Lecture 5 — 113 — Artificial Media 



mine pB. effects over a range of reactions. If a full 

 nutrient solution were employed for this purpose, pre- 

 cipitation of calcium phosphate, or other salts, would 

 take place at markedly alkaline reactions and so the 

 general composition of the solution would not be the 

 same over the whole range of pH. The basic solution 

 was therefore regulated in its composition in such a 

 way that all the salts remained in solution at the 

 highest pR value it was sought to study. Also, to 

 assure a supply of iron and manganese at alkaline 

 reactions, these metals were furnished as synthetic 

 humates. In other words, insofar as this is feasible, 

 hydrogen ion concentration was made the chief va- 

 riable throughout the entire set of cultures in certain 

 of the series of experiments. The plants were grown 

 in large volumes of nutrient solution, thoroughly 

 stirred by aeration, so that within limits the desired 

 hydrogen-ion concentration was maintained at root 

 surfaces. Adjustments of reaction were also made as 

 necessary, by addition of acid or alkali. 



For all species of plants investigated, a pH main- 

 tained at 3.0 in the culture solution was markedly 

 injurious, whatever other modifications were made in 

 the solution. Just above the range of acid reaction 

 that could not be tolerated by the plant the effect of 

 hydrogen-ion concentration could be significantly in- 

 fluenced by the calcium ion concentration. In the 

 weakly acid (pH 6) or in the slightly alkaline solu- 

 tions no pronounced effect of varying calcium con- 

 centrations was apparent within a considerable con- 

 centration range. At the more acid reactions, how- 

 ever, the growth and vigor of the plants were superior 

 at a high calcium concentration, as compared with a 

 relatively low one. These relations have an interest- 

 ing bearing on some of the problems of acid soils. In 

 mineral soils a high hydrogen-ion concentration is 

 likely to be accompanied by a low supplying power for 

 calcium where, if the water culture experiments are 



