4 Morris J. Blish 



silver nitrate or litmus paper to test the final washings. The soil 

 is then transferred to a container with 500 c.c. of 4 per cent, 

 ammonium hydroxid, and allowed to remain, with occasional 

 shaking, for 24 hours. The suspended matter is allowed 12 

 hours more in which to settle, after which the supernatant liquid 

 is drawn off and filtered, and an aliquot of this is evaporated to 

 dryness in a tared platinum or quartz dish. The residue is then 

 dried, either from 12 to 24 hours at 100° C. or from 2 to 4 hours 

 at 110° C, and weighed. It is then ignited and weighed again. 

 The difference in weight after ignition represents the weight of 

 humus, while the difference between the first weight of the dish 

 and the final weighing represents the so-called humus ash. The 

 source of error in this method is the considerable amount of very 

 finely divided clay which remains in suspension even after filtra- 

 tion. Accordingly when the residue obtained by the evaporation 

 of the solution is ignited, this clay is dehydrated, and the differ- 

 ence in weight, caused by this loss of water, is recorded as humus, 

 while the aluminum silicate remaining is weighed up as humus 

 ash. Accordingly, it is obvious that as far as humus alone is con- 

 cerned, this method is inaccurate and unreliable, and for this 

 reason it is now commonly accepted as unreliable. 



THE HILGARD METHOD 



The Hilgard method is generally accepted as being more reli- 

 able than the " Official " method, although it is much longer and 

 more tedious, often involving ten to twenty days. In this process 

 the lime and magnesia are extracted in the same manner as in the 

 " Official " method, but the extraction with the 4 per cent, ammo- 

 nium hydroxide is carried on in a different manner. After the 

 extraction of lime and magnesia, the soil is treated on the filter 

 with 4 per cent, ammonium hydroxide, and this treatment is car- 

 ried on either continuously or at intervals, until the liquid comes 

 through practically colorless, on which the extraction of humus 

 is assumed to be complete. This method not only frequently 

 takes several days, but often a very large amount of humus solu- 

 tion is obtained and a correspondingly large amount of it must be 

 used in the evaporation, which is then carried out as in the 



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