588 



PRINCIPLES OF SOIL MICROBIOLOGY 



tion definitely. Until our methods are more accurate, the question 

 cannot be answered in a positive way. It has been stated 121 that the 

 apparent gain of nitrogen in the soil is often due to drifting dust and 

 plant residues or to soil variability. 



Certainly the field results of A. Koch 122 do not speak for any nitrogen 

 fixation in the soil, following the addition of celluloses and even straw. 

 Positive fixation was obtained only when soluble sugars were added, as 

 seen from table 58. 



By determining the amount of nitrogen fixed in the soil per gram of 

 sugar added, it was found that although 720 grams of cane sugar had 

 been added per pot of soil and 9.75 mgm. of nitrogen has been fixed per 

 gram of sugar added, the plants utilized only 1.78 mgm. of nitrogen per 

 gram of sugar added. The larger part remained in the soil in a complex 



TABLE 59 

 Influence of cellulose upon crop yield 



form not readily utilized. The addition of cellulose exerted a decided 

 injurious effect upon crop yield due to competition for the available 

 nitrogen between the microorganisms and the plants. The following 

 years the nitrogen is made available again; however, one cannot speak 

 here of any nitrogen fixation. Similar results were obtained with straw. 



SYMBIOTIC NITROGEN FIXATION 



Relation between the bacteria and the host plant. "Virulence" in con- 

 nection with nodule bacteria has been defined as the ability of the organ- 

 ism to penetrate into the root tissues of the host plant, to multiply there, 



121 Hopkins, C. Soil fertility and permanent agriculture. Gin & Co., New 

 York. 1910. 



122 Rippel, A. Versuche aus dem Nachl&sz von Alfred Koch. Jour. Landw., 

 72: 17-52. 1924. 



