APPENDIX 633 



sufficient.) Burn the filter and ignite the precipitate in the blast to con- 

 stant weight. Weigh as calcium oxid and compute to calcium. 



Magnesium. Evaporate to dryness the filtrate and washings from the 

 calcium determination; wash the residue with hot water into a long- 

 neck Jena flask; remove ammonium salts by boiling to a small volume 

 with nitric and hydrochloric acids (about 25 cc. of each), adding more of 

 the acids (10 to 15 cc. of each) two or three times, and finally evaporating 

 nearly or quite to dryness. Transfer the remaining salts to a beaker 

 with hot water ; add enough ammonium chlorid to prevent the precipita- 

 tion of magnesium hydroxid, make alkaline with ammonia, and then add 

 .5 g. ammonium persulfate; digest on the steam bath for 30 minutes, 

 taking care that there is always an excess of ammonia present; filter, 

 and wash free from chlorids with hot water. Concentrate the filtrate and 

 washings to about 50 or 75 cc., acidify with hydrochloric acid, and add 

 5 to 10 cc. of a normal solution of NH 4 NaHPO 4 to insure the precipita- 

 tion of all magnesium. While stirring vigorously and taking care not to 

 strike or rub the sides of the beaker, slowly add enough ammonia to 

 make the solution distinctly alkaline. After 30 minutes, add 10 cc. of 

 strong ammonia, slowly and with vigorous stirring ; cover closely to reduce 

 the escape of ammonia, and let stand for 12 hours. Then filter, and 

 wash the precipitate free from chlorids, using i\ per cent ammonia water. 

 Dry the filter, burn at a moderate heat, and then ignite intensely to con- 

 stant weight, using the blast. Weigh as magnesium pyrophosphate and 

 compute to magnesium. 



NOTE. In all cases the above determinations relate to the total amounts 

 present in the soil. If much alkali or other soluble salts are present, the amount 

 may be determined by extracting with water, evaporating, and weighing the 

 residue, which may be analyzed subsequently if desired. The author does not 

 advise the analysis of soils by determining only the "acid-soluble" portion of 

 the constituents, a method which involves five arbitrary conditions: (i) the 

 choice of solvent, (2) the strength of solvent, (3) the relative amounts of soil and 

 solvent, (4) the time of digestion, and (5) the temperature of digestion. To 

 change any one of these arbitrarily fixed conditions may change the amount of 

 soil constituents dissolved. Such analyses furnish little information, and they 

 tend to discredit the very exact and highly serviceable science of chemistry- 



Pot-culture experiments by the Illinois Experiment Station have shown that 

 crops can be grown in the "insoluble residue," from ordinary acid digestion of 

 soils, without addition of potassium; and they markedly improve by green 

 manuring. 



