SOIL BACTEEIOLOGY AS A FACTOR IN CROP PRODUCTION. 7 



into the soil, provided conditions for bacterial growth are favorable. 

 In this connection it is interesting to note that the Subtilis group, a 

 subdivision of the group of ammonifiers comprising four or five 

 distinct species, occurs probably in every cultivated country. It 

 has been found in every country where any attempt has been made 

 to isolate these species. The products formed by the various species 

 of bacteria in the ammonifying group, in addition to the ammonia 

 formed from the proteid nitrogen, are not well known. It is there- 

 fore a possibility that some of these bacteria may produce substances 

 injurious to the growth of crop plants. The so-called soil toxins, 

 which may or may not be important factors in limiting the develop- 

 ment or continued cultivation of certam crops, may be produced 

 by these bacteria; there is as yet, however, no satisfactory evidence 

 that such is the case. 



NITRIFYING BACTERIA. 



The nitrifying bacteria are in reality two groups of organisms. 

 The first group when supplied with mineral plant food and with 

 ammonia nitrogen transforms the ammonia into nitrite. This group, 

 which formerly was considered to consist of a few very highly special- 

 ized organisms, is now known to number many species, most of which, 

 however, have the power of making the transformation of ammonia 

 to nitrite only to a moderate degree. The large number of organisms 

 having this power makes it seem probable that usually they may be 

 more important than the few species that are more active and also are 

 more seldom found. The second group of nitrifying bacteria is able 

 to further oxidize the nitrogen in the form of nitrites to nitrate nitro- 

 gen. Nitrate nitrogen is the form of nitrogen usually considered to 

 be of the highest value as plant food, while nitrite nitrogen, except 

 in very minute quantities, is an actual poison to crops. This special- 

 ized group of bacteria, therefore, which is able to feed on nitrate 

 nitrogen and as fast as it is produced transform it into desirable food, 

 is of extreme importance in all cultivated soils. Like the ammoni- 

 fying bacteria, however, the bacteria of this group also are widely 

 distributed and usually are present in sufficient numbers to perform 

 their proper functions. They develop best in soils that are kept in 

 good tilth, and usually grow very sparingly, if at all, in water-logged 

 acid soils, and but sparingly in the fine-grained clays and gumbos that 

 are almost impervious to the air. 



DENITRIFYING BACTERIA. 



The denitrifying bacteria, many of which are also ammonifying bac- 

 teria, are those which have the power almost diametrically opposed 

 to that of the two groups just described. These organisms trans- 



[Cir. 113] 



