139 



Berlhelot shows that veo^etable soils continually absorb nitrogen 

 from the air, aiul very much more than exists in the air as ammonia 

 or nitrooenous compounds, so that it must be taken directly from 

 the free nitrogen and this, too, although the soil contains no growing 

 vegetables. (Agr. Sci., Vol. I, p. 120.) Apparently this absorption 

 is the work of the microbes preparing the soil for future i)lant growth, 

 and much of the irregularity in our crop reports depends not u|)on 

 the climate or the fertilizer, but upon the activity of this form of life. 



Berthelot (1887) shows that the fixation of gaseous nitrogen of the 

 atmosphere by the soil takes place continualh" even when no vegeta- 

 tion is presented and that it is greater in soil exposed to rain than in 

 soil protected from the rain, this being undoubtedly due to the fact 

 that in the exposed soil the minute forms of life by means of which 

 nitrogenous compounds are formed can operate more intensely because 

 of the greater quantity of air dissolved in and carried down to them 

 by the rain. (See AVollny, X, p. 205.) 



A parallel investigation by Heraeus shows that probably tht^ bac- 

 teria may be divided into two classes — those which oxidize and those 

 which reduce the oxides, and that in general where an abundance of 

 nutj'ition exists, as in rich soils, the reducing bacteria are in excess, 

 and that, on the contrary, where these do not find a sufficiently favor- 

 able soil there the oxidizing bacteria have the upper hand. 



Salkowsky (188-7), as the result of his ow^i experinients, considers 

 it indubitably established that processess of oxidation in water can 

 onl}' be due to the vital activity of bacteria, and that this is equally 

 true of water permeating the soil, and therefore of the oxidation 

 l)rocesses in the soil itself. 



Warington (1887), having shown that the process of nitrification 

 goes on by means of organisms that are rather uniformly distributed 

 at the surface, and that they are less frequent at depths of 9 and 18 

 inches, depending on the porosity of the soil, and that none could be 

 found at depths of from 2 to 8 feet, has now revised these early 

 (•xi)eriments and finds a few nitrifying bacteria at depths at from 5 

 to English feet, but that in general they are less numerous and 

 have a feebler activity the deeper they are in the earth. Under 

 natural conditions nitrification occurs principally in the highest layer 

 of soil, because the conditions of this process — viz, accessibility of the 

 ir and quantity of nitrogenous compounds — are more favorable here 

 than in the loAver strata. (See AVollny, X, p. 211.) 



As our views as to the relation of the nitrogen of the atmosphere to 

 vegetation have been entirely remodeled within the past livo years, 

 the following summary by Maquenne (1891) has been selected as 

 showing the slow progress of our Ivuowledge up to the brilliant suc- 

 cess of Hellrieffel and Wilfarth. 



