152 Mineral Nutrition of Plants 



brought into solution, after about 2 to 3 hours and during the next 

 15 to 20 hour period, an amount of potassium which, in all cases, was 

 greatest for the undried materials and which diminished as the tem- 

 perature of drying increased. 



In explanation, Gorbunov regarded the heating and drying as re- 

 sponsible for a process of aging of the colloidal material during which 

 much of the diffuse part of the electrical double layer was destroyed. 

 That is, combination of potassium ions with residual negative charges 

 on the absorptive material made the latter less electronegative. He ap- 

 pears to have regarded this change as in large part irreversible and 

 accompanied by the formation of microaggregates with reduced dis- 

 persibility. According to this hypothesis, aggregation may hinder the 

 replaceability of the ion concerned, in this case potassium. Gorbunov 

 made no statements concerning the size of the "microaggregates." On 

 the other hand, he considered that organic matter plays an important 

 role in the fixation process. Gorbunov's hypothesis does not satisfac- 

 torily explain the relationships reported by Rubashov (61) between 

 aggregate size and replaceable potassium. Martin, Overstreet, and 

 Hoagland (49), on the other hand, found that potassium fixation may 

 be considerable in moist soils and, contrary to Gorbunov, do not con- 

 sider that drying is directly involved. The clarification of these apparent 

 inconsistencies awaits more work by chemists and physicists on the 

 mechanisms of the aggregation process. 



Experiments were recently conducted by Greacen (57) upon the 

 applicability of Boyd's equations (75) to cation exchange in columns 

 of soil aggregates. Greacen gave special attention to mechanisms which 

 might control rates of exchange between soil-adsorbed cations and 

 cations present in dilute, permeating solutions. Rates at which soil 

 aggregates of different sizes released their cations were measured by 

 passing solutions at controlled velocities through very short columns 

 of aggregates. 



Cation exchange for the Yolo and Aiken soil aggregates within the 

 "fine" size class, 0.3-0.1 mm., was found to be an equilibrium process 

 during passage, through a column of calcium aggregates, of N/50 mag- 

 nesium chloride solutions at macroscopic velocities of less than 0.3 cm. 

 per second. For coarser aggregates, > 0.6 mm., of Aiken soil, diffusion 



