9 6 



SOIL CONDITIONS AND PLANT GROWTH 



For each gram of dextrose supplied in the small doses about 8 

 milligrams of nitrogen were fixed during the first eight weeks ; but only 

 4 or 5 milligrams later on. In larger doses the sugar was less effective, 

 only 5 to 6 milligrams of nitrogen being fixed per gram of sugar at 

 first and 3 milligrams later. 



Pot experiments showed that the nitrogen thus added to the soil 

 became available for plant food. Dextrose and sucrose first depressed 

 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 15 C. The 

 optimum temperature lies between 25 and 30 C. 



Pfeiffer and Blank (226^), however, were unable to obtain any 

 beneficial results from sugar. The Rothamsted trials showed increases 

 for autumn applications but decreases for spring dressings. 



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

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

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

 Antigua. 2 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 



1 See The Agricultural News, 1908, vii., 227; 1910, ix., 339, and 1911, x., 179. 

 2 See Manurial Experiments with Sugar Cane in the Leeward Islands, 1908-09 and 

 1909-10. (Pamphlets 64 and 68, West Indian Department of Agriculture). 



