160 



able for plant food. Thus three distinct organisms are necessary 

 to make the nitrogen of complex nitrogenous substances available 

 for plants. This association of various organisms in bringing 

 about important results I shall refer to later ; it is one of the 

 most interesting phenomena which modern science has revealed. 



Now these various organisms may be deficient in soils in par- 

 ticular cases, and attempts have been made to supply them arti- 

 ficially. For this purpose the bacteria have been cultivated in 

 ■certain media, and sold under the name of " nitragin." Experi- 

 ments have been made with this material in Germany and else- 

 where, but with very variable results. In some cases great benetit 

 is said to have resulted, in others little or none. More experi- 

 ments are required before any definite conclusions can be drawn. 

 It is obvious that such a material might fail owing to improper 

 preparation, or that it might appear to fail, not from any want 

 of virtue in itself, but because the soil to which it is added already 

 contains the organisms in sufficient numbers, their activity 

 perhaps being hindered by unfavorable conditions. But there 

 are many other organisms present in soil besides those already 

 referred to, and among these are usually to be found some which 

 have a directly opposite effect, that is, they break up nitrates 

 into nitrites, or may go still further, and destroy them altogether, 

 with liberation and consequent loss of free nitrogen. They are 

 hence known as denitrifying bacteria. Obviously great com- 

 plexity is here introduced, and the question has to be considered 

 whether these denitrifying organisms can cause serious loss of 

 nitrogen from the soil, and if so under what conditions. It is 

 practically agreed that in some cases excessive quantities of fresh 

 horse or cattle manure exercise an injurious rather than a bene- 

 ficial effect, and especially that they lessen the effect of nitrogen- 

 ous manures, such as sulphate of ammonia and nitrate of soda, 

 and it has further been shown that manure of this kind fre- 

 quently contains these denitrifying bacteria in large numbers. 

 The conclusion was drawn, apparently too hastily, that it is the 

 oresence of numbers of these organisms in the excreta of horses 

 and cattle which causes the deleterious effect. Recent investiga- 

 tions, however, have thrown considerable doubt on this conclusion. 

 It has been shown that the denitrifying bacteria are already 

 present, at least usually, in the soil, and it is asserted that the 

 mere addition of a larger number will not account for the pheno- 

 mena. A large number of experiments have been made to 

 elucidate this question, the results of which have been to .show 

 that certain substances — starch, for instance — greatly increase 

 the activity of these bacteria, doubtless because they are particu- 

 larly suited for food for the organisms, and it has been asserted 

 that some of these food substances are always present in the 



