SOIL-PLANT RELATIONSHIPS AND PLANT NUTRITION 379 



with an isotopically enriched solution containing the same ion. Such "accumu- 

 lated" nutrients are not subsequently free to move as long as the cells are 

 active. 



The evidence for the existence of ion carriers is wholly indirect. It finds 

 support in kinetic studies and in the parallelism shown with enzyme-substrate 

 mechanisms. The synthesis of the carriers is assumed to be dependent on 

 the respiratory process in the cell, but interactions are suspected, thus com- 

 plicating the picture; that is, one ion may be essential in the synthesis of a 

 carrier for another ion, or even may constitute a part of the carrier complex 

 for another ion. 



This theory calls for the uptake of all ions to be dependent upon meta- 

 bolically synthesized ion carriers. A more restricted theory links anion uptake 

 with the operation of the cytochrome system and regards cation uptake as 

 incidental by exchange for the hydrogen ions liberated in the cell. Inhibitors 

 of cytochrome oxidase activity quench active uptake, but cellular metabolic 

 processes are so interrelated and interdependent that caution has to be ex- 

 erted in ascribing specific roles to particular mechanisms. 



There still remains much to be clarified in the basic process of nutrient 

 uptake and in accounting for root behavior in the nutritional environment 

 provided by soil. There seems to be some doubt among some plant physiolo- 

 gists as to whether the cation-exchange capacity of roots is of any consequence 

 and whether exchange to such sites constitutes any essential part of the ion- 

 absorption process. Because they lack specificity, the cation-retaining sites of 

 plant roots are not believed to be those involved in active uptake; moreover, 

 some are external and some are internal. Most experimentation on nutrient 

 uptake is carried out using very young seedlings immersed in simple salt solu- 

 tions containing a large excess of the ion under study. By diffusion the free 

 space rapidly comes into equilibrium, and by exchange the cation-retention 

 sites become extensively filled with the cations supplied. The situation in soil 

 may be very different because there the external sites at least will directly 

 reflect the cation array on the soil colloids; through equilibration this will 

 result in a modification of the composition of the soil solution in immediate 

 contact with the external surface of the roots, and therefore by diffusion that 

 of the solution in the free space. 



Ultimately it should be found possible to reconcile and incorporate in a 

 unified scheme the soil-solution theory of plant nutrition in soils and the 

 contact-exchange theory. As with the soil population where that in the im- 

 mediate vicinity of the roots is recognizably different from that at a distance, 

 so it is likely that the soil solution in the immediate vicinity of roots may be 

 substantially modified by the presence of ions on the surfaces of the root and 

 the micro-organisms thereon. The equilibrium reached is likely to be a com- 

 plex one between the surfaces of clay and organic colloids, on the one hand, 

 and actively metabolizing root tissues and micro-organisms, on the other. 



