94 SOIL CONDITIONS AND PLANT GROWTH 



the crop, then caused an increase, and finally left the soil richer in 

 nitrogen at the end of the experiment than at the beginning (Table 

 XXXIX.). 



Table XXXIX. — Effect of Dextrose and Sucrose on the Productiveness and 

 Nitrogen Content of the Soil. Koch (151). 



But if the soil temperature fell too low nitrogen fixation ceased : it was 

 not observed at 7° C. although it appeared to go on at 1 5° C. The 

 optimum temperature lies between 25° and 30° C. 



Increased yields of sugar-cane have followed the application of mo- 

 lasses to soils at the Station Agronomique and on Mr. Ebbel's estate ^ 

 in Mauritius, where the residual effect is well shown, and also in 

 Antigua (192). Peck in Hawaii, on the other hand, observed marked 

 losses of nitrate. 



An increase in crop following the application of sugar, or starch, to 

 the soil is not evidence of nitrogen fixation, but might equally well be ad- 

 duced to show that sugar and its decomposition products are direct plant 

 nutrients. Only when an actual gain in nitrogen is demonstrated by 

 analysis does the proof become satisfactory. As a practicable scheme 

 the addition of sugar to the soil would be out of the question for field 

 work. Pringsheim (231) has shown, however, that certain decomposi- 

 tion products of cellulose also serve as sources of energy to Clostridium 

 and presumably also to azotobacter. These particular products (which 

 were not identified) are apparently not always formed in the soil (152), 

 but are readily produced in culture solutions under the action of the 

 mixed bacterial flora from soils, composts, dung and river mud. The 

 difficulty of material might therefore be overcome because large quanti- 

 ties of cellulose are available on the farm in the form of straw. But 

 there still remains the question of temperature. Azotobacter, as we have 

 seen, requires more warmth than many other organisms, and according 



' See The Agricultural Netvs, igo8, vii., 227 ; 1910, ix., 339, and 1911, x., 179. 



