JOURNAL OF AGRtCOLTORAL RESEARCH 
Vol. VIII Washington, D. C., January 15, 1917 No. 3 
CALCIUM COMPOUNDS IN SOILS 
By Edmund C. Shorey, Scientist in Charge, and William H. Fry and William 
Hazen, Scientists, Chemical Investigations, Bureau of Soils, United States Depart- 
ment of Agriculture 
INTRODUCTION 
The term “chemical composition/' as applied to soils, is frequently, 
perhaps it should be said generally, taken to mean the ultimate composi¬ 
tion or total quantity of each of the elements present. If, however, a 
knowledge of the chemical composition of a soil is to be of any aid in 
studying chemical changes, the meaning of the term should be extended 
to include the kinds of compounds present—not only the quantity of 
each element present, but how they are combined. 
The desirability of obtaining such information has been recognized by 
agricultural chemists for a long time, as is evidenced by the numerous 
methods that have been proposed by which soil compounds containing 
certain elements could be classified. Methods involving digestion with 
dilute acids, methods for the determination of humus, for organic phos¬ 
phorus and organic sulphur, as well as methods for determining 
“availability," all belong to this category. None of these methods 
have been aimed at the identification of any definite compound, but 
rather at a grouping of compounds having some property in common. 
This is, however, a step in the direction of identification. 
Without implying any limitation as to methods applicable to the 
determination of kinds of compounds in soils, two methods may be 
mentioned here: The petrographic and the analytical. 
The use of the petrographic microscope in soil investigations has been 
fully discussed in a previous publication from this Bureau, 1 and it is only 
necessary to state here that this method has a serious limitation in that 
as at present developed it is not applicable to particles smaller than silt 
(0.005 mm - in diameter), and throws no light on the chemical nature of 
the fine material in the soil that no doubt is most active in any chemical 
changes that may take place. 
The petrographic method is, however, a valuable adjunct to any other 
method that may be applied to the determination of the chemical nature 
1 McCaughey, W. J., and Fry, W. H. Microscopic determination of soil-forming minerals. XT. S. Dept. 
Agr. Bur. Soils Bui. 91, 100 p., 11 fig. 1913- 
Journal of Agricultural Research, 
Washington, D. C. 
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Vol. VIII. No. 3 
Jan. is, 1917 
Key No. H—4 
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