360 Jones and Skive.—Influence of Ammonium Sulphate 
The dry weight yield values of this second experiment showed that the 
cultures of the Tottingham series (series A) were much superior to those of 
the series containing ammonium sulphate (series B). With the changing of 
one factor, that of the source of iron, a reversal of the superiority of the two 
types of solutions was brought about. When ferric phosphate is used as 
a source of iron, the solutions containing ammonium sulphate can make 
soluble enough iron from the amounts added to meet the requirements 
of the plants, but when ferrous sulphate is used in the solution containing 
ammonium sulphate it gives evidence of a toxic character not present in the 
Tottingham solutions. 
Thus the indications are that the choice of a source of iron for plants in 
a nutrient solution must be very carefully made, with respect to the chemical 
composition of such a solution, in order to avoid iron toxicity and still supply 
sufficient available iron to prevent chlorosis in the plants. 
Gris ( 11 ) in 1844 was the first to show the necessity of iron for the 
formation of chlorophyll in plants. The exact manner in which the plant 
utilizes iron, the most efficient form, and the proper amount to be supplied 
under a given set of experimental conditions, are yet to be determined. 
Tottingham and Beck (38) have shown that the response to iron during the 
early stages of growth by plants is dependent upon the amount of iron stored 
in the seed. Corson and Bakke (3), employing both ferrous and ferric forms 
of phosphate as sources of iron in nutrient solutions, found differences in their 
efficiency, and also differences in the response of different plants. 
Jones and Shive (19) with Shive’s three-salt solution R-C 2 (36), and 
employing different increments of iron in the forms of ferrous sulphate and 
ferric phosphate, have shown that the insoluble ferric phosphate is not 
suitable for use with spring wheat in this culture solution. Ferrous sulphate, 
on the other hand, gave excellent results when supplied in quantities of 
°75 to 3*0 milligrams of iron per litre of nutrient solution. In a later 
publication ( 20 ) it was shown that in a solution containing ammonium 
sulphate the ferrous form of iron was quite toxic, and the ferric form 
sufficiently available to prevent chlorosis in the plants. 
Gile and Carrero ( 8 , 9), in solution cultures, have shown that the 
reaction, concentration of the solution, and amount of iron used have 
a marked influence upon the availability of iron for rice plants. Maze (28), 
in a complete nutrient solution with ferrous sulphate (100 milligrams per 
litre) as a source of iron, found that the presence of ammonium salts of 
500 milligrams per litre produced a toxic condition for maize. Hartwell 
and Pember (13) found ferrous sulphate toxic to barley and rye seedlings in 
Knop’s solution in quantities of five parts per million of iron or over, while 
Ruprecht (35) in another nutrient solution found ferrous sulphate to be toxic 
to clover seedlings with four parts per million. 
To demonstrate more definitely the effect of these two forms of iron, 
