734 PRINCIPLES OF SOIL MICROBIOLOGY 



surface of the mixture; the abundance of the colonies serves as an index of the 

 activity of the soil. 



4 and 5. A large silica-gel plate, containing 2 grams of mannite is inoculated 

 with 1 gram of soil. After 48 hours incubation, the number of colonies on the 

 plate is determined; after 5 days incubation, the contents of the plate are ana- 

 lyzed for total nitrogen. An active soil will show 2,000 to 3,000 colonies per 1 

 gram of soil and will fix 20 mgm. of nitrogen for the 2 grams of mannite. 



THE CATALYTIC ACTION OF SOIL 



The catalytic action, or the catalytic power, of a soil is its ability 

 to produce oxygen from hydrogen peroxide; this has often been found 

 to be an index of the fertility of the soil. This action can be determined 

 by adding 5 grams of soil to 20 or 40 cc. of a 1.5 or 3 per cent solution 

 of H 2 2 and collecting the oxygen liberated; a 100 cc. gas-measuring 

 tube filled with a dilute solution of NaOH or KOH and inverted into 

 a bath of the same solution is used for collecting the gas. The soil 

 is usually placed in a large test tube or in a 300-cc. Erlenmeyer flask 

 and may be suspended in a little water, before adding the peroxide. 

 One portion of soil is used untreated and another portion is previously 

 sterilized in the autoclave so as to determine the role of the living or- 

 ganisms in the process; a third portion of soil may be ignited and then 

 used in the test, so as to determine the role of the organic matter in 

 the process of H 2 2 decomposition. The period of incubation is 

 usually 5 to GO minutes and the temperature 17° to 37°C. An increase 

 in concentration of substrate, temperature and period of incubation 

 will all lead to an increase in the amount of oxygen liberated. 



Konig 44 found that the decomposition of H 2 2 by soil was due chiefly 

 to the enzyme catalase produced by the soil microorganisms and plant 

 materials, and to some extent to the inorganic part of the soil 

 and to organic colloids. On sterilizing the soil by heat or treat- 

 ment with chloroform, iodine, mercury bichloride and especially 

 hydrocyanic acid, the liberation of oxygen was greatly diminished. 

 The catalytic action was found to be further increased by a similar 

 action of manganic oxide, iron and aluminum oxides. The forma- 

 tion of oxygen by heated soils was ascribed to the increase in 

 alkalinity due to the formation of CaO from CaC0 3 . When heated 

 soil is moistened and allowed to remain one day in a desiccator filled 

 with C0 2 , its catalytic power is greatly diminished. Konig found a 



44 Konig, J., Coppenrath, E., and Hasenbaumer, J. Beziehungen zwischen 

 den Eigenschaften des Bodens und der Nahrstoffaufnahme durch die Pflanzen. 

 Landw. Vers. Sta., 66: 401-461. 1907; 53: 472-476. 1906. 



