98 Mineral Nutrition of Plants 



tions, or with nutrient solutions maintained at constant composition, 

 definitely show that up to concentrations in the neighborhood of 

 0.001 N, the absorption rate of an ion is a linear function of its con- 

 centration in the culture medium (24,37). Since, in nutrient solutions 

 as ordinarily used, the activities of ions are not appreciably different 

 from their molalities, it is not possible to discern in these cases whether 

 the rates of absorption of the ions by plants are determined by the 

 concentrations or by the activities of the ions. This point, while per- 

 haps not of great importance in the study of plants growing in nutrient 

 solutions, becomes a question of major concern in the estimation of 

 ion availability from soil systems. This becomes evident when the 

 physical chemistry of soil ions is considered. 



Some conclusions regarding the activities of soil ions 



The characteristic chemistry of soil ions becomes apparent when 

 a soil suspension or gel in equilibrium with its filtrate or "soil solution" 

 is considered. This approach has practical as well as theoretical impli- 

 cations, since many soil fertility tests are based on the chemical com- 

 position of the soil solution rather than of the whole soil. Such an equi- 

 librium system can be represented as outlined in Figure 4. 



SOIL 



SOIL SOLUTION 





Figure 4. Diagram showing a soil suspension or gel in equi- 

 librium with its filtrate or soil solution. 



