Hoagland —168— Plant Nutrition 



the trees receiving an adequate supply of potassium, 

 hardly any of the calcium was present in dissolved 

 form and only part of the magnesium. In the par- 

 ticular circumstances sodium was not an important 

 constituent of the tissues. 



The concentration of potassium was very low in 

 the expressed sap of the leaves of the potassium 

 deficient trees and the deficit of potassium base was 

 not compensated for in this system except in some 

 measure by magnesium. This means that in the nor- 

 mal trees potassium and magnesium were the prin- 

 cipal bases in the leaf sap, with potassium predom- 

 inating, while in the potassium deficient trees the total 

 content of fixed bases was low and the relation of 

 potassium to magnesium was greatly altered (mag- 

 nesium increasing in relation to potassium) , with what- 

 ever influence these alterations in base relations have 

 on the protoplasmic colloids in contact with the sap. 

 Another result was that the pB. of the expressed leaf 

 sap of the deficient trees was significantly lower than 

 that of the normal trees. As bearing on these rela- 

 tions, the organic acid system discussed in the preced- 

 ing lecture should be kept in view. The effects I men- 

 tion were apparent before the leaves reached a stage 

 at which external signs of injury became evident but 

 one can reasonably assume that changes in the chem- 

 ical constitution of the leaf were the cause of the 

 subsequently manifest injury. 



The relations of potassium, sodium and rubidium 

 in the physiology of the plant have received extensive 

 discussion during the history of plant nutrition. Potas- 

 sium and rubidium are closely similar in many chem- 

 ical properties, so much so that their analytical sep- 

 aration is difficult. Yet rubidium cannot be substituted 

 for potassium in the growth of higher plants and 

 the former element may have a toxic effect not pro- 

 duced by potassium. For algae and fungi statements 

 have occasionally been made that rubidium can take 

 the place of potassium, but there is apparently no 



