VALUE OF CERTAIN NUTRITIVE ELEMENTS 303 
More recently thorough studies on the physico-chemical relations 
existing between the various compounds and elements in culture 
solutions have been made in an attempt to understand some of the 
factors neglected in the earlier work. Livingston (1906), Breazeale 
(1905), Osterhout (19066), Tottingham (1914), and Shive (1915a, b) 
have contributed to our knowledge of balanced nutrient solutions. 
It has been the purpose of the experiments herein recorded to 
study the effect of some of the essential nutrient elements upon the 
development and composition of plants when other physico-chemical 
factors, such as unequal osmotic pressure and the addition of new 
chemical elements, were controlled as far as possible. 
The culture work from May to August, 19 15, and May to August, 
1 916, was carried on under climatic conditions different from those, 
that governed the latter part of the work, which differences may in 
some cases explain the differences between the data recorded for the 
two respective periods. In a later paper a comparison will be made 
of the meteorological conditions for the two periods. 
From September, 191 6, to date the work has been carried on in 
the laboratory of plant physiology at the University of Wisconsin. 
It is a pleasure to acknowledge my indebtedness to Professor James 
Bertram Overton for his advice and assistance in supplying the rather 
extensive apparatus necessary during the course of this investigation. 
Methods 
Knop's standard nutrient solution, slightly modified by adding 
0.1 gram of sodium chloride per liter, w^hich made a salt concentration 
of 1.5 grams per liter instead of 1.4 grams, was used as the standard 
nutrient solution throughout the series of experiments. Standard 
laboratory chemicals were used during the first two years, but on 
account of the large proportion of impurities, which caused consider- 
able difficulty in the physical study of the solutions and which might 
also be expected to produce important modifications in the results of 
the experiments, it was thought best to use the purest chemicals 
obtainable for all later work. Kahlbaum's analyzed chemicals were 
finally used, with the exception of potassium nitrate and calcium 
chloride, of which Merck's "Blue Label" brand was used, and with 
the exception also of calcium nitrate, which was prepared from a 
special grade of nitric acid and Iceland spar (CaCOs). All chemicals 
