Hoagland —120— Plant Nutrition 



able conditions. Therefore the plant forms innumer- 

 able points of intimate contact with colloidal soil par- 

 ticles. 



Some of the contact effects between root and soil 

 colloid may be interpreted in terms of soil solution 

 phenomena, but several of my colleagues, especially 

 Jenny and Overstreet (1939), have recently pro- 

 posed that a more direct contact relation operates.* 

 They have sought to study this question by a com- 

 bination of a water culture and colloid system. Puri- 

 fied clay colloids with different proportions of adsorbed 

 ions were prepared and suspended in water or in nu- 

 trient salt solutions. Excised barley roots of high 

 salt absorbing capacity, like those already described, 

 were immersed in the suspensions. In many instances 

 the movement of ions between root and colloid was 

 investigated usefully by radioactive tracer methods, 

 particularly with reference to cation movement. 



The theory underlying the explanation of some of 

 the results is that ions adsorbed on colloidal surfaces 

 may undergo surface migration, one ion exchanging 

 for another within the oscillation volumes of the ions. 

 There is direct evidence that this can happen in a soil 

 colloid system. Jenny and Overstreet go further 

 and believe that such an ion exchange can occur 

 between the root and soil colloid and form an efficient 

 mechanism for the removal by the plant of ions ad- 

 sorbed on the soil colloids, through exchange of meta- 

 bolically produced hydrogen ions for calcium, mag- 

 nesium, potassium, or other ions held by the soil 

 colloids. The chief immediate nutrient reserve for 

 some ions is found in these adsorbed ions. Never- 

 theless, the soil solution mechanism by no means 

 loses its interest because of the possibilities of contact 

 absorption. Certain ions, notably nitrate ions, are not 

 adsorbed to any important degree by the soil colloids 



*An early suggestion was made by Comber (1922) for a 

 modification of the soil solution hypothesis. 



