18 METHODS OF ANALYSIS. 



6. Acid-Insoluble Materials. Official. 



The residue from acid soluble may be analyzed by the usual methods for 

 silicates. If it is desired to determine the silica soluble in alkalies, the residue 

 must be dried at 100 C. and an aliquot removed before ignition for treatment 

 with sodium carbonate solution, as described under determination of inorganic 

 plant constituents (d), page 22. Another aliquot, or the remainder of the 

 residue, is ignited and weighed. 



7. Total Alkalies. Official. 



Determine in a separate portion of the soil by J. Lawrence Smith's method, 

 given in Crookes's Select Methods, second edition, pages 28-40, and Principles 

 and Practice of Agricultural Analysis, 1894, volume I, pages 378-381, or, prefer- 

 ably, determine by this method the alkalies in the insoluble residue from " 5. (a) 

 Method of preparing soil solution," page 14, and add the amount obtained from 

 the hydrochloric-acid solution. 



8. The More Active Forms of Phosphoric Acid in Soils. Provisional, 

 (a) SOLUTIONS REQUIRED. 



Prepare a large stock solution of normal hydrochloric acid by titrating against 

 a standard potassium hydroxid solution containing little or no carbonate, using 

 phenolphthalein as the indicator. Also prepare a fifth-normal solution of hydro- 

 chloric acid. 



(b) DETERMINATION. 



Digest 10 grams of air-dried soil, in a stoppered flask, with 100 cc of fifth- 

 normal hydrochloric acid, for exactly five hours in a water bath kept at a 

 temperature of 40 C. Filter the solution through a dry paper, cool to the 

 room temperature, and titrate 20 cc of the filtrate with standard carbonate-free 

 potassium hydroxid solution, using phenolphthalein as the indicator. From 

 the data thus secured calculate the exact number of, cubic centimeters of normal 

 acid of the stock solution and of water to make exactly one or two liters of 

 acid of fifth-normal strength after allowing for the amount neutralized by the 

 amount of soil to be used in the following procedure. 



Place 200 grams of the air-dried soil in a large, dry, glass-stoppered bottle 

 and add exactly 2,000 cc of fifth-normal hydrochloric acid corrected for neutral- 

 ization as above described. In the case of soils known to be rich in available 

 phosphoric acid 100 grams of soil and 1,000 cc of acid will be sufficient. Place 

 the bottle in a large water bath and keep at a temperature of 40 C. for exactly 

 five hours, shaking thoroughly each half hour. At the end of the digestion 

 shake contents of bottle well and pour quickly upon a large, dry, ribbed filter 

 of two thicknesses of paper and of sufficient size to receive the entire contents 

 of the bottle. The filtrate is to be received in a dry vessel and the solution 

 poured back through the paper until entirely clear. Evaporate 1,000 cc of the 

 filtrate if 200 grams of soil be used, or 500 cc if 100 grams be employed, to 

 dryness in a porcelain dish, after adding a few cubic centimeters of nitric acid 

 to oxidize organic matter, etc., moisten the residue with hydrochloric acid, take 

 up with water, and filter into a flask of about 500 cc capacity. Add 15 grams 

 of ammonium nitrate in solution, then strong ammonium hydroxid until a 

 permanent precipitate forms, and then concentrated nitric acid until the precipi- 

 tate dissolves. Dilute to about 100 cc, if less than that volume, place a ther- 



