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DUGGAR—H-ION CONCENTRATION AND NUTRIENT SOLUTIONS 9 
The problem was then to shift the hydrogen ion concentration 
of the solutions used without disturbing any more than necessary 
the composition of the medium. From preliminary tests of 
solutions A and B it was inferred that the chief interest might 
attach to the addition of alkali to solution A and of acid to 
solution B. The Py exponent of the former is low, but in the 
latter it more nearly approaches neutrality. It is seen that the 
active acidity of the Shive solution is due to dihydrogen potas- 
sium phosphate, and the relatively slight variation in the pro- 
portions of the other practically neutral salts may be assumed 
not to change materially the hydrogen ion concentration. In 
this sense, therefore, the 108 solutions tested by Shive are prob- 
ably nearly identical and determine, in respect to growth, the 
values of the proportions of the nutrient ions present in the 
solutions only in relation to this one value of Py. This value 
might, of course, be assumed to be most favorable under all 
environmental conditions, but it is equally possible that it is not. 
This is a factor which should be given most careful considera- 
tion in all mineral nutrient, or salt-balance, studies as well as in 
studies upon toxic action. 
As a convenient index to the content of the various cultures a 
simple scheme of notation has been devised, which, with very 
` slight call upon the memory, enables one to see at a glance the 
constitution of the culture medium. This involves a system of 
letters and numerals as explained below. The initial letter of 
a culture refers to the general constitution of the nutrient solu- 
tion, and as stated above there are three such solutions. The 
modified Shive solution employed is designated A; the solution 
remotely based on the Crone formula is solution B; and the 
Livingston-Tottingham solution, C. The next letter is invari- 
ably a small letter and denotes the plant employed; thus w desig- 
nates wheat; p, Canada field peas; and e, corn; while x is em- 
ployed where more than a single kind of plant is designated, or 
where no particular plant is specified. A culture index such as 
ApO, BpO, or CpO, etc., refers to the use of peas with the solu- 
tions mentioned without the addition of acid, alkali, or other 
constituent. When acid or alkali is added an inclined line repre- 
sents, in this case, neutrality, and when figures are given above 
the line mentioned they represent ec. of 0.092 molar phosphoric 
