Lecture 7 — 169 — Potassium Nutrition 



conclusive proof of this. Any preparations of rubidium 

 salts that have been used probably were not free of 

 potassium. There is no evidence that sodium can 

 replace potassium except in part. 



A distinction should be made between partial and 

 complete substitution of chemical elements. The latter 

 is meant in connection with the remarks just made. 

 Even though complete physiological replacement cannot 

 be effected, it would still be possible that a degree of 

 substitution could occur. There is some reason to be- 

 lieve that in this sense sodium can decrease the potas- 

 sium requirements of plants. It is not difficult to 

 understand that sodium might partially take the place 

 of potassium as a component of the sap buffer system, 

 but there is the limitation in this action, as already 

 noted, that many species of plants do not absorb 

 sodium from dilute solutions as rapidly as they absorb 

 potassium. 



These various arguments point to the importance 

 of potassium in many plant buffer systems. At the 

 same time it is necessary to recognize that some kinds 

 of plants do ordinarily contain a large amount of cal- 

 cium and magnesium in their sap and that these ions 

 are likewise of importance in the plant buffer systems. 

 The alfalfa plant might be mentioned as one that nor- 

 mally not only has a high total calcium content, but 

 also a considerable amount of calcium in soluble form 

 and presumably present in the cell sap. 



Other Functions of Potassium : — The impression 

 should, of course, be avoided that the function of 

 potassium is restricted to a role in buffer systems. 

 There must be other functions of this element and 

 they are, unfortunately, still to a large extent obscure. 

 In the general literature of plant science a favorite 

 idea is that potassium is essential for photosynthesis 

 or for carbohydrate metabolism. It is perhaps not 

 inappropriate to remark at this point that since potas- 

 sium is an essential element for plant growth it is 



