526 
Journal of Agricultural Research 
Vol. VII, No. 12 
rially affecting the appearance of the plants. It is, therefore, not suffi¬ 
cient to judge the adequacy of the iron supply by the mere color of the 
leaves. Also a sufficiency of iron can not be insured by simply increasing 
the quantity of iron added to the solution as this may even diminish the 
amount of available iron. 
Although this work demonstrated chiefly the reaction of the solution 
and the source of iron as factors influencing the availability of iron, the 
importance of other conditions was indicated. Undoubtedly the fre¬ 
quency with which the nutrient solution is changed influences appreciably 
the availability of the iron. The results in Table XII showed that the 
precipitated iron increased with the age of the solution and also with the 
growth of plants in the solution. This increase in precipitated iron prob¬ 
ably accompanied a certain decrease in soluble iron, as there was doubt¬ 
less some balance between the precipitated, colloidal, and soluble iron. 
Doubtless absorption also affected the quantity of soluble iron in certain 
cases. The diminution in growth following the addition of carbon black 
to the solution with 0.002 gm. of iron per liter in experiment 10 was prob¬ 
ably caused by a decrease in soluble iron through simple adsorption. In 
experiment 4 the decrease in available iron following an increase in the 
quantity of iron added to the alkaline solution may have been partially 
due to the adsorption of soluble iron by the larger precipitate of iron. 
On the basis that hydrolysis was the chief factor determining the 
amount of soluble iron in the solutions, one would not expect that increas¬ 
ing the phosphate^ in the neutral solution would appreciably affect the 
iron assimilated by the plants. Experiment 11 confirmed this. 1 
This work furnished no evidence of rice being able to assimilate other 
than soluble iron, but tended to confirm previous work showing that even 
colloidal iron is unavailable (6). It did show, however, that rice used 
iron, which must have been present in exceedingly low concentrations. 
The facts established concerning the availability of iron in these nutrient 
solutions help explain results obtained by certain investigators with other 
nutrient solutions. It is realized, however, that the availability of iron 
in each nutrient solution probably varies according to the concentration 
and composition of the solution as well as according to the method of con¬ 
ducting the cultural test. 
The chlorosis of peas in certain nutrient solutions observed by Maze et 
al. (10) was evidently not due to the excretion of calcium carbonate by 
the roots, but to salts in the solution depressing the availability of the 
iron. Probably the potassium silicate, which is strongly hydrolyzed into 
potassium hydroxid, was chiefly responsible for the nonavailability of 
the iron. The chlorosis observed by Von Crone (3) with plants in cer¬ 
tain nutrient solutions was not due to the soluble phosphates, but prob¬ 
ably to a deficiency of iron. In certain solutions the lack of iron was 
1 If the concentration of phosphate ions had been very low, such as would be afforded by tricalcium or 
ferric phosphate, doubtless the addition of soluble phosphates would have depressed the available iron. 
