174 Soil Analysis [294-296 



Details of the Analysis 



294. Silicates. — Weigh out about 1 gram of the air-dried 

 substance; dissolve in HC1, as described in paragraph 10, and 

 evaporate to dryness on the water bath. When quite dry, heat 

 on the sand bath for a few minutes. Allow to cool, and 

 moisten with 2 c.c. of strong HC1. Stir to break up clots. 

 Add 25 c.c. of dilute HC1 (1-3), evaporate for five minutes, 

 then filter. Wash thoroughly with hot water, transfer the 

 precipitate to a weighed platinum dish, ignite, and weigh. 



295. Iron Oxide and Alumina. — The estimation of 

 oxide of iron and alumina is complicated by the presence of 

 the large excess of lime salts in the liquid. When ammonia is 

 added a considerable quantity of CaC0 3 will be precipitated, 

 together with the hydrated oxides. 



Raise the filtrate from the silicates to boiling-point, remove 

 from the flame, and add dilute ammonia until just alkaline. 

 Boil again, and allow the precipitate to settle. Decant off the 

 liquid through a filter. Wash once by decantation ; then re- 

 dissolve in dilute HC1, as described in paragraph 273. Boil, 

 and re-precipitate with ammonia. Should the precipitate now 

 be small (only a few milligrams) it may be weighed, but 

 usually it should be dissolved and re-precipitated a third time. 



After igniting and weighing, the precipitate should be dis- 

 solved and tested for P 2 5 , according to the method described 

 in paragraph 239. Should it be present, it must be estimated. 



296. Lime. — In a good agricultural limestone there is 

 little difficulty about this estimation, which is carried out as 

 described in paragraph 274. In dolomitic limestones, how- 

 ever, it is more difficult. When magnesic salts are present 

 and a large excess of ammonium oxalate is used, the calcic 

 oxalate precipitate is largely contaminated with magnesic 



