Dec. i7,1917 
Soil Acidity and the Hydrolytic Ratio in Soils 
669 
At this point the possible combinations of the alkali earth elements 
and iron and aluminium with other elements in soils may well be con¬ 
sidered. The principal mineral salts of the alkali earths are carbonates, 
silicates, phosphates, sulphates, chlorids, and nitrates. Of this list 
only the carbonates and silicates of calcium and magnesium give a 
red color on boiling their solutions with phenolphthalein. The principal 
iron and aluminium salts found in soils are silicates and phosphates, 
while sulphates, chlorids, and nitrates may be present. Theoretically 
these salts hydrolyze and give acid solutions, and will react with car¬ 
bonates of silicates of the alkali earths; but these reactions proceed to 
an equilibrium and not to the limits of chemical equivalents. 
Experiment iv. —Dilute solutions of the carbonate and silicate of 
calcium were boiled with phenolphthalein until the red color appeared. 
Small quantities of iron and aluminium salts were then added until the 
red color disappeared. Several salts of iron and aluminium were used in 
this manner, and the red color could be obtained or dispelled at will by 
increasing in order the calcium or iron and aluminium salts. Similar 
results were obtained by using solutions from alkaline and acid soils. 
If a soil solution gives a red color with phenolphthalein when boiled 
and concentrated, a free alkali hydroxid is undoubtedly present, proba¬ 
bly coming from the decomposition of a carbonate or silicate of an alkali 
earth element. On the other hand, if a soil solution gives no red color 
under the above-named conditions, an acid salt may be present in suffi¬ 
cient quantity to prevent the formation of free alkali. 
From the ratios given in the preceding pages two general conclusions 
can be drawn: (1) The quantities of acid-soluble calcium are proportional 
to the quantities of acid-soluble iron and aluminium in neutral soils, and 
also to the unreacting acid-soluble iron and aluminium in the acid soils; 
and (2) the quantities of calcium required to neutralize the acid soils are 
proportional to the quantities of acid-soluble iron and aluminium in ex¬ 
cess of the ratio 1:1.3 or to the reacting iron and aluminium. These 
results can not be explained on the basis of absorption, unless iron and 
aluminium compounds are the only absorbing substances in the soils, or 
that these compounds are proportional in acid-soluble quantities to the 
total amounts of absorbing compounds; also, if soils contain other ab¬ 
sorbing substances, their absorbing power is satisfied to an equal degree; 
or that Nf 5 hydrochloric acid dissolves an amount of absorbed lime pro¬ 
portional to the unreacting iron and aluminium in the acid soils; and also 
soils exhibit two kinds of absorption, one kind independent of the con¬ 
centration of limewater and the other dependent upon it. According to 
present knowledge of soil composition and absorption, these considerations 
are untenable. 
The proportions found are in direct accord with the theory of hydroly¬ 
sis and chemical equilibrium. When a neutral equilibrium is established 
between hydrolyzing compounds, the reacting substances bear certain 
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