Jan. 15,1917 
Calcium Compounds in Soils 
73 
The interpretation of the analytical data in this way is, of course, 
open to some objections, and all the analytical data have been presented 
separately in order that any reader may have the opportunity to offer 
intelligent criticism. 
Among the objections to the interpretation just presented the follow- 
' ing are probably the most obvious : 
Carbonates. —Where the dilute-add extract contained CaO equiva¬ 
lent to the C0 3 found, all the C0 2 was calculated to caldum carbonate; 
but where such equivalent was not found, the CaO in the dilute-add 
extract was calculated to calcium carbonate. This is practically an 
arbitrary decision that in some cases all the C0 2 was derived from cal¬ 
cium carbonate and in other cases it was not. An examination of the 
figures will show, however, that any error introduced through this 
assumption is small. 
In this connection it should be noted that it is a question whether or 
not the small quantity of C0 2 found in many instances was really derived 
from carbonates. It is possible that in some cases these small quanti¬ 
ties of C 0 2 were derived from the indpient decomposition of some organic 
constituent brought about by the treatment involved in the method, 
or that it may simply have been occluded in the soil. 
Sulphates. —The presence of Ca and S 0 4 ions in a water extract of 
a soil is not proof that calcium sulphate existed as such in the soil. This 
is, of course, true, but gypsum was found in two samples by the petro¬ 
graphic method, and the small quantities stated as calcium sulphate 
have been presented as an assumption supported only by that fact. 
Phosphates. —No provision has been made for caldum phosphates. 
Apatite was found in one sample by petrographic examination, but it 
does not seem feasible to attempt a separation of calcium phosphates by 
analytical methods until some reliable method for distinguishing be¬ 
tween organic and inorganic phosphorus is developed. 
Calcium phosphates are probably present in many soils, but the 
quantity must be small and the error introduced by ignoring this in the 
present scheme also small. Where this error would fall would depend 
on the character of the phosphate. Apatite would probably appear as 
difficultly decomposable silicates, and dicalcium hydrogen phosphate 
probably as calcium combined with humus. 
Easily, decomposable silicates. —The differences between the quan¬ 
tity of calcium obtained by 4 per cent acid (method A) and that ob¬ 
tained by 2 per cent (method B), stated as easily decomposable sili¬ 
cates, is in some cases very small; such differences as 0.02 and 0.01 
per cent are such as would be allowed in duplicates by the same method. 
This is recognized, and such quantities are not significant, but are stated 
for uniformity of presentation. It should be noted in this connection 
that, while these differences are often very small, they are always in the 
same direction, the quantity of calcium obtained by the more dilute 
acid always being smaller. 
67906 °— 17—2 
