Aug. 15, 1921 
Effect of Ammonium Sulphate upon Plants 
721 
Experience has, of course, shown that culture solutions may not be 
expected to produce good growth of green plants if they contain more 
than a very small quantity of available iron, and observations of toxicity 
produced by iron under various experimental conditions are frequent 
in the literature. Thatcher ( 16 ) makes the broad statement that— 
Only soluble ferric compounds seem to serve as a suitable source of supply of the 
element; ferrous compounds being usually highly toxic to plants. 
In this connection, Hartwell and Pember (7) found ferrous sulphate to 
be toxic to barley and rye seedlings when added to Knop’s solution in 
concentrations of N/ 5,000. This concentration, however, is much higher 
than the concentrations of ferrous sulphate used in the present experi¬ 
ments. These authors state that the absolute effect of a given strength 
of ferrous sulphate would be expected to depend upon the frequency of 
the renewed applications and upon the nature of the nutrient solution 
employed. Rupreclit ( 14 :) also found that ferrous sulphate when present 
in culture solutions above four parts per million of iron exerts a toxic effect 
upon clover seedlings. 
From a consideration of the data presented in the preceding pages 
it is evident that the form and quantity of iron in a medium for plant 
growth is a very important factor. In experiment I of this study the 
iron supplied to the plants in the form of ferric phosphate was entirely 
inadequate to support plant growth in the Tottingham solutions. When, 
however, iron in this form was supplied to the plants in the solutions con¬ 
taining ammonium -sulphate which was substituted for the potassium 
nitrate in the Tottingham solutions, iron did not become a limiting factor 
for growth and these solutions produced excellent plants which were 
vigorous and healthy throughout the early stages of growth extending 
over a period of five weeks. On the other hand, when iron was supplied 
in the form of the soluble ferrous sulphate, the Tottingham solutions 
supported excellent growth and produced very high yields of both tops 
and roots, while the solutions containing ammonium sulphate were 
exceedingly toxic to the plants, and this toxicity was intensified by the use 
of higher concentrations of the iron salt but disappeared entirely when 
only a small trace of iron (0.01 mgm. per liter) was supplied. It appears, 
therefore, that the injury to the plants did not result from the direct 
influence of the ammonium salt here used but was caused by the iron 
which was made available in excessive quantities through the presence 
of the ammonium salt. 
YIELDS OF TOPS AND ROOTS IN RELATION TO SALT PROPORTIONS 
The relation between yields and salt proportions will here be con¬ 
sidered for two series only, these being the Tottingham series (A) of 
experiment II and the ammonium-sulphate series (B) of experiment I. 
In these two series the plants were at all times healthy and vigorous and 
suffered no apparent injury. The plants of the Tottingham series (A) 
