7 12 Journal of A gricultural Research v 0 i. xxi, no. io 
The graphs of figure 2 show at once that the yields of tops from the 
cultures of the ammonium-sulphate series are much superior to the yields 
from the corresponding cultures of the Tottingham series. In general 
this is true also with respect to the yields of roots from the two series as 
is indicated by the graphs of figure 3. However, the root yields from 
three cultures of the ammonium-sulphate series are actually lower than 
are the yields from the corresponding cultures of the Tottingham series. 
The superiority of the yields from the ammonium-sulphate series is 
clearly not due to a greater efficiency of the four main salt constituents 
or combinations of the solutions of the series but is the direct result of 
the difference in the availability of the iron in the form here used in the 
solutions of the two 
series during the first 
five weeks of growth. 
In the Tottingham 
series the growth of 
the plants was greatly 
retarded after the 
third week of growth 
by a deficiency in the 
supply of available 
iron, while no such 
deficiency of iron was 
at any time apparent 
in the culture solu¬ 
tions of the ammo¬ 
nium-sulphate series. 
The nature of the nu¬ 
trient solutions with 
respect to their salt 
constituents and hydrogen-ion concentration here appears to determine 
the availability and the efficiency of the ferric phosphate as a source of 
iron for the plants. In connection with this study the work of Wolkoff 
(21) is of interest. In a sand culture procedure with soybeans this 
author used the same series of nutrient solutions employed in the pres¬ 
ent study, but the solutions had an osmotic concentration value of 2.5 
atmospheres and iron was supplied in the form of iron rust. A marked 
superiority of the yields from the culture containing ammonium sul¬ 
phate over those from the Tottingham solutions was shown, but the 
author states that the most pronounced differences in the plants from 
the two series of cultures were those of color, the plants from the cul¬ 
tures containing ammonium sulphate always being much greener than 
those from the Tottingham series. 
Fig. 2.—Graphs of actual yield vaules of wheat tops for the Tottingham 
series and the ammonium-sulphate series of Experiment I. 
