728 PLANT GROWTH AND PLANT COMMUNITIES 



• TABLE II 

 Uptake of K and Ca by Young Soybeans from Chlorides and Bicarbonates 



tween the hydrogen ion and a soluble anion will shift the equilibrium 

 so as to favor uptake. Equally, one would expect that bonding of the 

 nutrient cation by any anion (colloidal or other) in the nutrient sub- 

 strate would decrease uptake. 



But here one must examine certain limitations in the application 

 of this conclusion. If a chloride and a bicarbonate system oflFer suffi- 

 ciently high concentrations of, say, potassium, then the initial uptake 

 will lie on the part of the total curve that is insensitive. Similarly, when 

 comparison is made between a highly bonded and a loosely bonded 

 metallic cation, if both systems o£Fer equal and sufficiently high ionic 

 activities, differences in initial uptake might well be small. Thus, in 

 general, bonding effects, like absolute activity levels, will show them- 

 selves markedly only at quite low concentrations, the ranges of sensi- 

 tivity being different for different elements. 



It will be noted that "initial" uptake is referred to above. This 

 might be defined as that part of the total uptake which operates with- 

 out significant change in the external medium. For such effects to be 

 unequivocally shown, stringent conditions must govern the renewal of 

 nutrient solution in relation to the accumulation products from the 

 roots. 



In the experiments reported by Marshall and Upchurch ( 1953 ) , 

 calculation of the bonding energy balance, as between the hy- 

 drogen ion and a metallic cation on a given exchanger, revealed sev- 

 eral cases in which this was a critical factor. It was apparent that dif- 

 ferences upwards of 1,000 calories per mole were necessary in order to 

 show such effects clearly. (In comparing these with other factors it 



