THE ABSORPTIVE PROPERTIES OF SOILS 359 



for the availability of phosphorus in soils to which a 

 superphosphate fertilizer had been applied many months 

 previously. It might explain also the availability of 

 a superphosphate on soils devoid of calcium carbonate. 

 Although such availability is always less than where 

 this carbonate exists, it is greater than would be ac- 

 counted for by the solubility of ordinary iron phosphate. 



261. Adsorption. — There is a physical fixation, termed 

 adsorption, due to the concentration of the soil solution 

 in immediate contact with the surface of the particles. 

 The phenomenon is familiarly exemplified in the clarify- 

 ing effect of the charcoal filter. This process results in 

 the retention, in fine-grained soils, of considerable soluble 

 material that would otherwise be washed out. In the 

 case of nitrates, which are not retained by the zeolites, 

 adsorption is an important factor (par. 244). If a 

 solution of a known quantity of nitrate of soda is added 

 to a clay soil, and an attempt is then made to extract 

 the nitrate from the soil with distilled water, it will be 

 found impossible to recover a very appreciable propor- 

 tion of the amount added. While adsorption probably 

 does not account for all the nitrates retained, there can 

 be no doubt that it plays an important part. Nutritive 

 salts held in this way are readily available to the plant, 

 whose root hairs come in contact with the soil particles. 

 It is not impossible that other fertilizer constituents are 

 held by the soil in this manner. 



262. Absorption by colloids. — According to Van Bem- 

 melen, 1 who has made a very exhaustive study of this 



1 Van Bemmelen, J. M. Die Absorptionsverbindungen und 

 das Absorptions vermogen der Ackererde. Landw. Vers. Stat., 

 Band 35, Seite 69-136. 1888. Also, Die Absorption, Seite 

 548. Dresden, 1910. 



