670 
Journal of Agricultural Research 
Vol. XI, No. 12 
fixed relationships to each other, which may be expressed in the form of a 
proportion. An equilibrium not neutral in reaction requires the addition 
of a quantity of acid or aklali proportional to the uncombined substances 
before*the neutral point is obtained. If a solution of an acid salt of iron or 
aluminium is titrated with a standard solution of limewater until the 
reaction proceeds to a certain point but not to neutrality, the quantity of 
calcium oxid used is proportional to the quantity of iron or aluminium in 
the portion of the salt decomposed, and the amount of calcium oxid yet 
required to neutralize the solution is in the same proportion to the quan¬ 
tity of iron or aluminium in the portion of the salt yet to be acted upon, 
as is also the total quantity of iron or aluminium in the salt neutralized, 
a condition similar to that found in the acid-soluble portions of the soils 
studied. 
When determinations for “acid consumed” were made on the soils pre¬ 
liminary to the solubility method, it was observed in case of the medium 
sands that more alkali was required to neutralize the acid solutions than 
was necessary to combine with the quantity of A/5 hydrochloric acid used; 
therefore these soils may have contained acid salts soluble in the acid in 
order to require this excess of alkali to neutralize the solutions. 
The reddening of blue litmus paper by soils has been explained from 
the standpoint of selective absorption, but the phenomenon can be as 
logically explained on the basis that these soils contain acid salts ab¬ 
sorbed by soil gels or relatively insoluble. Phosphates and hydrated 
silicates of iron or aluminium can be washed with water until the fil¬ 
trates do not react acid to litmus paper, but when the paper is brought 
in contact with the solid material it is strongly reddened. It is also well 
known that soils which do not redden blue litmus paper, as well as neu¬ 
tralized acid soils, absorb large quantities of calcium oxid from limewater 
solutions. Silica gels also hold soluble-acid salts of iron or aluminium 
so tenaciously that these salts can not be washed out. The fact that a soil 
solution is not acid to litmus is no proof that an acid salt does not exist 
in the soil. 
From the results obtained it is believed that the indirect-titration 
method as previously explained is adequate to determine the quantity of 
lime required to neutralize an acid soil. In determining the lime re¬ 
quirement of an acid soil due regard should be given to the factors influ¬ 
encing salt hydrolysis, solubilities of soil minerals, and mass action. 
Time, temperature, and concentrations of the reacting compounds are 
the principal factors involved. It is well known that in reactions con¬ 
cerning mass action, hydrolysis, and solubilities a certain length of time 
is required for equilibrium. Because of the effects of temperature on 
hydrolysis and solubility, a temperature considerably above or below 
that normal to field soils under plant-growing conditions should probably 
not be used. A point of importance in the procedure of the indirect- 
titration method is that the quantities of reacting substances, soil and 
