144 BACTERIA AND SOIL FERTILITY 



They determined the carbon dioxid set free by acids, the amount 

 of calcium dissolved by an ammonium chloride solution, the 

 behavior of the soil toward litmus, and the biological test. The 

 result of this test v^as that the biological test agreed with the 

 known condition in 90 per cent of the cases, the ammonium 

 chloride in 50 per cent, the litmus in 40 per cent, and the carbon 

 dioxid failed more often than not to indicate the correct con- 

 dition of the soil. 



What Do They Feed Upon? — These organisms probably 

 require for their nutrition the same elements as do the higher 

 plants, namely, carbon, hydrogen, oxygen, nitrogen, potassium, 

 sulfur, calcium, magnesium, and iron, and possibly aluminum 

 and manganese. 



They obtain their carbon and hydrogen from organic com- 

 pounds, preferably from carbohydrates. Oxygen is obtained 

 either from the atmosphere or from combined sources, depending 

 on the species and the conditions under which they are grown. 

 The other elements are obtained from the soil. 



A marked difference between these and the higher plants is 

 that they possess the power of obtaining their nitrogen from the 

 air. In the presence of combined nitrogen, however, they be- 

 come lazy and obtain their nitrogen in the easier way from the 

 soil. Small quantities stimulate, whereas large quantities dis- 

 courage nitrogen fixation since the organisms live on the nitrates. 

 This may be due to a number of causes: (i) direct toxic action 

 of the salt, (2) antagonism of other organisms which it favors, 



(3) the using up of the energy supply by these organisms, and 



(4) the discouragement of fixation by the use of sodium nitrate. 

 They must also have water and a suitable temperature. These 

 are the same as for the nitrifiers, hence need not be considered here. 



From Whence Their Energy? — The nitrogen-iixing organ- 

 isms differ widely from other plants in their energy requirements. 

 This is due to the fact that they are causing the lazy nitrogen 

 to combine with other elements. This requires energy in greater 

 quantities than is required by other bacteria. This must be sup- 

 plied by organic compounds, preferably one of the carbohydrates. 



