374 Jones and Skive.—Influence of Ammonium Sulphate 
to those from the other two series. On the whole, it appears that the 
modified Tottingham solutions of series G, in which nitrogen is available for 
the plants in the forms of both nitrate and ammonium, are somewhat more 
efficient than are the unmodified Tottingham solutions for the growth of 
soy bean plants during the early phases of development, but any of the 
solutions whose formulae appear in Table VI may be expected to produce' 
good growth of this species if supplied with a suitable form of iron in the 
proper concentrations. 
Summary. 
The experiments described in this paper were conducted for the purpose 
of studying, in a comparative way, the effects of ammonium sulphate upon 
the growth of soy beans in nutrient solutions during the early stages of 
development, and to determine the influence of this salt upon the availability 
for the plants of different forms of iron. A study was made also of the 
reaction change of the nutrient solutions induced by contact with plant 
roots. Two type-series of culture solutions were used. The first of these 
comprised twenty solutions selected from the Tottingham series of eighty- 
four, and the second consisted of the same solutions modified by substituting 
ammonium sulphate for the potassium nitrate in equivalent osmotic con¬ 
centrations. All the solutions had a total osmotic concentration value of 
approximately one atmosphere. Ferric phosphate in an aqueous suspension 
and ferrous sulphate in solution form were added in stated amounts of iron 
per litre of nutrient solution. The culture solutions were renewed at 
regular intervals of three and one-half days throughout the growth period 
of approximately five weeks. 
The main results of the experiments may be summarized as 
follows: 
1. The plants grown in the Tottingham solutions invariably produced 
a marked decrease in the hydrogen-ion concentration of the solutions. 
2. The plants grown in the solutions containing ammonium sulphate 
invariably increased the hydrogen-ion concentration of these solutions during 
the early stages of growth, the hydrogen-ion concentrations of these solutions 
being maintained at a much higher level than those of the unmodified 
Tottingham solutions, although the initial pH values of corresponding 
solutions of the two types were practically the same. 
3. The nature of the salt constituents determines the direction of the 
reaction change of the culture solutions in contact with the roots of the 
growing soy bean plants. 
4. Ferric phosphate in quantities of less than one milligram of iron per 
litre of nutrient solution was not sufficiently available in the Tottingham 
solutions to supply the needs of the plants for iron during the early stages 
