704 Journal of Agricultural Research voi.xxi, No. 10 
Iron was supplied to all the cultures in equivalent amounts either in 
the form of the so-called insoluble ferric phosphate or the soluble ferrous 
sulphate. These two forms of iron were chosen in order to test the 
effect of a soluble and an insoluble form of iron upon growing plants in 
connection with the two types of nutrient solutions used, and still not 
introduce anions different from those provided by the four main salts 
present in the solutions in considerable quantities. The ferric phosphate 
was added to the solutions from a stock supply prepared as described 
in a previous publication (12), while the ferrous sulphate was added in 
the form of an aqueous solution freshly prepared each time just before 
being used in the culture solutions. This form of iron does not precipi¬ 
tate so rapidly nor so completely from the culture solutions here used 
as do other forms of Soluble iron. By direct qualitative tests made 
from four to seven days after the ferrous sulphate was added to the 
culture solutions, the presence of dissolved iron could still be detected. 
As indicated by these qualitative tests, the capacity of iron citrate for 
maintaining its solubility in these solutions was equally as strong as that 
of ferrous sulphate. This is in accord with the view of Hoagland (zo), 
who states that iron citrate and tartrate appear to be the most efficient 
sources of iron. The availability and efficiency of a given iron salt in 
relation to plant growth are, of course, determined largely by the con¬ 
centration of the iron salt used and by the concentration, reaction, and 
composition of the nutrient solution in which it is used, as Gile and 
Carrero (5) have shown. Duggar (2) very successfully used what he 
calls “soluble ferric phosphate/’ which apparenlty consists of ferric 
phosphate with sodium citrate, as a single salt. He states that this 
salt combination possesses the advantage of solubility to a high degree 
and introduces no difficulties in the preparation of the solutions but 
may render their composition somewhat less definite. 
The plants used as indicators consisted of spring wheat of the Marquis 
variety. The seeds were germinated on a germinating net like that used 
by Sliive (15). All the seedlings used were carefully selected for uni¬ 
formity of size and general appearance as to health and vigor and were 
transferred to the culture solutions when about 5 cm. tall. Three 
seedlings were comprised in each culture. The seedlings were mounted 
in the double-piece paraffined cork stoppers as devised by Tottingham 
(18). These were of the proper size to fit the quart fruit jars which 
were used as containers. To exclude light from the roots and to pre¬ 
vent heat absorption when the cultures were exposed to sunlight the 
containers were covered with cylindrical shells black within and light 
in color on the outside like those described by Sliive (15). 
The solutions were renewed regularly at intervals of 3 % to 4 days. 
At the time of each renewal of the solutions tests of the hydrogen-ion 
concentration of each of the old solutions was made by the colorimetric 
