Feb., 1922] 
ARNDT — THE GROWTH OF FIELD CORN 
49 
the ferrous salt to the less toxic ferric salt. The addition of calcium salts 
and phosphates to toxic solutions of both salts also lessened the injurious 
effect by aiding the formation of insoluble iron salts. 
It has already been noted that most soluble iron and aluminum salts 
are readily hydrolyzed with an accompanying liberation of the acid radicle, 
Daikuhara ('14), Abbott, Conner, and Smalley ('13), Hartwell and Pember 
('18), and Mirasol ('20) state that all soils which indicate injury to plants- 
by aluminum are acid. This acidity is usually reported as the lime require- 
ment of the soil. Recent work by Joffe ('20) indicates that this lime re- 
quirement is closely related to the H-ion concentration. Duggar ('20) has 
shown that the H-ion concentration may be a limiting factor in plant growth 
and that the effect of any particular concentration upon plant growth 
varies with the plant used. Good yields were secured with corn with H-ion 
concentrations varying from a pH of 3.2 to one of 7.1. The optimum H-ion 
concentration depended upon the nutrient solution and possibly to some 
extent upon the environmental conditions. Hoagland ('17) reported that 
a H-ion concentration of 0.3 X lO"^ is toxic to barley seedlings. These 
facts emphasize the necessity of determining the depression caused by ai 
H-ion concentration in the nutrient solution, which lacks the toxic salt, 
equal to that which is produced by the hydrolysis of the iron and aluminum 
salts, if we wish to determine the toxicity of the salt itself. A comparison 
of the depression caused by the salt with that of an equal acidity produced 
by an acid whose anion is of little or no importance as a depressing factor, 
should give some indication of the toxicity of the salt itself. 
Because of the importance of a suitable source of iron in nutrient solu- 
tions, it was necessary to ascertain the proper source of iron and the amount 
necessary to secure the optimum growth of corn. Maze's ('19) experi- 
ments with corn lead him to recommend ferric sulphate rather than ferrous 
sulphate as a source of iron. Shive ('15), Tottingham ('14), and others, 
in the study of the salt requirements of plants, have used ferric phosphate 
as a source of iron. Duggar ('20) has noted that corn grown in Shive 's 
solution becomes chlorotic. The efficiency of ferric and ferrous phosphate 
has been compared by Corson and Bakke ('17). They secured the best 
yield with the ferric salt. Shive and Jones ('21) have found lately that 
the use of ferrous sulphate as a source of iron in wheat cultures gives superior 
growth to that secured in cultures in which ferric phosphate is used. Pre- 
liminary experiments showed that ferric phosphate is not a suitable source 
of iron for corn in solutions of Type i as recommended by the Committee 
of the National Research Council on the Salt Requirements of Representa- 
tive Agricultural Plants. Consequently, experiments were made to de- 
termine the form of iron necessary to assure a sufficient supply to prevent 
a lack of it from limiting the growth of the plants. 
