aluminum oxid, 34.59 per cent. ; 

 potassium, 0.89 per cent.; ammonia (NH3) 8.37 per cent. If a very large 

 excess of calcium salts had been present, instead of the ammonia, the cal- 

 cium could have replaced the potassium entirely in the silicate, as has act- 



1 Zeitschr. d. Geol. Ges. 1876, p. 318.. 



