EXPERIMENTS 269 



Experiment No. 15. 

 Absorption of Gases by Soils. 



Weigh 50 grams of soil into a wide mouthed bottle, add 50 cc. of 

 wgter and i cc. of strong ammonia. Note the odor. Cork the 

 bottle, shake, and after 24 hours again note the odor. To what is 

 the absorption of the ammonia due? Is this a physical or a chem- 

 ical change. Define fixation. 



Experiment No. 16. 

 Acid Insoluble Matter of Soils. 



Weigh 10 grams of soil into a beaker, add 100 cc. hydrochloric 

 acid (50 cc. strong acid and 50 cc. H 2 O); cover the beaker with a 

 watch glass ; heat on the sand bath in the hood for two hours, re- 

 placing the acid solution, if necessary, in case excessive evapora- 

 tion takes place. Filter, transfer and wash the residue, using 50 

 cc. distilled water. Note the appearance and quantity of insoluble 

 residue. Of what does it consist ? What is its value as plant food ? 

 How does it resemble the original soil and in what ways does it 

 differ? Save the filtrate for the next experiment. 



Experiment No. 17. 

 Acid Soluble Matter of Soils. 



Divide the filtrate from the preceding experiment into three 

 equal portions, (i) To one portion add ammonia until alkaline. 

 The precipitate formed consists of iron and aluminum hydroxid and 

 phosphoric acid. Note the color and gelatinous appearance of this 

 precipitate. When dried it occupies only a small volume. Filter 

 and remove this precipitate. This filtrate contains lime, magnesia, 

 potash and soda. To the filtrate add 20 cc. of ammonium oxalate, 

 warm on the sand bath and note any precipitate of calcium oxalate 

 that is formed. (2) Evaporate the second portion nearly to dry- 

 ness. Add 20 cc. distilled H 2 O and 3 cc. HNO 3 ; warm to dissolve 

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

 and shake. The precipitate is ammonium phosphomolybdate, 

 which contains the element P in chemical combination. (3) 

 Evaporate third portion in the evaporating dish on the sand bath. 

 What does the residue consist of and what elements does it con- 

 tain? 



