98 CHEMICAL ANALYSIS OF SOIL 



Pure caustic soda should be used for the estimation, 

 and a blank experiment should be carried out in order 

 to correct for : — 



(i.) The amount of COg originally present in the 

 lOO c.c. of caustic soda used in the Reiset tower. 



(ii.) The COg present in the air contained in the 

 absorption apparatus previous to addition of the soda 

 solution. 



(iii.) Any COg which may be present in the acid 

 used for decomposing the carbonates. 



"Total" Mineral Constituents. 



77. Hydrochloric Acid Extraction} 



Fifty grams of the fine earth are boiled in an open 

 flask with loo to 150 c.c. of concentrated hydrochloric 

 acid for a short time, with constant shaking, in order 

 that the acid may attain constant strength. The flask 

 is then loosely stoppered with a funnel or a sealed-ofl" 

 glass bulb, and the contents digested on the water-bath 

 for forty-eight hours. The mixture is filtered, while 

 hot, through a Buchner funnel, washed thoroughly 

 with hot water, and the filtrate and washings made 

 up to 1000 C.C. 



Unignited soil must be taken for extraction, since 

 ignition occasions drastic and variable alteration of the 

 soil — e.g., no constant proportion is found between the 

 potash extracted from ignited and unignited soil. 



78. Phosphoric Acid. 



Fifty c.c. of the solution are evaporated to dryness 

 in a platinum or porcelain dish, and ignited ; the residue 



^ If, as sometimes happens, only the phosphoric acid and 

 potash are to be estimated, it will be sufficient to use 20 grams of 

 soil, and make up the hydrochloric acid extract to 500 c.c. 



