Lecture 7. 



ASPECTS OF THE POTASSIUM NUTRITION OF 



PLANTS AS ILLUSTRATING PROBLEMS OF 



THE SYSTEM, SOIL-PLANT-ATMOSPHERE 



Introductory Statement 



About fifteen years ago several members of the 

 Agricultural Experiment Station in California under- 

 took an investigation of a disease of prune trees oc- 

 curring in the upper part of the Sacramento Valley, 

 which presented a problem of practical importance. 

 The trees in the affected districts often grew for five 

 to eight years in a satisfactory way and then, when 

 they came into heavy bearing, the so-called "die-back" 

 disease appeared, although at least one type of soil 

 produced injury at an early stage of growth. One of 

 the chief symptoms of injury was a scorching of the 

 leaves, occasionally preceded by a slight chlorosis. The 

 scorching of leaves assumes various patterns and 

 sometimes is accompanied by the dying of small cir- 

 cular areas of tissues, which fall out and leave a "shot- 

 hole" effect. After a period of severe injury the upper 

 part of the tree dies back. In the course of several 

 seasons severely injured trees may die or become 

 worthless. 



While some aspects of the injury to the trees sug- 

 gested potassium deficiency as one cause of the trouble 

 it was at first deemed doubtful that this factor was 

 concerned, since potassium deficiencies in California 

 soils were not expected, especially in the growth of 

 fruit trees. By general inspection the soils in which 



