Mineral Nutrients 161 



strated that there is no excretion of a mineral acid, and that 

 any organic acids present are beyond the sensitiveness of 

 litmus. Nevertheless, some investigators have found other 

 acids present under certain conditions. These conditions 

 are mainly poor oxygen supply. Stoklasa and Ernst, 

 for example, have identified traces of acetic acid and 

 formic acid with poor oxygenation of the roots of corn and 

 barley. Under such circumstances these may be regarded 

 as evidence of unfavorable surroundings, and not as excre- 

 tions beneficial to the plant. Under similar circumstances 

 oxalic acid was identified in the case of the sugar-beet 

 and of the hyacinth. From the preceding it seems safe 

 to assume that in the case of cultivated plants normal 

 solvent action is due to CO 2 (an excrete product produced 

 by every living cell ; cf . respiration) . 



It should be observed, however, that recent studies 

 by Schreiner and Reed call special attention to the 

 oxidizing action of roots. This seems to be brought 

 about by a peroxidase, and the process may be practically 

 important, since many cultural practices are designed 

 to promote oxidation. 



88. CO 2 excretion and the availability of phosphorus. 

 Stoklasa and Ernst have further given some data indi- 

 cating that the relative rate of excretion of CO 2 by the 

 roots per gram of dry weight of substance is directly im- 

 portant in determining the capacity of a plant to get phos- 

 phoric acid from the more insoluble substrata. The follow- 

 ing table exhibits side by side the excretion of CO 2 , as 

 shown by water cultures, and the absorption of phosphoric 

 acid when the same kinds of plants are grown in gneiss 

 and basalt : 



