Apr., 1923] TOTTINGHAM AND RANKIN-AVAILABILITY OF IRON 
209 
hydrogen-ion concentration of the iron-free nutrient solution at pH 4.2, 
the growth of young wheat plants shows ferric citrate to be decidedly the 
most available form of iron. In this form, 10 milligrams of iron per liter 
appear to be sufficient for the growth of these plants. 
With the exception of the less favorable results from FeSC>4 as com¬ 
pared with its high solubility, the variations of efficiency in these various 
forms of iron can be attributed either to differences in solubility or to differ¬ 
ent modifying effects upon the pH value of the nutrient solution. Thus, 
ferric citrate reduces the hydrogen-ion concentration and acts favorably, 
while Fe2(S04)3 increases the concentration of this ion and is an injurious 
source of iron. The inferiority of FeSCU as compared with ferric citrate 
may reside in toxic properties of the former, as suggested by Jones and Shive 
( 9 )- 
It seems probable that the formation of either Fe(OH) 3 or basic iron 
salts by hydrolysis of inorganic salts of iron will render these unavailable 
in general as sources of iron in nutrient solutions. Yet ferric hydroxide 
may be superior to ferric phosphate as a source of iron in some cases, as 
indicated by a previous observation (13) that barley seemed to acquire a 
much improved supply of iron from a nutrient solution containing suspended 
ferric phosphate when ferric hydroxide was also added. Apparently the 
use of FePCh should be discriminative, because of its low solubility. These 
results confirm previous observations of the favorable effects of iron applied 
to nutrient solutions in the form of salts of organic acids. The conclusions 
find support in both the appearance of plants, especially as to intensity of 
green color of the leaf, and the data of plant measurements. 
Summary 
1. Results are here given relative to the solubility and availability 
to young wheat plants of various compounds of iron in a particular form 
of nutrient solution. 
2. Solubility tests at different pH values of the nutrient solution have 
shown that ferric phosphate is relatively insoluble. This is true of ferric 
and ferrous sulphate at a hydrogen-ion concentration of the iron-free nu¬ 
trient solution approaching neutrality. While ferric citrate is not very 
soluble, it possesses the advantage of remaining soluble over a considerable 
range of pH values of the nutrient solution. 
3. Ferric sulphate increases the hydrogen-ion concentration of the nu¬ 
trient solution here used, while ferric citrate causes the opposite effect. 
The other forms of iron tested have little influence in this respect. 
4. The growth measurements of the young wheat plants show that 
ferric citrate was decidedly the most favorable form of iron here employed. 
The variation in efficiency of iron in the forms supplied is correlated with 
variation either in the solubility of this element or in the modification of 
the pH value of the nutrient solution. The results show clearly that ferric 
phosphate is likely to be inefficient because of its low solubility. 
