LABORATORY PRACTICE 321 



any residue. Add 5 to 7 cc. of ammonium molybdate, heat gently, 

 and shake. The yellow precipitate is ammonium phosphomolyb- 

 date, which contains the element P in mechanical and chemical com- 

 bination. (3) Evaporate the third portion in the evaporating dish 

 on the sand bath. Of what does the residue consist and what 

 elements does it contain? 



Experiment No. 20 



Extraction of Humus from Soils 



Place 10 gms. of soil in a bottle (preferably a glass-stop- 

 pered one) and add 200 cc. H 2 O and 5 cc. HC1. Shake and allow 

 10 to 24 hours for the acid to dissolve the lime so the humus 

 can be dissolved by the alkali. Filter the acid and wash the soil 

 on the filter with distilled water until the washings are no longer 

 acid to litmus paper. Transfer the soil to the bottle again, add 

 100 cc. H 2 O and 5 cc. KOH solution. Shake, and after two to 

 four hours filter off some of the solution which is dark-colored 

 and contains dissolved humus compounds. 



To 10 cc. of the filtered humus solution add HC1 until neutral. 

 The precipitate formed is mainly humic acid and soil humates. 

 Evaporate a second portion of 10 to 20 cc. to dryness ; the black 

 residue obtained is humus material extracted from the soil. 



Experiment No. 21 



Nitrogen in Soils 



Mix 5 gms. of soil and an equal bulk of soda lime in a mortar; 

 transfer to a strong test tube. Connect the test tube with a deliv- 

 ery tube which leads into another test tube containing distilled 

 water. Heat cautiously for from 5 to 10 minutes, with the Bunsen 

 burner, the test tube containing the soil and soda lime. Test the 

 liquid with litmus paper and note the reaction. Soda lime aided 

 by heat decomposes the organic matter of the soil and forms CO 2 , 

 H 2 O, and NH 3 . The nitrogen in the form of ammonia is distilled 

 Y 



