VI.] METHODS OF ANALYSIS 145 



in which case the soil should be attacked with a boiling 

 dilute solution of ammonium chloride. 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, nitrifica- 

 tion, 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. In a few soils ferrous carbonate 

 may be present; this is oxidised to ferric hydrate 

 when the powdered soil is boiled with water, and 

 may be so removed before determining the carbon 

 dioxide. In temperate climates, however, it is only 

 a few bog soils that need be examined for ferrous 

 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 20-2 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 cc and aliquot 

 portions are taken for the various determinations. 

 The analytical operations are carried out in the usual 



K 



