375 
on Plant Growth in Nutrient Solutions, &c. 
of growth. On the other hand, this form of iron in quantities of less than 
one-half milligram of iron per litre of solution was ample to supply the 
needs of the plants for this element in the solutions containing ammonium 
sulphate. The maintenance by the plants of a higher level in the hydrogen- 
ion concentration of the solutions containing ammonium sulphate, and the 
possible influence of this salt upon the permeability of the plant cells 
towards iron, undoubtedly account for the greater efficiency of iron in these 
solutions. 
5. Ferrous sulphate in quantities of from 0-25 to 0-50 milligram of 
iron per litre of nutrient solution was sufficiently available in the Tottingham 
solutions to satisfy the needs of the plants for iron. However, ferrous 
sulphate in the solution containing ammonium sulphate produces a condition 
very toxic to the plants, the degree of toxicity increasing with increase in 
the amounts of iron from 0-25 to 5-00 milligrams per litre of nutrient 
solution. 
6. Ferrous sulphate when used in too high concentrations produces on 
the leaves of soy bean plants in both series a characteristic brown specking 
which is more pronounced in the solutions containing ammonium sulphate 
than in the unmodified Tottingham solutions. 
7. The availability for the plants of a given iron compound and its 
efficiency appear to be determined in large measure by the composition of 
the nutrient solution and by the nature of the reaction change induced by 
contact with the plant roots. 
8. With the solutions containing the salts KN 0 3 , KH 2 P 0 4 , Ca(N 0 3 ) 2 , 
and MgS 0 4 (Tottingham solutions) the maximum yield of soy bean tops was 
obtained when these salts were present in the volume-molecular proportions 
o-oo2o, o-oo2ii, 0-00730, and 0-00711, respectively, with ferrous sulphate 
as the source of iron. The maximum yield of roots was obtained with 
a solution containing these salts in the volume-molecular proportions 
o-co2o, 0-00633, 0-00438, and 0-00711, respectively, with ferrous sulphate as 
the source of iron. Good yields of tops and roots were obtained with only 
a narrow range in the proportions of potassium nitrate, but with relatively 
wide ranges in the proportions of the other salts. 
9. With the solutions containing (NH 4 ) 2 S 0 4 , KH 2 P 0 4 , Ca(N 0 3 ) 2 , and 
MgS 0 4 the maximum yield of both tops and roots was obtained when these 
salts were present in the volume-molecular proportions 0-0014, 0-00211, 
0-00146, and 0-01659, respectively, with ferric phosphate as the source of 
iron. Good yields of tops and roots were obtained with only the lowest 
proportions of ammonium sulphate but with relatively wide ranges in the 
proportions of the other salts. 
10. In general, high yields of tops were associated with high yields of 
roots. 
