14 



PRACTICE 6. 

 FIXATION OF BASES IN SOILS. 



(a) Place a small bunch of glass wool in a percolator, cover with i cm. 

 of clean sand, and add 100 gm. of clayey soil. Upon this carefully pour 

 250 cc. of dilute ammonium sulfate solution (50 cc. of the solution pre- 

 pared in Practice 4, plus 200 cc. of ammonia-free water). When percola- 

 tion ceases, mix the percolate thoroughly and determine nitrogen in two 

 50 cc. portions. 



Titrations (1) cc. NH 3 



(2) cc.NH 3 



Average , cc. NH 8 



Mg. N per cc. in solution used 



Mg. N per cc. in percolate 



Percent N fixed by soil 



(b) Repeat the experiment, using 200 gm. of the same soil. 



Titrations (1) cc. NH 3 



(2) cc.NH 3 



Average cc. NH 3 



Mg. N per cc. in solution used 



Mg. N per cc. in percolate 



Percent N fixed by soil 



(c) Repeat the experiment, using 200 gm. of sandy soil. 



Titrations (1) cc. NH 3 



(2) cc. NH 3 



Average cc. NH 3 



Mg. N per cc. in solution used ... 



Mg. N per cc. in precolate 



Percent N fixed by soil 



Give a general reaction for the fixation of bases by soils and explain 

 fully what chemical elements that are important in soil fertility may be 

 retained in soils by means of this reaction. 



