May 19.19*3 Nitrogenous Fertilizers Influencing Chlorosis in Rice 625 
acid condition of the medium. Similarly, when nitrates were used, the 
reaction of the medium was changed but in the opposite direction. 
The effect of these residues on the insoluble nutrients in the soil had been 
demonstrated by Shulov {21) , who found that when sulphate of ammonia 
was used as a source of nitrogen the insoluble phosphates in the soil were 
rendered more available to plants than when nitrogen was supplied as 
sodium nitrate. 
Maz^, Ruot, and Lemoigne (77), too, suggested that the chlorosis, 
which developed in com with which they experimented, was caused by 
the precipitation of iron by calcium carbonate that was liberated in the 
culture solution when the plant assimilated the acidic radicle of a calcium 
salt. 
More recently, Jones and Shive (jo) have shown that, in solution 
cultures of wheat plants, the availability of ferric phosphate, as indicated 
by the susceptibility of the plants to chlorosis, was governed by the degree 
of acidity of the medium which increased or decreased as a result of plant 
growth according to whether ammonium sulphate or calcium nitrate 
was used as a source of nitrogen. 
It is probably true that in a solution culture in which there is no close 
contact between the plant roots and the precipitated material the reac¬ 
tion of the entire medium must be sufficiently acid to render the iron 
soluble in order to prevent chlorosis, and that the hydrogen-ion concen¬ 
tration at the threshold of chlorosis is the critical reaction governing the 
availability of the form of iron in that particular culture. In the denser 
media, however, in which the roots are in contact with insoluble material 
and in which circulation of the nutrient solution is retarded and sedi¬ 
mentation of precipitated matter is prevented, there may be, as a conse¬ 
quence of plant growth and the rejection by the plant of nonassimilable 
compounds, a very significant modification of the composition and 
reaction of the nutrient, the extent and intensity of which it is not easy 
to determine. Under such conditions it is evident that only that part 
of the culture which is in contact with the plant roots is the true medium 
in which the plant grows, and that the reaction of the unassimilated 
residue of the nutrient salts may be of more significance than is the reac¬ 
tion of the mass of the medium. 
EFFICIENCY OF NITRATES AND AMMONIUM SALTS AS NUTRIENTS 
FOR RICE 
. It has long been known that rice {Oryza sativa) differs from the majority 
of other economic plants in that it does not seem adapted to the utilization 
of nitrate nitrogen as a fertilizer when it is grown under swamp condi¬ 
tions. Ammonium salts, on the other hand, have generally been found 
to serve as a suitable source of nitrogen, but Daikuhara and Imaseki (j} 
have noted that the difference between the efficiencies of the two forms 
of nitrogen disappears when rice is grown on soil that is not submerged. 
Many attempts have been made to explain this characteristic prefer¬ 
ence of the rice plant for ammonium nitrogen. Nagaoka (79) suggested 
that three factors were involved in interfering with the utilization of the 
nitrates: (i) Loss of nitrogen by denitrification; (2) the formation of 
poisonous nitrites in the process of denitrification; and (3) the absence 
in the plant of sufficient sugar to transform the nitric nitrogen absorbed 
into protein. 
Daikuhara and Imaseki (j) showed that the last theory was untenable 
when they proved by analysis that the sugar contents of paddy rice and 
of upland rice were approximately equal. Their results, however, indi- 
