vt.] METHODS OF ANALYSIS 165 



the organic constituents of the soil, especially in acid 

 soils rich in humus. It is not sufficient in such cases to 

 estimate the calcium dissolved by dilute acids from the 

 soil, because there are always present other compounds 

 of calcium, e.g., silicates and sulphates, which are soluble 

 in the acid and would be reckoned as calcium carbonate. 

 The factor that is required is not the calcium, but the 

 amount of carbonate which will serve as a base in the 

 soil and combine with the acids liberated by decay, 

 nitrification, or from some of the artificial manures. 

 To this end it is not necessary to discriminate between 

 the carbonates of calcium and magnesium, accordingly 

 the carbon dioxide evolved is calculated back to calcium 

 carbonate. 



(5) For the determinations of soluble constituents 

 20 grams of the powdered soil are placed in a flask 

 of Jena glass, covered with about 70 c.c. of strong hydro- 

 chloric acid, and boiled for a short time over a naked 

 flame to bring it to constant strength. The acid will now 

 contain about .202 per cent, of pure hydrogen chloride. 

 The flask is loosely stoppered, placed on the water- 

 bath, and the contents allowed to digest for about forty- 

 eight hours. The solution is then cooled, diluted, and 

 filtered. The washed residue is dried and weighed as 

 the material insoluble in acids. 



The solution is made up to 250 c.c. and aliquot 

 portions are taken for the various determinations. 

 The analytical operations are carried out in the usual 

 manner, but special care must be taken to free the 

 solution from silica and organic matter. For phos- 

 phoric acid a portion of the solution is evaporated to 

 dryness and ignited at a dull red heat, with occasional 

 stirring for quarter of an hour. The residue is taken up 

 with hydrochloric acid, filtered, again evaporated to 

 dryness, and heated in an air-bath for half an hour at 



