136 
Journal of Agricultural Research 
Vol. VII, No. 3 
Many investigators consider soil acidity as being primarily related to 
adsorptive phenomena. Cameron (5) has explained the reddening of 
blue litmus paper by many soils as due to # selective absorption of the 
base from the litmus base and does not correlate the reddening of the 
litmus paper with the presence of soluble acid except in very rare cases. 
Harris (15) also ascribes soil acidity to the selective adsorption of bases 
by the soil. Bogue (3) and others have expressed a similar opinion 
concerning soil acidity. The phenomena of adsorption, as is well known, 
has also been invoked to explain the fixation of various ions by the soil. 
Another view of soil acidity attributes it to an exchange of bases in 
which a weak base, as aluminium, has been replaced by a strong base 
such as potassium. The resulting hydrolysis of the aluminium salts 
produces an acid reaction in the solution. This view has been advanced 
most recently by Rice (26), Conner (8), Loew (20), Daikuhara (10), and 
Veitch (35). This is a plausible explanation for the acidity of soils 
which haye been treated with salts, although it does not indicate the 
reaction of the soil previous to the salt treatments. The results already 
presented in regard to the addition of salts show that in the case of acid 
soils this treatment markedly increased the H-ion concentration in every 
case. One slightly alkaline soil was also found to give an acid reaction 
after the salt treatments. These data are in direct accord with the 
accumulated evidence concerning the acidity developed when a strong 
base reacts with the soil constituents replacing a weak base. In the 
light of these observations it is quite unnecessary to have recourse to 
adsorption theories to account for the acidity of many salt-treated 
soils, even though the soils originally might have had an alkaline reac¬ 
tion. This is illustrated in the case of soil 14, already cited. Obviously, 
the principal reactions involved in producing the acidity in such cases 
are of a chemical rather than of a physical nature. The importance of 
such an exchange of bases is emphasized by the work of Sullivan (32), 
Rice (26), and Conner (8). 
Truog (33), Hanley (14), Gillespie (13), and Loew (20) have recognized 
that the acidity of a soil is due to the presence of soluble acids. The 
experimental evidence reported in this paper has brought the writers 
to a similar conclusion, in which case it is not necessary to associate 
soil acidity with physical adsorption. 
A great many obscure phenomena in the fields of biochemistry and 
soil chemistry, as well as many others, have been classified under the 
indefinite terms “absorption” and “adsorption.” The indiscriminate use 
of these terms has not served to clarify the problem involved. Thus, the 
fixation of plant foods by soils has been explained by some investigators 
entirely on the basis of physical adsorption. Frequently, the importance 
of the chemical exchange of bases has been disregarded. Moreover, the 
possibility of addition compounds as suggested by Sullivan (32) has not 
received sufficient consideration. The results given in Table VII show 
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