Hoagland — 20 — Plant Nutrition 



tion of the nature of the process of absorption of 

 salt by the plant that appealed to us as especially de- 

 serving of attention in the attempt to advance our 

 understanding of the interrelations of soil and plant. 

 I hope to return to these and related questions in later 

 lectures. 



In the early studies on soils and artificial nutrient 

 solutions, almost always attention was centered on the 

 seven classical essential elements that the plant had 

 to obtain from its nutrient medium (nitrogen, calcium, 

 magnesium, potassium, sulphur, phosphorus and iron). 

 The essential role of other elements was unregarded, 

 save by a few investigators, who themselves generally 

 had no conclusive evidence. The development of knowl- 

 edge of certain chemical elements needed by plants in 

 minute amounts constitutes a striking feature of the 

 study of plant nutrition, both practical and scientific, 

 during the past decade. I intend to devote the next 

 lecture to this subject. 



Climatic Influences : — Finally, in all efforts to elu- 

 cidate the relation of nutrient solutions to plant 

 growth, the climatic complex enters into the equation. 

 Experiments with culture solutions had all been made 

 in greenhouses, subject to great fluctuations of tem- 

 perature, light and humidity. Most experiments of 

 necessity must be carried out in this way. In the 

 modern period, however, various developments have 

 occurred in the direction of efforts to impose for 

 special experimental purposes as complete control as 

 possible of the plant's environment; that is, control 

 of the nutrient solution, illumination, temperature, 

 humidity, and air movement. If I may mention 

 only one of these developments for purposes of 

 illustration. Dr. Davis and I, a considerable number 

 of years ago, became interested in the use of air- 

 conditioned chambers illuminated by artificial light, 

 and succeeded in growing young wheat plants with 

 such control of the environment of the solution and 



