FERMENTATIVE ACTION OF SOILS 207 



The work is at present of a tentative character : there 

 is great difficulty of obtaining small samples of soil 

 which are strictly comparable and representative of the 

 whole field or garden, and further research is necessary 

 to ascertain the most suitable solutions to be used in the 

 work. 



It is hoped, however, that investigations along these 

 lines may ultimately lead to the discovery of a rapid 

 means of comparing the fertility of soils. 



Ex. 101. Dig up a block of soil with a spade : break it, and 

 from the untouched portion remove about a Ib. with a knife or 

 trowel sterilized in the field with a spirit flame. Store the sample 

 in a wide-mouthed sterilized bottle. 



Take 300 gr. of the soil, and add it to a flask containing 

 300 c.c. of sterile tap water : shake the contents for five minutes. 



(i) Test the putrefactive power or peptone-destroying power 

 of the soil. 



Prepare a ij per cent, solution of peptone in tap water, and 

 sterilize it. Place 10 c.c: of this solution in a test-tube, and add 

 to it by means of a wide-bored pipette 5 c.c. of the above turbid 

 soil and water mixture. 



Incubate at 25 C. for four days. Then pour the contents of 

 the tube into a flask to which is added i or 2 grams of calcined 

 magnesia, and a drop or 'two of paraffin, to prevent subsequent 

 frothing of the liquid. 



Distil off the ammonia from the compound produced by the 

 action of the soil bacteria on the peptone into ~$ normal 

 sulphuric acid. 



(ii) Test the urea-splitting power of the soil. 



Take TO c.c. of Beijerinck's urea solution (p. 225). Add to 

 it 10 c.c. of the above soil and water mixture, and incubate at 

 22 C. or three days. 



Estimate the amount of ammonium carbonate produced by 

 titration with normal hydrochloric acid. 



