Over street and Dean 97 



less, it is hoped that it gives a picture of the present state of uncertainty 

 concerning these compounds and indicates a wide field for future re- 

 searches. 



Information concerning the reversibility of absorption reactions 



As will be emphasized in a later section, a knowledge of the reversi- 

 bility of ion absorption processes is essential for the formulation of soil 

 factors affecting ion availability. Here again our information is very 

 meager. In a study of the effects of suspensions of hydrogen clays on 

 barley roots, Jenny and Overstreet (24) obtained results that perhaps 

 serve as evidence for the reversibility of the reactions 



R • H + K + > R • K + H + 



and 



2 R • H + Ca ++ > R 2 • Ca + 2 H + 



The plant roots were observed to lose significant fractions of their 

 potassium and calcium contents after a few hours of contact with 

 dilute hydrogen-bentonite suspensions. Evidently this loss was not the 

 result of permanent injury, since subsequently it could be shown that 

 the tissue absorbed normally from standard nutrient solutions. 



In so far as can be determined, evidence for the reversibility of anion 

 absorption is even less substantial than in the case of cation absorption. 

 In some unpublished work by J. M. Heslep, plant roots were placed 

 in suspensions of clays of the kaolinite type. In a few hours significant 

 amounts of phosphate were observed to leave the roots and become 

 fixed on the clay. This may be taken as presumptive evidence for the 

 reversibility of the reaction 



R' • OH + H 2 P0 4 " > R' • H 2 P0 4 + OH". 



A series of experiments testing the reversibility of absorption re- 

 actions in general are being undertaken in the Divisions of Soils and 

 Plant Nutrition at Berkeley. 



The effects of concentrations and activities on 

 absorption rates in nutrient solutions 



Ion-absorption experiments conducted with flowing nutrient solu- 



