Dec. 17,1917 
Soil Acidity and the Hydrolytic Ratio in Soils 
671 
limewater, are not changed during the reaction. Generally speaking, 
quantities of the reacting compounds are chosen that give a neutral equi¬ 
librium at a normal temperature. In this way the true point of neutral¬ 
ity is obtained, a fact undoubtedly known but not fully appreciated by 
many investigators, because in investigations on the subject and in de¬ 
veloping lime-requirement methods an excess of soil or reagents is com¬ 
monly used without apparent regard to the effects of mass action. Con¬ 
sideration should also be given to the nature of the products formed dur¬ 
ing the reaction. It will not be sufficient to determine the degree of 
acidity in a soil and calculate the amount of lime required to neutralize 
this acidity, because soil acidity undoubtedly results from the hydrolysis 
of soil compounds and enough reagent should be used to force the hydro¬ 
lytic action to the neutral point. If calcium carbonate is used as a re¬ 
agent, it should be in a very finely divided condition; otherwise the parti¬ 
cles become coated with gels of iron, aluminium, or silica at the point of 
reaction with the soil, and their rate of solution is retarded. Considering 
all the aspects of the case, the logical reagent to use in a lime requirement 
method is calcium hydroxid. 
SUMMARY 
(1) A definite relationship was found between the ratios of calcium to 
iron and aluminium soluble in N/5 hydrochloric acid and the soil reaction. 
All the acid soils have ratios CaO: (Fe 2 0 3 -f A 1 2 0 3 ) above 1:1.3, and all the 
alkaline soils have ratios below this figure. 
(2) It is believed that the reactions of the soils studied depend chiefly 
upon the hydrolytic ratios existing between hydrolyzing compounds of 
the alkali earths and iron and aluminium. 
(3) A method for determining the calcium oxid required to neutralize 
a soil by indirect tiration is described in which certain fixed quantities of 
a standardized calcium-hydroxid solution are allowed to react separately 
with varying amounts of soil. The concentrations of soil and solution w 
giving a neutral reaction are chosen from the series by comparing the 
electrical resistance of the several solutions, also by means of phenol- 
phthalein indicator. The greatest electrical resistance and faint color of 
the indicator is coincident with the concentration giving a neutral 
reaction. 
(4) The quantities of lime required to neutralize the acid soils may be 
determined by computing the quantities of calcium oxid necessary to 
add to the acid soluble calcium oxid found in the soils to bring the ratios/ 
CaO : Fe20 3 +AljOg to 1 : 1.3. The quantity of calcium oxid required 
by this factor method corresponds closely to the quantities required when 
determined by means of the indirect titration method, and it appears 
that the titration method is accurate and convenient. 
