71 



Considering first the influence which cultivation exerts on the nitrifying 

 power of soils to which ammonium sulphate has been added, evidence is 

 presented which shows that of the 44 pairs of virgin and cultivated samples, 

 27 gave higher nitrate formation by the cultivated soil; in 15 the virgin 

 sample excelled ; and in two there were no nitrates formed by either mem- 

 ber of the pair. Comparing the total amount of nitrates formed by the two 

 series, the cultivated soils show again a superiority over the virgin samples, 

 and the average increase, when taking place in the cultivated sample, was 

 9.43 mgs. as compared with 6.98 mgs. in the virgin sample. Table No. II 

 is given to summarize these points. 



TABLE II. Comparative Nitrifying Efficiency of 44 Virgin and 

 44 Cultivated Soils after Six Weeks. 



Number of pairs in which cultivated soils excelled 27 



Number of pairs in which virgin soils excelled 15 



'Number of pairs in which no nitrification took place : 2 



Total increase in nitrates by cultivated soils 489.27 mgs. 



Total increase in nitrates by virgin soils 347.00 " 



Average increase where cultivated soils excelled 9.43 tf 



Average increase where virgin soils excelled 6.98 ' ' 



The evidence which points to a more efficient nitrifying flora in culti- 

 vated soils than is present in corresponding virgin types is unmistakable. 

 The process of cultivation undoubtedly tends to develop the nitrifying 

 organisms chiefly, it may be supposed, through superior aeration which it 

 affords. The relationship which is here brought out is very similar to that 

 noted in the fixation tests, practically the same number of soils showing 

 superiority of the cultivated samples. More significant still is the fact that 

 the increases, when induced by the cultivated flora, average considerably 

 higher than when resulting from the virgin flora which would indicate a 

 higher degree of virulence of the bacteria which produced the respective 

 processes as well as preponderance in occurence. 



As was the case with the fixation tests an effort was made to trace a re- 

 lationship between soil type and its influence on nitrification. Only the 

 virgin soils were noted in this connection and the general classification of 

 all the soils into five different types as previously made was used in the 

 present instance. The nitrates formed by the virgin soils of these respective 

 types were averaged and are presented in Table III. 



