Hoagland —112— Plant Nutrition 



solution — that of what might be termed "economical" 

 utilization of nutrients. By this is meant the prep- 

 aration of a solution from which the plant will absorb 

 the various ions in the same proportions as they are 

 originally present in the solution. This is a goal that 

 is seldom possible of achievement, because of the 

 dynamic nature of the plant system and the varied 

 inter-relations between ions in absorption, also because 

 of the effects on absorption of nutrients of the physio- 

 logical age of the plant. Yet we have had some success 

 in preparing a nutrient solution along this line of 

 approach for the growth of tomato plants (Arnon 

 and Hoagland, 1940). It is scarcely necessary to say 

 that the adjustment was only a rough one, but still 

 it was useful as a practical method of managing the 

 culture solution. 



In previous lectures several observations on hydro- 

 gen ion effects in nutrient solutions have been made. 

 Something more on this point should be added now. 

 None of our experiments suggest that plant growth, 

 at least of any of the numerous species studied, is 

 notably influenced by small variations in the pH of 

 the culture media. In fact there seems to be a rela- 

 tively wide range of hydrogen ion concentrations over 

 which this factor per se has no highly significant 

 effect on the plant. To isolate the hydrogen-ion va- 

 riable under ordinary conditions of plant growth is 

 not easy and perhaps never completely effected, even 

 with more or less controlled artificial cultures. Thus, 

 at a relatively high pH, plants grown in a nutrient 

 solution may become chlorotic when inorganic iron 

 and manganese salts supply these elements for the 

 reason that precipitations may occur. 



Considerable importance can be attached to the 

 inter-relations between hydrogen ion and calcium ion 

 concentrations. An example is found in some recent 

 experiments conducted in our laboratories by Arnon 

 and his coworkers (1942). It was desired to deter- 



