on Plant Growth in Nutrient Solutions , &c. 363 
effect it may have on permeability, makes the iron phosphate so available that 
the iron requirements of the plants are satisfied with a very small amount of 
iron in this form. With the Tottingham solutions greater amounts of the 
ferric phosphate must be added to the solutions to obtain maximum yields. 
The fact must not be overlooked, however, that different indicator plants 
may have different iron requirements, that a growth period of thirty-five 
days is required to bring soy bean plants to the flowering stage, and that 
during the reproductive and the later growth phases the iron requirements 
Fig. 1. Graphs of actual yield values of soy bean tops and roots grown in Tottingham’s 
solution TqiqCs (series C), and in the ammonium-sulphate modification of this solution (series D) r 
supplied with varying amounts of iron in the form of ferric phosphate. 
of these plants may be entirely different from the demands during the 
vegetative phases of growth. 
The dry-weight yield values from the cultures supplied with ferrous 
sulphate, as given in Table II, are graphically represented in Fig. 2. It will 
be observed from this figure and from the data of Table III that ferrous 
sulphate has a very depressing effect upon the dry weight yields of both tops 
and roots in the ammonium-sulphate series D. That this is a toxic effect is 
very evident from the general appearance of the plants, and from the fact 
that the yields are very much lower than the control in which no iron was 
supplied. On the other hand, the soluble ferrous sulphate is a very good 
source of iron for the plants in the solutions of the Tottingham series C, even 
in very small amounts. 
The toxicity which occurs with plants grown in solutions containing 
