ADSORPTION 301 



reduced in productiveness under cultivation respond 

 to the application of organic matter in a remarkable 

 manner. Humus in these soils seems to be the chief 

 conserver of readily available plant-food materials. 

 Ferric and aluminum hydrate aid in the retention 

 of acids, notably phosphoric, by forming highly in- 

 soluble compounds. 



165. Adsorption. There is a physical absorption, 

 termed adsorption, due to the concentration of the soil 

 solution in contact with the surface of the particles. 

 The phenomenon is familiarly exemplified in the clari- 

 fying effect of the charcoal filter. This process results 

 in the retention of considerable soluble material in fine- 

 grained soils, that would otherwise be washed out. 

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

 zeolites, adsorption is an important factor. (See page 

 325.) If a solution of a known quantity of nitrate of 

 soda be added to a clay soil, and it is then attempted to 

 extract the nitrate from the soil with distilled water, 

 it will be found impossible to recover a very appreciable 

 per cent of the amount added. While adsorption prob- 

 ably does not account for all of 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. 



166. Occlusion. According to Wiley, clay in a col- 

 loidal state has the property of dissociating to a certain 

 extent potash salts, and entangling the basic ion in the 

 meshes of the colloid structure. How extensive or 

 important this action is has not been demonstrated. 



