Hoagland — 18 — Plant Nutrition 



culture solutions in order to avoid the complexities of 

 soil conditions. In view of impressions gained from 

 the literature of the time, it was with some surprise 

 that we observed that at a pH of 5 barley plants 

 showed no injury and in fact this reaction was highly 

 favorable. Later there followed in research in soil 

 science and plant nutrition what might be called a 

 hydrogen ion or pH period. Thousands of pH meas- 

 urements on soils were made, and some enthusiasts 

 sought to explain ecological distribution of plants and 

 agricultural suitability of soils almost solely in terms 

 of hydrogen ion effects.* But it has been learned 

 that physiological relations of plants to their media 

 are not so simple as that. Hydrogen ion concentration 

 must be considered in relation to many other inter- 

 reacting factors. It seems to be the internal reaction 

 in plant cells rather than the external reaction that 

 must be closely regulated. An opportunity will be 

 afforded in subsequent lectures to say more on hydro- 

 gen ion relations of the culture medium to plant 

 growth, but what I have already stated with reference 

 to adsorbed calcium and hydrogen ions may be recalled 

 now as pertinent to the general question. 



Experiments by Artificial Culture Methods : — In 



addition to the hydrogen ion studies performed by the 

 water culture technique, we inaugurated other water 

 culture studies to ascertain if we could to some degree 

 imitate soil solutions under more highly controlled 

 conditions than a soil medium could afford. About 

 this time a project was sponsored by the National 

 Research Council, in which numerous investigators 

 were invited to participate, for making water culture 

 studies on plants with the objective of finding out the 



*The observation should also be offered that the interpre- 

 tation of a hydrogen ion value on a complex system like that 

 of the soil with its liquid and solid phases opens many questions. 

 When measurements are made at low moisture contents, doubts 

 may arise that the actually measured potentials really signify 

 hydrogen ion concentrations or activities. 



