7io 
Journal of Agricultural Research 
Vol. XXI, No. io 
It appears that the type of the culture solution as determined by the 
nature of ion constituents is in a large measure responsible for the direction 
of the change in reaction produced by the growing plants. Hoagland 
( io ) in a recent publication has emphasized the point that a nutrient 
solution is an exceedingly complex system and that it does not appear 
possible at the present time to determine quantitatively the ions and 
undissociated salts which it may contain during or after the growth of 
plants in it, much less does it appear possible to determine the exact 
relationship existing between the different components of such a complex 
system. In an earlier publication (8) the last-named author came to the 
conclusion that plants may regulate the reaction of the nutrient medium 
in such a way that excessive hydrogen or hydroxyl ions can not occur. 
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!•—Graphs of Ph values of culture solutions after contact with 
plant roots during growth interval between solution renewals; aver¬ 
ages of tests during growth period of experiment I. Also diagram 
of score values for yellowness. Broad and narrow vertical lines 
represent score for plants of Tottingham series and ammonium- 
sulphate series, respectively. 
In later work (9) he 
found that barley 
plants grown for seven 
weeks in a favorable 
nutrient solution when 
transferred to single 
salt solutions of va¬ 
rious salts did not 
bring about an un¬ 
favorable condition of 
acidity or alkalinity. 
In this work, however, 
he noted that ammo- 
nium-chlorid solutions 
retained an acid reac¬ 
tion and the hydrogen- 
ion concentration was 
slightly increased after 
contact with the 
plants. Toole and 
Totting-ham (77), 
working with Knop’s solution, call attention to the fact that growing 
barley seedlings always had a marked neutralizing effect upon the 
medium. It is of interest here to note that plants grown in the Shive’s 
( J 5) 3-salt solution always show a marked tendency to decrease the 
hydrogen-ion concentrations, but when ammonium sulphate, even in 
relatively small amounts, is superimposed upon this solution the direction 
of the change in reaction is reversed, just as it is when this salt is substi¬ 
tuted for potassium nitrate in the Tottingham solutions. This is true of 
plants during at least the first four weeks of growth. 
Inspection of the data in Table IV brings out the fact that the initial 
P n values of the corresponding solutions of the two series are approxi¬ 
mately the same. The averages of the P H values determined at the end 
of the various growth intervals are, however, always much higher for the 
