176 § 99. ANALYSIS OF SOILS AXD ROCKS. 



If the soil is very rich in organic matter, it will be bet- 

 ter to treat 500 c.c. of the solution with sodic carbonato 

 and potassic nitrate, or with chlorine, as directed in 

 § 93, A, and use ^1^ of the solution finally obtained for 5, 

 omitting, of course, the determination of the alkalies 

 in this portion of the solution unless the oxidation was 

 effected with chlorine, and ^1^ for c. 



Sometimes, however, when there is not a very large 

 proportion of organic matter present, and the above 

 treatment for oxidation is not followed, traces of organic 

 matter are contained in the solution of ferric oxide ob- 

 tained in ^, for estimation with permanganate; where 

 great accuracy is required therefore, it would be well, 

 after having titrated the ferric solution once, to reconvert 

 the ferric oxide into ferrous, with zinc or sulphurous 

 acid, titrate the solution again, and to repeat this until 

 a constant result is obtained. The same mode of pro- 

 cedure should be followed, also, in estimating the strength 

 of the permanganic solution. 



In accurate soil analyses, the phosphoric acid should be 

 estimated twice. 



In a complete soil analysis, it is desirable to determine 

 the sUlcic acid, which, after treatment of the soil with 

 cold concentrated hydrochloric acid, is soluble in a con- 

 centrated solution of sodic carbonate. 



For this purpose, take 5-10 grms. of the residue that 

 was insoluble in the acid, in case of a soil rich in carbon- 

 ates ; or, digest 25 grms. of the air-dried soil with three 

 times the quantity of cold concentrated acid, 48 hours in 

 the cold, filter, and wash the contents of the filter perse- 

 veringly, first with cold and afterAvards with hot water, 

 and use this residue. 



Boil this insoluble substance and also an equal amount 

 of the original air-dried soil, with sodic carbonate, in the 

 manner described for the separation of sand and silica 

 (§ 58, a, 2). The difference between the amounts of 



