(Vor. 7 
42 ANNALS OF THE MISSOURI BOTANICAL GARDEN 
Hoagland and his associates (717, '18, '19) in a series of articles 
have pointed out some misinterpretations and discrepancies in 
the earlier work, and among other things have shown that with 
certain proportions of salts in solution cultures and in sand cul- 
tures having an initial acid reaction, this reaction was changed 
with the growth of the crops until it was approximately neutral. 
In certain cultures with an initial reaction approximately neutral, 
plants were grown to maturity without change of solution, and 
with the reaction remaining constant throughout. A nutrient 
solution strongly alkaline from the presence of K;PO, became 
approximately neutral. Hoagland has also emphasized an 
important point appreciated likewise by some earlier investi- 
gators, namely, that the equivalence of positive and negative 
ions in the solutions is maintained, and the state of equilibrium, 
the recognition of which is often too vague, is of necessity kept in 
mind in any discussion of the absorption of ions. 
In this paper it has been pointed out that there is generally a 
decline in growth in solutions A and C when the H-ion concen- 
tration is approximately Px 6. This may be due to the relative 
insolubility of the phosphates. On the other hand, the generally 
more favorable growth in the B solution at or approaching neu- 
trality may be related in part to the better distribution of phos- 
phate ions or particles, due to the presence of certain substances 
in a state of greater dispersity. In this connection it will be 
recalled that Bonazzi (19) and Allen (19) have contributed 
interesting data on the favorable effects of shaking or agitation, 
on the growth of Azotobacter chroococcum, an organism neces- 
sarily grown in alkaline solutions. 
Toole and Tottingham's (18) results, showing increased 
yield with the addition of ferric hydroxide to Knop's solution, 
are also of particular interest in considering the data obtained 
in this work with solution B. At present there seems to be no 
basis for a final opinion on the róle of aluminium in promoting 
growth in these experiments. The high adsorptive property of 
the compound used, together with its buffer action, may be con- 
cerned in the explanation. The maximum effect of this com- 
pound occurred with wheat, but in view of the complication of 
factors involved this may not be significant. 
In the various series of experiments here reported there is 
