822 PRINCIPLES OF SOIL MICROBIOLOGY 



but corresponding unlimed soils showed a deficiency of bacteria even 

 when the host plant had been grown a year previously. Artificial 

 inoculation of such a soil was found 17 to lead to a considerable increase in 

 nodule formation. 



The use of large quantities of soil for purposes of inoculation involves 

 great expense and trouble in transportation and handling, aside from the 

 introduction, with the old soil, of weed seeds and injurious microorgan- 

 isms, such as the fungi causing various wilts and nematodes. This 

 led again to the use of pure cultures. More reliable cultures are now 

 produced, both on artificial media and in sterile soils, as a result of 

 the increased knowledge on the cultivation of the organisms. 18 



Commercial cultures and their preparation. The commercial prepara- 

 tions of nodule bacteria commonly found on the market are in the form 

 of liquid, agar, or soil and peat cultures. The historical process of 

 development of the artificial culture of these bacteria is as follows: 

 Gelatin —> cotton — » liquid — * agar — » organic — * inorganic material. 10 



Two media are used at the United States Department of Agriculture, for the 

 preparation of the legume cultures. One is a soil extract medium, made from 10 

 kgm. of field soil, 40 grams CaO and 100 liters ofjtap water. Ten grams of cane 

 sugar and 0.5 gram K 2 HP0 4 are added for each liter of the extract. The reaction 

 is adjusted to slight acidity to prevent the precipitation of the phosphate. The 

 other medium is a modification of Ashby's medium for aerobic nitrogen assimilat- 

 ing organisms: 



Cane sugar 2000 grams NaCl 20 .0 grams 



K 2 HP0 4 20.0 grams Calcium carbonate . . . 100 grams 



MgS0 4 -7H 2 20.0 grams Tap water 100 grams 



CaS0 4 -2H 2 10 grams 



The organism is grown in square bottles containing about 200 cc. of medium, 

 which is the quantity used for one bushel of seed. The cultures are not kept 

 for more than a month and their efficiency is tested by inoculating plants grown 

 in sand cultures. 



17 Wilson, J. K. Effect on nodulation of supplementing the legume bacteria 

 of the soil with artificial cultures. Jour. Amer. Soc. Agron., 18: 280-294. 1926. 



18 The movement of legume bacteria in the soil is discussed by Frazier, W. C, 

 and Fred, E. B. Movement of legume bacteria in soil. Soil Sci., 14: 29-35. 

 1922. The application of inoculated soil to legume seed by Amy, A. C, and Mc- 

 Ginnis, F. W. Method of applying inoculated soil to the seed of leguminous crops. 

 Jour. Amer. Soc. Agron., 13: 289-303. 1921. A comparative study between the 

 inoculating power of artificial cultures with inoculated soil has been made by 

 v. Feilitzen, 1909 (p. 821) and Teisler, E. Azotogen, Nitragin oder Naturimp- 

 ferde? Centrbl. Bakt. II, 34: 50-56. 1912; also Ktihn, A. Ibid., 30: 548. 1911. 



19 Rural New Yorker, April 20, 1915. 



