Vol. XXI, No. io 
702 Journal of Agricultural Research 
attempted. Mention should be made, however, of the important work 
of Hutchinson and Miller (it) in which they summarize a review of the 
more definite contributions to the literature on this broad subject. As 
a result of their own careful research on the direct assimilation of ammo¬ 
nium salts by higher plants, these authors conclude that agricultural 
plants can develop normally when supplied with nitrogen only in the 
form of ammonium salts, that some plants grow equally well with ammo¬ 
nium or nitrate as a source of nitrogen, and others, while capable of 
assimilating ammoniacal nitrogen, appear better able to utilize nitrates. 
They express the doubt, however— 
whether ammonium salts can ever produce better final results than nitrates. 
In some preliminary experiments with nutrient solutions of the 
Tottingham {18) type in comparison with these solutions in which the 
potassium nitrate (KN 0 3 ) was replaced by ammonium sulphate in 
equivalent osmotic concentrations, it was found that the solutions con¬ 
taining ammonium sulphate were toxic to wheat and soybean plants 
when soluble ferrous sulphate (FeSOJ in quantities of 1 mgm. of iron per 
liter of nutrient solution was added as a source of iron for the plants. 
When, however, equivalent amounts of iron in the form of the insoluble 
ferric phosphate (FeP 0 4 ) were added to the solutions containing ammo¬ 
nium sulphate, the toxic effect upon the plants entirely disappeared. 
The phenomena observed in connection with the use of the two forms of 
iron (soluble and insoluble) in the presence of the ammonium ion in com¬ 
plete nutrient solutions appeared to be of sufficient importance to war¬ 
rant careful investigation. A comparative study was, therefore, made 
of the physiological effects produced upon wheat and soybean plants 
during the early stages of development by the two types of nutrient 
solutions just mentioned—the Tottingham solutions unmodified and 
these solutions in which ammonium sulphate was substituted for the 
potassium nitrate—with traces of a soluble ferrous compound and an 
insoluble ferric compound added as sources of iron for the plants. A 
study was also made of the influence of the plants upon the media in 
which they were grown. The results obtained with wheat will alone be 
presented in the present paper. Those for soybeans will appear in a 
later publication. 
METHODS OF PROCEDURE 
The experiments considered in the following pages each comprised two 
series of cultures. In the first of these two series, 20 representative solu¬ 
tions chosen from Tottingham’s ( 18) complete series of 84 were used. The 
20 solutions chosen are uniformly distributed throughout the series and 
will be designated by the culture numbers referring to the positions 
which they occupy in the series and on the 4-coordinate diagrammatic 
scheme employed by Tottingham. The second series was like the first 
in every respect, except that ammonium sulphate in equal or- 10 tie con- 
