14 METHODS OF ANALYSIS. 



arable soil should be sampled, which would make three composite samples from 

 the field one from the surface or arable soil, one from the subsurface soil 

 that is, from the stratum between the plow line and the true subsoil line, and 

 one from the true subsoil. Dry the samples in a well-aired, cool place. 



2. Preparation of Sample. Official. 



Prepare the laboratory sample by putting it through a 1 mm sieve, using a 

 rubber-tipped pestle to rub up the lumps. Weigh and discard the detritus. 

 Keep the sample in a cool place and stopper to prevent change in condition. 

 Use a 3 mm sieve for preparing that portion of the sample intended for deter- 

 minations requiring 100 grams or more of soil. 



3. Moisture. Official. 



Dry 2 or more grams in a tared platinum dish for 5 hours at the temperature 

 of boiling water; cover the dish, cool in a desiccator, and weigh. Repeat the 

 heating, cooling, and weighing at intervals of 2 hours, until the material ceases 

 to lose weight. The loss of weight is reported as moisture. Weigh rapidly, to 

 avoid absorption of moisture from the air. 



4. Volatile Matter. Official. 



Heat the dish and dry soil from the above determination to full redness, 

 occasionally stirring, until all organic matter is burned away. If the soil con- 

 tain appreciable quantities of carbonates it is moistened, after cooling, with a 

 few drops of a saturated solution of ammonium carbonate, dried and heated to 

 dull redness to expel ammonium salts, cooled in the desiccator, and weighed. 

 The loss in weight represents the organic matter, water of combination, salts 

 of ammonium, etc. (See Appendix, p. 234, for Total Organic Carbon.) 



5. Strong Acid Digestion of the Soil, 

 (a) METHOD OF PREPARING SOIL SOLUTION. OFFICIAL. 



Digest a quantity equivalent to 10 grams of moisture free soil (this facili- 

 tates the subsequent calculations). Make the digestion preferably in an Krlen- 

 meyer flask of nonsoluble glass of 200 or 300 cc capacity. The flask should 

 have a ground-glass stopper terminating in a reflux tube 20 inches or more in 

 length. A rubber stopper carrying a tube may be substituted if glass-stoppered 

 flasks are not available. Use 100 cc of hydrochloric acid of a constant boiling 

 point (sp. gr. 1.115), made approximately by diluting 1,350 cc of ordinary acid 

 (sp. gr. 1.20), with 1,000 cc of water. Digest continuously for 10 hours on a 

 steam or water bath, shaking the flask every hour. After settling, decant the 

 solution into a porcelain dish or hard glass beaker. Very small quantities of 

 the sediment passing over will do no harm. Wash the insoluble residue o:. 

 filter with hot water and continue the washing until free from chlorals, adding] 

 the washings to the original solution for evaporation. Oxidize the organic j 

 matter present in the solution with a few drops of nitric acid and evaporate 

 to dryness on a water bath. Take up with hot water and a few cubic centi- 

 meters of hydrochloric acid and again evaporate to complete dryness. When] 

 the final evaporation is complete and the dish cooled, add a few drops of stroi 

 hydrochloric acid, sufficient only to saturate the residue. Add 10 to 20 cc ol 

 water, warm on the bath to secure complete solution, and filter, washing until 

 free from chlorids. Again evaporate this solution to dryness. to render insoli 

 ble any silica that may yet be in solution, and treat as above. The filtrate 



