102 Mineral Nutrition of Plants 



Information as to whether activities or concentrations determine the 

 absorption rates of anions is scanty indeed. Appropriate systems for 

 testing this point would be soils that show large concentration differ- 

 ences of anions between the soil suspension and its filtrate. These con- 

 ditions presumably would obtain in soils which show a large negative 

 absorption of anions or in soils that adsorb phosphate, arsenate, or 

 molybdate in large amounts. The experiments of Dean and Rubin 

 (//) with a soil high in fixed phosphate would seem to indicate that 

 the absorption rate by plants of phosphate is a function of the activity 

 of the phosphate ion rather than of its concentration. That is, the 

 adsorption rate from a soil high in adsorbed phosphate was apparently 

 no greater than from its filtrate. 



It is evident that a great deal more experimentation is warranted on 

 this point. Nevertheless, the fact that the absorption rates of such im- 

 portant soil ions as Na+ and K+ are controlled by concentration rather 

 than activities gives rise to the question as to what functions of con- 

 centrations in the soil are appropriate indications of the availability of 

 these ions. 



Relationship of ion availability to certain 

 concentration factors in the soil 



For many years attempts have been made to correlate the availa- 

 bility of an ion with its concentration in the exchange complex of the 

 soil. On the whole this correlation has not proved very satisfactory. 



Fairly recently, Jenny and Ayres (2j) and others have attempted to 

 correlate the availability of Na+, K+, and Ca++ with their equivalent 

 percentage in the exchange complex of the soil (degree of saturation). 

 In a number of cases this procedure has resulted in fairly satisfactory 

 correlations; however, as yet the results are so limited that generaliza- 

 tions are not justified. 



In the case of anions that are not adsorbed by soil particles, it is 

 customarily assumed that an adequate assessment of their availability 

 is given by their concentrations in the soil water. Whether this assump- 

 tion is justified in soils that exhibit a large negative adsorption of anions 

 has not been determined. The availability of anions such as phosphate, 

 arsenate, and molybdate that are adsorbed in large amounts by some 



