7H 
Journal of Agricultural Research 
Vol. XXI, No. IO 
in the size of the plants of the different cultures in this series as a result 
of variations in the salt proportions of the culture solutions, as the dry 
weights in Table III will show. 11 ms, by the use of the soluble ferrous 
sulphate in small quantities, instead of the insoluble ferric phosphate 
used in the solutions of the preceding experiment, the general health 
and vigor of the plants in the Tottingham series was very greatly im¬ 
proved, while the opposite effect was experienced by the plants in the 
cultures of the ammonium-sulphate series. 
INFLUENCE OF THE PLANTS UPON THE, IIYIIROGEN-ION CONCENTRATION OF THE MEDIA 
. The summarized data of the hydrogen-ion concentration determina¬ 
tions of this experiment are given in Table VI in terms of P„ values. 
The data of this table correspond in every respect to the similar data of 
Table IV. From the data of Table VI it is again evident that the 
growing plants in the Tottingham series showed a marked tendency to 
change the reaction of the solutions toward neutrality, while the direction 
of this change in reac¬ 
tion was reversed by 
the growing plants in 
the solutions of the 
ammonium-sulphate 
series. In the latter 
series the average of 
the P H values obtained 
at the end of the sev¬ 
eral growth intervals 
was lower for each cul¬ 
ture solution through¬ 
out the entire series 
than was the corresponding initial P n value, although the increase in 
the hydrogen-ion concentration was not very great in any case. 
The averages of the P n values determined at the end of the various 
growth intervals are always much higher for the cultures of the Totting¬ 
ham series than they are for the corresponding cultures of the ammonium- 
sulphate series. This is clearly brought out by the graphs of figure 4. 
A direct comparison of these graphs with the corresponding ones of the 
preceding experiment (fig. 1) shows that the two sets of graphs are gen¬ 
erally in good agreement. This indicates that the influence of the 
growing plants upon the reaction of the culture solutions here used was 
practically the same when the iron was supplied in the form of the 
insoluble ferric phosphate or in the form of the soluble ferrous sulphate. 
The detailed hydrogen-ion concentration data for the culture solutions 
producing highest, medium, and lowest yield given in Table VII show 
in a general way the direction and the magnitude of the change in 
6.0 
6.5 
6,0 
4.5 
Series A - 
Se 
ries 
: B ■ 
- - - 
— 
- 
^ * 
* *** % 
T- 1 1 1 1 1 1 1 3 1 1 3 3 3 5 3 ! 7 3 5 5 
R- 3 6 6 1 1 3 7 5 3 13 1 3 3 11 1 1 1 1 
C- 33136611171331511131 
Fig. 4.—Graphs of P H values of culture solutions after contact with 
plant roots during growth interval between solution renewals; 
averages of tests during growth period of experiment II. 
