on Plant Growth in Nutrient Solutions , cfc. 35.7 
more closely and under controlled conditions the effect of the ammonium- 
ion on the plants, on the reaction of the nutrient medium, and on the 
availability of other nutrient elements, especially that of iron. 
Methods of Procedure. 
The experimental work of this study was carried out with nutrient 
solutions as the culture media. Two series of solutions were used, each 
comprising twenty cultures. In the first series twenty representative 
solutions were chosen from the eighty-four of Tottingham’s ( 37 ) complete 
series. The twenty solutions selected are uniformly distributed throughout 
the series, and are designated by the culture numbers referring to the 
positions which they occupy on the four-co-ordinate diagrammatic scheme 
employed by Tottingham. The second series was like the first in every 
respect, except that ammonium sulphate in equal osmotic concentrations 
was substituted for the potassium nitrate in the Tottingham solutions of the 
first series. Two control solutions serving as standards for comparison were 
added to each experiment. These consisted of Tottingham’s best solution 
for wheat, number ToR^C^ with a total osmotic concentration of 2*5 atmo¬ 
spheres, and Shive’s ( 36 ) best solution for wheat, number R 5 C 2 with a total 
osmotic concentration of 1-75 atmospheres. The latter contains no 
potassium nitrate. Thus any marked differences in the response of the 
plants towards the nutrient media in the corresponding cultures of the two 
series compared with the controls could be attributed to the influence of the 
ammonium sulphate upon the plants either directly or indirectly, assuming 
the cultures to be subjected alike to all other experimental conditions. 
Baker’s analysed salts were used to prepare the half-molecular stock 
solutions from which the culture solutions were made up. Table I gives 
the numbers of the cultures, which correspond to the numbers designating 
the solutions selected from the Tottingham series, and the partial volume- 
molecular concentrations of the salts as they occurred in the solutions of the 
two series to give a calculated total osmotic concentration value of one 
atmosphere. Cryoscopic determinations showed that this concentration was 
closely approximated in the solutions. 
The essential element 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 salts contain no anion different from 
those provided by the four main salts which are contained in each solution. 
These two sources of iron were used to determine which was the more 
efficient form of iron in the solutions of the two types. The ferric phosphate 
was supplied as a suspension in water from a stock supply prepared as 
described in a previous publication ( 19 ), and the ferrous sulphate was added 
in the form of a freshly prepared aqueous solution. The latter does not 
precipitate so rapidly nor so completely from the culture solutions here used 
