58 Journal of Agricultural Research voi. viii, No. 3 
of the compounds in soils, and may be used both to suggest other methods 
of attack and to confirm results obtained by other means. 
The application of analytical methods to the problem of determining 
the identity and quantity of soil compounds presupposes some knowledge 
of these compounds. The method has the limitation common to all 
analytical operations—namely, that the differences sought to be deter¬ 
mined may be smaller than the unavoidable error in the method used, 
and, furthermore, that the value of the results obtained depends on the 
accuracy of the information regarding the kind of compounds present. 
It has seemed that information regarding compounds of calcium in the 
soil might be more satisfactorily obtained by the analytical method than 
in the case of other elements, because (1) there is available considerable 
information regarding the kinds of calcium compounds that may be 
present in soils, and (2) among these compounds several classes are 
represented having quite different properties and capable of being 
separated by analytical means. 
The calcium compounds known or supposed to be present in soils are 
as follows: Calcium carbonate, or caldte; calcium magnesium carbonate, 
or dolomite; calcium sulphate, or gypsum; calcium phosphate with cal¬ 
cium chlorid or fluorid, apatite; and calcium silicates, a class represented 
by a number of minerals, nearly all containing metals in addition to 
calcium. All of these are known to occur in the rocks from which 
soils are formed, and all have been shown to be present in soils. In addi¬ 
tion to these, most soils are supposed to contain compounds of calcium 
with the organic bodies commonly designated as humus. The assump¬ 
tion of the presence of such calcium-humus compounds is based, first, on 
the fact that when such humus bodies are isolated from soils they appar¬ 
ently combine with lime, forming compounds insoluble in water or dilute 
alkalies; and second, in the case of many soils such humus bodies can be 
leached from soils by dilute alkalies only after leaching with dilute 
acid, whereby the calcium is removed as a salt of the acid used, the 
humus bodies remaining in a condition to be leached by dilute alkali. 
With this information the calcium compounds present or likely to be 
present in soils may be classed as follows: Calcium as carbonate, as sul¬ 
phate, as phosphate, as silicates, and as combined with humus com¬ 
pounds. 
The present paper presents results obtained in attempting to classify 
the calcium compounds in soils as just outlined by the application of 
analytical methods supplemented by such information as was furnished 
by the petrographic method. 
SOILS 
The samples of soil selected for this work, 63 in number, represent 23 
soil types from 24 locations in 19 States. Each location is represented 
by from two to four samples, according to the number of layers encoun- 
