NITRIFICATION IN ARABLE SOIL. 361 



Koch' observed in the soils whi(;li he ex;iiiiine<l that inieToorganisms 

 rapidly decreased in number as the depth at which the samples were 

 taken increased. 



Nitrifiable organic substances. — It will be readily understood a priori 

 that all the nitrogenous substances which the soil contains naturally, 

 or which have been introduced in the form of fertilizers, are not acted 

 upon by nitrifying ferments with equal readiness. In soils cultivated 

 without fertilizers the nitrifying material is humus, which, on account 

 of the resistance it i)resents to the action of ferments, is abundant in 

 certain soils and present in all, since no soil bearing vegetation is 

 absolutely devoid of nitrogenous matter. This resistance to nitrifica- 

 tion, however, is not absolute, and it is interesting to study to what 

 extent the microorganisms overcome it. During recent years the 

 author has devoted much time and study to the investigation of the 

 formation of nitrates in soils from different sources and in different 

 states of fertility," and since barnyard manure is to-day more largely 

 employed than any other fertilizer in the majority of farm operations, 

 the author has also investigated to what-extent the nitrogenous matter 

 supplied by it to the soil is transformed into the highly assimilable 

 nitrates. 



Nitrates readily filter tlrrough the soil, and are so completely removed 

 in the drainage that a study of the drainage water affords an accurate 

 measure of the activity of nitrification in a soil. The leaching of a 

 soil by rain water is by no means as complete as that obtained in the 

 laboratory on small quantities of soil. Nevertheless, analysis of the 

 drainage water of soil without vegetation carried on throughout the 

 year shows such .small quantities ot nitrates during winter that it 

 appears certain that the xain water removes practically all the nitrates 

 formed during the more favorable seasons. Furthermore, by allowing 

 the experimental soils to be-exposed to rain, sun, and cold, andcollect- 

 ing the water which passes through, we undoubtedly obtain more precise 

 information regarding nitrification in soils in place than*can be procured 

 by studies in the laboratory. It is by examining the drainage water 

 thus obtained that the author has studied nitrification in arable soils. 



STUDY OF DRAINAGE WATERS — METHODS OF RESEARCH. 



In these investigations 2 methods were followed. At first pots were 

 used, but afterwards these were discarded for vegetation boxes. The 

 first were large, slightly conical pots, of glazed earthenware, standing 

 on iron tripods. The holes in the bottom were fitted with tubes, 

 through which the drainage water ])assed into large flasks placed beneath 

 to receive it. Before filling with soil the bottoms of the pots were covered 

 with a layer of gravel to facilitate drainage and prevent clogging of 



■ Jahresber. Agr. Chem., 81, p. 69. 



2 These rcsearcLes have beeu jiubhshed iu Auu. Agrou., 14-20. 



