PLANT ROOT-SOIL INTERACTIONS 671 



root. Similar relationships existed for sodium systems. Ion uptakes, as 

 usually measured, are net-influxes. 



Ratner (Jenny, 1952) also has observed extensive desorption by 

 clay of potassium from roots, and even from stems and leaves. It is im- 

 portant to record that in all trials the roots and plants always appeared 

 healthy and revealed no physiological signs of root injury. 



An interesting valence effect of ion uptake from colloidal systems, 

 advanced by Mattson ( Mattson, 1948; \\Tklander, 1955), states that 

 in any colloidal system— root or soil— the adsorption of bivalent ions in 

 relation to univalent ions is favored by high exchange capacities. Since 

 legume roots often have considerably higher exchange capacities than 

 grass roots do, the former should have, and do have, higher Ca/Na 

 ratios than the latter, when grown in the same nutrient medium. Con- 

 versely, for a given plant species, its Ca/Na ratio should decrease as 

 the soil's exchange capacity increases. Elgabaly and \\'iklander 

 (Wiklander, 1955, 1957) noted that the Ca/Na ratio of barley roots 

 was 0.72 when grown in kaolinite suspension (low exchange capacity), 

 and only 0.45 when grown in bentonite suspension (high exchange 

 capacity). It is somewhat surprising that this theorem operates with 

 living roots, since the uptake of ions is strongly controlled metabolically 

 whereas the valence effect operates non-metabolically. 



Though significant from the point of view of growth, these ob- 

 servations on plant-soil interactions provide no clue to the mechanisms 

 involved in the counter flow of ions. The aforementioned hydrolysis of 

 exchangeable ions may play a part, also the exchange with H2CO3 

 produced by respiration. 



Jenny and Overstreet ( 1939 ) suggested an additional mechanism 

 of interaction— the contact model, or contact-exchange theory. Briefly, 

 if root surfaces and clay particles are brought together so closely that 

 their electric double layers (cation swarms) intermingle, a mutual 

 transfer of ions is facilitated. In other words, the exchangeable ions of 

 root and soil are credited with reactivity in the root-soil contact zone, 

 independently of the soil-solution processes. 



Tivo-phase experiments 



In a strict sense the contact model can neither be proved nor dis- 

 proved. But certain consequences of surface interaction may be ex- 

 amined by separating the participating phases in one way or another. 

 For instance, plants may be grown in a clay suspension and in a solu- 

 tion which is in equilibrium with it, the two resembling the two phases 

 of a classical Donnan system. In fact, two-phase experiments have be- 

 come the key criterion for assessing the direct contribution of exchange- 

 able ions in plant nutrition and the plausibility and range of applica- 



