Lecture 7 — 167 — Potassium Nutrition 



up of a high potassium content is effected with special 

 faciUty. We speak of a "luxury" absorption when the 

 content of potassium in the tissues increases without 

 increase of growth. 



There is also another physiological aspect of the 

 interrelations of bases after these have been absorbed 

 by the plant. The major part of the potassium in 

 plant tissues is located either in the cell sap or, if not, 

 is at least easily soluble in water; although a small 

 proportion of the potassium can exist in difficultly 

 soluble form in some plants. On the other hand, a 

 much larger percentage of the calcium and magnesium 

 is found in difficultly soluble form. It has long been 

 known that in some species of plants calcium oxalate 

 is present and forms crystals or precipitates. Pectin 

 or protein compounds are capable of combining with 

 or adsorbing calcium or magnesium ions. 



Some of these facts have a significance with refer- 

 ence to plant sap buffer systems that is not always 

 appreciated. If the supply of potassium from the nu- 

 trient medium is low and the potassium content of 

 the plant tissues is diminished as a result, the content 

 of fixed base in the sap decreases if much of the cal- 

 cium and magnesium goes out of solution in the plant, 

 even though the content of total calcium and magne- 

 sium in the tissues as a whole increases by amounts 

 equivalent to the decreased absorption of potassium. 

 Many species of plants do not absorb sodium rapidly 

 enough to substitute for potassium for this purpose, 

 or sodium may not be available in sufficient amounts 

 in the medium. 



I should like to present a specific consideration 

 from the studies on the potassium nutrition of prune 

 trees to which I referred at the opening of this lecture. 

 The observations to be described were made on trees 

 grown in Berkeley in cylinders of soil of one thousand 

 pounds capacity. While the potassium deficient trees 

 usually contained in their leaves a higher percentage 

 of total calcium and magnesium than was found in 



