MICROBIOLOGICAL ANALYSIS OF SOIL 



731 



bringing to optimum moisture, incubating, and determining the residual man- 

 nite every five days by oxidation with KAln0 4 . This method can serve as an 

 index of the activities of the nitrogen fixing flora of the soil, as well as of the 

 amount of phosphorus available. 



The method of determination of soluble organic matter in the soil is carried 

 out as follows. Five grams of soil is withdrawn and allowed to air-dry; the air- 

 dry soil is then weighed again and extracted for two hours, with occasional 

 shaking, with 200 cc. of water. The extract is filtered through paper and 10 cc, 

 or an amount equivalent to 0.25 gram of soil, is placed in a 400-cc. beaker 

 with 50 cc. of 0.05 N potassium permanganate solution and 3 cc. of dilute (6 :100) 

 sulfuric acid. The beaker is placed in boiling water for twenty minutes, 50 cc. 

 of 0.05 N oxalic acid is then added and the solution is titrated with 0.02 N potas- 

 sium permanganate solution. The number of cubic centimeters of the latter 

 expresses the amount of organic matter (residual mannite + soluble soil organic 

 matter). 



TABLE 81 

 Nitrogen fixed in 100 cc. of mannite solution -f- 10 grams of soil 



A definite correlation was found to exist between nitrogen fixation 

 in mannite solution and soil fertility. 39 A certain correlation was also 

 reported 40 between crop productivity of a soil and its ability of fixing 

 nitrogen, when mannite is added to it (soil method). By adding a 

 definite amount of soil (10 grams) to a mannite solution, free from 

 phosphates, then inoculating with a culture of Azotobacter and deter- 

 mining the amount of nitrogen fixed, after a definite period of incuba- 

 tion, an approximate index of the presence or absence of available 

 phosphorus in the soil can be "obtained, 41 as shown in table 81. 



39 Lohnis, F., and Pillai, N. K. tlber stickstofffixierende Bakterien. III. 

 Beitrag zur Methodik der bakteriologischen Bodenuntersuchung. Centrbl. 

 Bakt. II, 20: 781-795; Green, H. Investigation into the nitrogen metabolism 

 of soil. Centrbl. Bakt. II, 41: 577-608. 1914; Burgess, 1918 (p. 716). 



40 Brown, P. E. Bacterial activities and crop production. Iowa Agr. Exp. 

 Sta. Res. Bui. 25. 1915. 



41 Niklewski, 1912 (p. 726). 



