Jan. i Sl 1917 
Calcium Compounds in Soils 
75 
cium and that this calcium compound must be broken up by treatment 
with a mineral acid before the humus compounds can be extracted with 
dilute alkali. Experience in extracting a large number of American 
soils with dilute alkali is, however, quite in keeping with the analytical 
results here presented, and indicates that there are many soils in which 
such calcium humus compounds do not occur. 
There are four samples with high CaO content in which the calcium 
carbonate content is also high (samples 39, 48, 58, 59). These are all 
samples of subsoils of soils derived from material high in calcium car¬ 
bonate where the carbonate has not been leached from the lower strata. 
There are five samples in which the CaO content is above 3 per cent, 
but where the calcium carbonate content does not exceed <^14 per 
cent (samples 1, 27, 57, 60, 61). In these the greater part of the calcium 
is present as difficultly decomposable silicates. 
In considering the calcium-carbonate content of these samples, the 
question regarding the lower limits of the effervescence test for car¬ 
bonate arises. All the samples were tested for effervescence by the 
addition of dilute hydrochloric acid, and no effervescence was observed 
where the calcium-carbonate content was below 0.40 per cent. The 
next lowest in the series contains 0.25 per cent, and these results would 
indicate this test as ordinarily used in the field would be negative where 
the calcium-carbonate content was less than 0.30 or 0.40 per cent. 
The data, however, are not sufficient to warrant the fixing of any limit. 
Perhaps the most interesting fact brought out by these figures is that 
it is possible to have two soils with the same or approximately the 
same calcium content, but containing quite a different assortment of 
calcium-containing compounds. A few comparisons of this kind have 
been made and are presented in Table V. 
Table; V .—Comparison of 12 soil samples containing nearly equal amounts of 
calcium in various forms 
Sample No. 
Total 
CaO. 
CaO as 
CaCOs. 
CaO as 
easily 
decom¬ 
posable 
- sili¬ 
cates. 
CaO as 
diffi¬ 
cultly 
decom¬ 
posable 
sili¬ 
cates. 
CaO 
with 
humus. 
24.. 
I. 98 
1. 02 
O. 12 
. J.0 
0. 02 
7 4 
1.84 
. 9 8 
in 
36 . 
O. II 
•47 
. 11 
in.. ;. 
-l. y^ 
.98 
. 9 8 
I. 41 • 
I. 40 
I. 26 
. OO 
* O' 4 - 
2 7 
46 .. . 
. 17 
' A O 
on 
. J.y 
. 6l 
I. 27 
C T 
14. 
* A / 
.08 
in 
. vy 
. 04 
.46 
04 . 
47 . 
0 0 
10 ... 
. J.y 
. 20 
. i’c 
• o A 
1. OI 
• 59 
2. 33 
A 7 4 
• 
. 01 
.29 
42. 
I. 26 
27 
58 . 
5 * 37 
4. 64 
I, 12 
2. 50 
.06 
02 
* ^0 
• 54 
. 11 
CiO 
61 . 
•13 
18,. 
n- 04 
I. 08 
7 7 
45. 
1.14 
07 ' 
* 
'll 
71 
• 61 
• 66 
* 0 7 
