88 Mineral Nutrition of Plants 



Retention of boron by soils 



Apparently the mechanism for retention of boron by soils does not 

 parallel that of the other soil anions (CI, S0 4 , P0 4 ). Boron retention is 

 lowest in acid soils, but increases rapidly in the range pH 6-10. Olson 

 and Berger (^5) have shown that fixation is not affected by calcium 

 addition except when this addition influences the soil pH. Boron fixa- 

 tion by soils is readily reversible. Clays which have had the free iron 

 and aluminum oxides removed from them fix more boron than un- 

 treated samples. 



Negative anion adsorption 



There is accumulating evidence that in neutral and alkaline soils 

 there is negative adsorption of nitrate and chloride ions. Reitemeier (41) 

 and others have observed decreases in soluble nitrate and chloride in 

 soils on dilution. In other words, there are lower concentrations of these 

 ions in the solution immediately surrounding soil particles than at some 

 distance from them. It has been suggested that this phenomenon may 

 be caused by the insolubility of nitrate and chloride in the "unfree" or 

 bound water. Another concept of the mechanism of this negative ad- 

 sorption is that it is caused by a diffused anion swarm, the distribution 

 of which depends on the Donnan distribution principle. 



ABSORPTION RATES AS A MEASURE OF ION AVAILABILITY 



A widespread experimental method for the study of ion availability 

 is the measurement of absorption rates with selected test plants, or 

 plant tissues under varying conditions in the soil or culture medium. 

 While providing a facile means for investigation, this approach, in 

 order to be productive, requires an adequate comprehension of the 

 absorption process in plants and a knowledge of the physical chemistry 

 of ions in soil. Unfortunately, this essential knowledge is not available 

 to us, although a number of pertinent facts begin to appear. It will be 

 our purpose in the following sections to list the established facts con- 

 cerning ion absorption in plants as well as certain physical-chemical 

 observations regarding ions in soil. Particular attention will be paid to 

 anions, although the majority of the considerations apply to cations as 

 well. 



