BACTERIA OF THE SOIL 65 



Exercise 94. Nitrification in SoU 



For some reasons it is more satisfactory to study the process 

 of nitrification in soils than in solutions. Where comparisons 

 are desired between the capacity of different soils for nitrifica- 

 tion, onlj' soils can be used. ^Moreover, Stevens and Withers 

 (Centralbl. f. Bakt., 2te Abt., 23: 355. 1909) have shown that 

 soils which show capacity for nitrification often give no result 

 when their suspensions are used as inocula hi solutions. The capac- 

 ity of a soil for nitrification is not, however, a simple proposition 

 depending only upon the number and vigor of the necessary bac- 

 teria; it is profoundly influenced by soil conditions — for ex- 

 ample, quantity and quality of organic matter, reaction of soil, 

 moisture content, porosity, etc. — as well as by other factors 

 less readily recognized. 



1. With a sterile trowel procure soil samples about 10 cm. 

 below the surface. 



2. Put 100 g. of the freshly collected soils into glass tumblers. 

 Reserve one or more tumblers of soil for controls. Add to the 

 others 5 ce. of a 2 per cent solution of sterile ammonium sul- 

 phate (equivalent to .100 g. (NH^)2S0^). Add 5 cc. of sterile 

 distilled water to the control tumblers. Weigh each tumbler and 

 record the weight ; cover the tumblers with glass plates. Once 

 each week weigh the tumblers and add sterile distilled water to 

 bring the weights back to their original value. Incubate the 

 tumblers at 80°-35° C. for four to six weeks. 



3. At the end of the experiment transfer the soil from the 

 tumblers to agate-ware pitchers or to large, wide-mouthed glass 

 bottles. Use 1000 cc. of distilled water for the transfer of each 

 sample. Give thorough agitation for five minutes ; then allow 

 the soil to settle for thirty minutes. Filter off the supernatant 

 extract. Filtration may be accomphshed with a Pasteur-Cham- 

 berland bougie, using the apparatus described in Bulletin Ko. 31, 

 Bureau of SoUs, U.S. Department of Agriculture (see Appendix), 

 or, since only a portion is required for analysis, the extract may 

 be decanted through a folded filter paper. The filtration through 

 paper may be hastened by adding 5-10 g. of alum (potassium 



