110 Influence of Plant Residues on Nitrogen Fixation 



and the results have for this reason been widely discounted, it still 

 appears to be fairly evident from the field experiments described above 

 that soil nitrates may be definitely and adversely affected when appreci- 

 able quantities of sugar and starch, and therefore of straw, are appEed to 

 field soils in spring. The experiments cited by Warington(33), in which 

 it was shown that an annual application of straw at the rate of 2000 lbs. 

 per acre to some of the Rothamsted plots did not result in any definite 

 decrease of crop^ cannot be regarded as indicating the complete absence 

 of any adverse changes, but rather that under certain conditions an 

 ecjuilibrium may obtain between destructive and constructive processes. 



Summary. 



The foregoing experiments give definite evidence, corroborative of 

 the work of Koch, Remy and others, that the nitrogen content of sand 

 or soil may be appreciably increased by the activity of Azotobacter when 

 some suitable source of energy is supplied. For this purpose sugars such 

 as dextrose and saccharose are suitable, but distinct gains have also been 

 obtained by the use of plant residues. In laboratory experiments an 

 increment of upwards of 6 mgrm. of nitrogen per gram of plant residues 

 occmTed, but in pot experiments gains of 9 mgrm. per gram of substance 

 were obtained. 



It is al.so shown that on the field scale, and in spite of the entrance of 

 complicating factors, definite increases of crop (equal to 20-54 per cent.) 

 resulted from the application of carbonaceous compounds (sugar) when 

 the soil conditions were favourable. Since the difference between the 

 action of sugar and plant residues is largely one of degree and not of 

 type, it is reasonable to suppose that such substances as stubble, leaves, 

 and other complex organic materials may also serve to contribute 

 indirectly to the reserves of soil nitrogen. 



The general soil conditions making for the successful operation of 

 nitrogen fixation processes are, in addition to the supply of some .source 

 of energy, a suitable temperature, the presence of phosphates and a supply 

 of basic material such as calcium carbonate. Even under the most 

 favourable circumstances for nitrogen fixation, there occurs a period 

 during which adverse processes come into play, and it is not advisable 

 that a crop be introduced before these have run to completion. 



Under unfavourable conditions and particularly during periods of low 

 temperature, these adverse changes may persist without any subsequent 

 entrance of soil gains. 



