458 
E. H. TOOLE AND W. E. TOTTINGHAM 
these two values varying in opposite directions; but the variations in 
acidity are so small that they can hardly, in themselves, explain the 
differences in yield. Even the comparatively high hydrogen-ion 
concentration of the solution which contained carbon black was 
decidedly below that found by Hoagland (13) to be favorable to barley 
seedlings in his nutrient solutions with controlled hydrogen-ion con- 
tent. He found that a hydrogen-ion concentration of o.7xio~^N. 
(Ph, 5.2) was not injurious, while our culture with the highest acidity 
was 0.2 X io~^ N. (Ph, 5.7). Attention is called to the fact that, in all 
cases, the growing barley seedlings had a marked neutralizing effect on 
the nutrient solution, and that the solutions in which the plants had 
been grown were more nearly uniform in hydrogen-ion concentration 
than the original solutions. Hoagland (13) found that acid solutions 
decreased slightly in hydrogen-ion concentration, that alkaline solu- 
tions decreased markedly in hydroxy 1-ion concentration, and that 
neutral solutions tended to remain constant during the growth of barley 
seedlings. 
The correlation of the very small amount of soluble phosphorus in 
the solution containing ferric hydrate with the increased yield from 
that solution suggests that this paucity of soluble phosphates may be 
the critical factor responsible for the observed results, possibly enhanc- 
ing growth by rendering the solution nearly neutral. From these re- 
sults, it would seem that the nutrient solution proposed by Crone (14), 
using the slightly soluble ferrous and tri-calcium phosphates instead of 
potassium phosphate, deserves more extended trial than it appears to 
have received. Our results are also in accord with the observations of 
Truog (15) that the barley plant can get a sufficient supply of phos- 
phorus from precipitated ferric phosphate. From the uniformity of 
root growth in all cultures, it would seem that we have here, not a 
case of the direct toxicity of the phosphate to the root, but rather a 
difference in the use of the materials within the plant under some 
difference in external conditions. Although a sufficient supply of 
phosphorus is necessary for the full development of plants, the presence 
of an excess of the phosphate ion in the culture solution would seem 
to be, sometimes, a limiting factor in growth. Among others, McCall 
(16) has also noted this effect in his sand cultures. The very dark 
color of the leaves of all our cultures to which ferric hydrate had been 
added might indicate a greater utilization of the iron under these con- 
ditions. 
