720 
Journal of Agricultural Research 
Vol. XXI, No. io 
The graphs representing the yields from the Tottingham solutions 
(series C), on the other hand, show that in these solutions the variations 
in the amounts of iron within the range here used (o.oi mgm. to 5.0 mgm. 
per liter of solution) did not produce marked differences in the yields. 
All the solutions of this series produced relatively high yields, and there 
were no evidences of specific toxicity, although, as previously stated, the 
0.01-mgm. and the 0.10-mgm. portions of iron were not sufficient to 
prevent chlorosis in the plants grown in these solutions. 
It will be observed that the control culture (without iron) in the ammo¬ 
nium-sulphate series (series D) produced a much higher yield than did 
the control culture in the Tottingham series (series C). This agrees with 
the general appearance of the plants in these two cultures, the plants in 
the former being green and apparently healthy while those in the latter 
were chlorotic. The large difference in the dry-weight yields of the 
plants from these control cultures may possibly be accounted for by the 
fact that the magnesium-sulphate and monopotassium-phosphate 
crystals contained small amounts of iron (less than 0.001 per cent by 
analysis) as an impurity. This very small trace of iron was perhaps 
soluble in the solution containing ammonium sulphate and available to 
the plants and insoluble and, therefore, unavailable to the plants in the 
Tottingham solution. This is offered only as a suggestion and may not 
explain the real cause of the large difference in dry top yields, since the 
difference in the root yields from these control cultures is very slight. 
In this connection it might be suggested that the toxic influence of the 
iron in the solutions containing ammonium sulphate is perhaps directly 
related to the hydrogen-ion concentrations of the culture solutions. 
The initial hydrogen-ion exponents of the solutions used in series C and D 
are approximately the same (Pn 4.7); but the hydrogen-ion concentra¬ 
tions of the Tottingham solutions change rather rapidly toward the 
neutral point when in contact with the plant roots, while under the same 
conditions the hydrogen-ion concentrations of the solutions containing 
ammonium sulphate are increased, or at least maintained at their initial 
values, during the several growth intervals between successive solution 
changes throughout the experiment period as is indicated by the data in 
Table VIII. Since iron compounds are less soluble in culture solutions 
with lower hydrogen-ion concentrations, it is possible that in the Tot¬ 
tingham solutions with decreasing concentrations of these ions sufficient 
iron was perhaps removed from solution by precipitation to prevent the 
toxic influence of this element. The suggestion here made appears to 
gain some support from actual observation. Precipitates invariably 
occurred in both the Tottingham solutions and the solutions containing 
ammonium sulphate, in which the proportions of iron as here used were 
relatively high, after standing in contact with the plant roots; but these 
precipitates always appeared first in the Tottingham solutions and were 
always much more pronounced in these than they were in the solutions 
containing ammonium sulphate. 
