62 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9 
of the other iron salts. This behavior of ferric phosphate was very interest- 
ing when it formed as a precipitate when ferric nitrate was added directly 
to solution ''A." A 0.001 N concentration did not produce plants of a 
normal green color. A 0.002 N concentration was decidedly toxic. It is 
singular that growth should be depressed by the addition of an iron salt, 
before sufficient iron is available for the growth of the plant. When the 
efficiency of ferric nitrate as a source of iron is compared with that of 
ferrous sulphate, the result is striking. At 0.0005 ^> the yield in the latter 
is 68 percent of that in the former; while at 0.001 A"", the ferrous salt be- 
comes very toxic and the ferric salt gives the better yield. This greater 
physiological activity of the ferrous salt may be correlated with its greater 
influence on catalytic activities. Other factors, however, may be con- 
cerned. The ferrous salt is less readily precipitated than the ferric. There 
may also be a difference in the solubility of the two phosphates which may 
be influenced by the concentration of phosphorus and calcium in the solu- 
tion. Any attempt at an explanation of this difference can be only specu- 
lative' until more data are available. 
The concentration of ferrous sulphate necessary to give the maximum 
yield with corn in solution ''A" is probably close to 0.0005 N. This is 
approximately 14 mg. per liter. The iron requirements of corn are evi- 
dently very different from those of wheat and probably also from those of 
rice as determined by Shive and Jones ('21) and Gile and Carrero ('20). 
The findings of Corson and Bakke ('17) apparently are not applicable to 
corn. It is somewhat difficult to interpret Maze's recommendation of 
a ferric rather than a ferrous salt. He used a concentration of 50 mg. of 
ferrous sulphate per liter, an amount which would have been extremely 
toxic in solution "H." Maze ('13) noted that everything depends on the 
relative proportion of all elements. This is well illustrated by the behavior 
of ferrous sulphate in solutions "H" and "A." A concentration of it in 
solution ''H," two fifths of that required for optimum growth in solution 
*'A," reduced the yield 25 percent to 30 percent. A doubling of this con- 
centration in solution H " reduced the yield almost 50 percent. The three 
ferric salts are about alike toxic, and their effect upon growth in solution 
"H," when compared to ferrous sulphate, is practically of the same order 
as the relative efficiency of ferric nitrate and ferrous sulphate in promoting 
growth in solution "A." At a 0.0002 N concentration they are practically 
without effect upon the yield ; and at 0.0004 ^» they gave about a 15 percent 
better yield than the ferrous salt. 
The toxicity of aluminum salts at the higher concentrations does not 
seem to be a function of the concentration. The average depressions are: 
0.0001 N 10 percent, 0.0002 TV 28 percent, 0.0004 ^40 percent, 0.0006 iV42 
percent in the solution cultures, and 0.0002 N 26 percent, 0.0004 ^ 39 
percent in the sand cultures. There is an increase in toxicity of aluminum 
salts, as measured by yield of tops, with increasing concentration; but it is 
