Hoagland — 132 — Plant Nutrition 



from a potassium salt solution, in an initial period 

 of absorption, more equivalents of potassium than of 

 even a mobile anion like bromide. The solution be- 

 comes more acid, although the increase in hydrogen 

 ion concentration does not account for all the excess 

 of potassium absorbed. The balance of cations and 

 anions is in part maintained by entrance into the 

 solutions of calcium and magnesium ions, possibly 

 derived from protoplasmic or cell wall phases. From 

 solutions of potassium sulphate, potassium ions have 

 always been observed to enter the roots much more 

 rapidly than the sulphate anions, which are absorbed 

 extremely slowly. These selective absorptions of ca- 

 tions lead to increased organic acid synthesis by the 

 tissue. 



Decidedly striking is the effect observed when 

 potassium is accumulated from a dilute solution of 

 potassium bicarbonate. Under aerobic conditions 

 potassium is absorbed as readily from the bicarbonate 

 as from the bromide.* The entering potassium is 

 approximately balanced by newly formed organic 

 acids. If potassium moves inward accompanied by 

 bicarbonate ions the latter are later broken down and 

 the carbon dioxide given off. Bicarbonate ions do not 

 appreciably accumulate in the cells. Some investi- 

 gators would prefer to regard the absorption of potas- 

 sium as a potassium-hydrogen ion exchange, accom- 

 panied by formation of organic acids by the roots. 

 The net effect would be the same for either mechanism. 



Less readily demonstrated but frequently signifi- 

 cant are decreases in organic acid content of roots 

 found when anions are absorbed in excess of cations. 

 The anions are then replaced in the culture solution, 

 directly or indirectly, by bicarbonate ions derived in 

 part, it would appear, from oxidation of organic acids. 



* Since bicarbonate ions are not considered to stimulate 

 respiration, it is difficult to understand how potassium accumu- 

 lation can be related to the theory of anion respiration under 

 these conditions. 



