254 PLANT PHYSIOLOGY 



Such a concentration on the surface of sohd particles of soil 

 is shown by molecules of electrolytes dissolved in the soil solu- 

 tion. Physical adsorption is the cause of the lack of uniformity 

 of the soil solution, which is more concentrated near the surface 

 of the soil particles and becomes more diluted farther away from 

 them. Some substances show a negative adsorption. Their 

 particles are not attracted but on the contrary are repelled by 

 the soil particle. Some of the anions, e.g., chloride and nitrate 

 ions, behave in this way, and therefore they are easily filtered 

 through the soil, not being retained by it. 



The physicochemical base-exchange capacity of the soil is of 

 major importance for the nutrition of plants. It is the capacity 

 of the soil to exchange a part of the cations retained on its sohd 

 particles for an equivalent amount of cations present in the 

 surrounding soil solution. The cations adsorbed disappear 

 from the solution and are deposited on the sohd particles of 

 the soil. An equivalent amount of other cations passes from 

 the soil particle into the solution. The organic substances of the 

 soil, such as humus, represent adsorption complexes of organic 

 colloids, products of the decay of plant and animal residues with 

 various cations, chiefly Ca and Mg. The inorganic, or " zeolite," 

 part of the soil was found by Gedroitz to be a complex alumino- 

 silicate aggregate capable of exchanging its bases for the bases of 

 salt solutions. Both the inorganic and organic particles par- 

 ticipate in base exchange. The cations, or bases, present in 

 the adsorbing complex of the soil that are capable of base 

 exchange are known as the ''exchange bases." Calcium and 

 magnesium in most cases are exchangeable, as well as sodium 

 in alkali soils and hydrogen in acid soils. The adsorptive 

 force of the soil particles is great, and exchangeable bases can 

 be extracted from the soil only by means of displacement by 

 other cations. 



Chemical binding of substances by the soil is observed in 

 cases where the substance introduced into the soil reacts with 

 soil compounds to give insoluble products, which precipitate 

 and thus associate with the solid phases of the soil. Such 

 phenomena are observed, for instance, in introducing phosphates 

 into a soil that is rich in calcium salts, when an insoluble tri- 

 calcium phosphate, Ca3(P04)2, is formed. Chemical binding 

 chiefly determines the holding of phosphates and in a lesser 



