726 PLANT GROWTH AND PLANT COMMUNITIES 



particular cation, in the nature of the complementary ions and in the 

 nature of colloidal particles ( Barber and Marshall, 1951, 1952; Marshall 

 and Patniak, 1958; Marshall and Upchurch, 1953). In terms of plant 

 nutrition, we can get an idea of its usefulness by reference to Arnon's 

 and Grossenbacher's observation ( 1947 ) that plants grown upon the 

 calcium form of the sulphonic acid exclianger Amberlite IR 100 suf- 

 fered from calcium deficiency. Much exchangeable calcium was pres- 

 ent, but apparently the plants could not successfully compete for it. 



However, this type of experiment needs very careful examination 

 before the bonding of a single ion can be isolated as the main factor in 

 plant response. Actually, Arnon and Grossenbacher used a mixed resin 

 system which contained all the nutrient cations and anions in exchange- 

 able form. Was the observed calcium deficiency due to an unfavorable 

 potassium-calcium ratio in the equilibrium solution, or was the ab- 

 solute level of calcium activity in the whole resin system responsible? 

 If both factors need to be considered, how would it be possible to dis- 

 tinguish them experimentally? Before examining the situation in 

 greater detail, certain experiments in solution cultures must now be 

 considered. 



Absolute activity levels 



First we must see at what point in the cationic concentration or ac- 

 tivity scale definite signs of deficiency appear. This varies, of course, for 

 diflFerent elements, but in all cases it is low. To fix it definitely is not 

 simple. The rate of growth and the relationship to other nutrients both 

 need consideration. Furthermore, since cations can readily pass from 

 root to substrate, very good conditions for renewal of the nutrient 

 media must be maintained. 



In this laboratory we have, on three occasions, carried out such 

 experiments on soybeans. In the first series (McLean, 1948) relatively 

 simple comparisons of bicarbonate solutions with Wyoming bentonite 

 suspensions were made. In one set of comparisons, only calcium was 

 ofiFered in the substrate. In four other comparisons, varying proportions 

 of calcium and potassium were employed; at each level the respective 

 calcium and potassium activities of the bicarbonate and bentonite sys- 

 tems were equal. In no case was there evidence of calcium or potassium 

 deficiency. The lowest calcium activity— namely, 0.7 x 10"^ mols/L— 

 was evidently ample, since four times this amount caused no significant 

 increase in uptake. The lowest potassium activity was 1.6 x 10"^ mols/L, 

 and this also gave close to the maximum uptake. 



In the second series (Upchurch, 1953) chlorides, bicarbonates, 

 Wyoming bentonite, Putnam clay, and Amberlite IR 120 were used as 

 substrates. In the mixed cationic systems (chloride, bicarbonate, and 



