^62 HOW CROPS FEED. 



As to the mode in which the soil thus assimilates free 

 nitrogen, several hypotheses have been offered. One is 

 that of SchiJnbein, to the effect that in the act of evapora- 

 tion free nitrogen and water combine, with formation of 

 nitrite of ammonia. In a former paragraph, p. 79, Ave 

 have given the results of Zabelin, which appear to render 

 this theory inadmissible. 



A second and adequate explanation is, that free nitrogen 

 existing in the cavities of the soil is directly oxidized to 

 nitric acid by ozone, which is generated in the action of 

 ordinary oxygen on organic matters, (in the same way as 

 happens when ordinary oxygen acts on phosphorus,) or is, 

 perhaps, the result of electrical disturbance. 



Experiments by Lawes, Gilbert, and Pugh {Phil. 

 Trans. ^ 1861, II, 495), show indeed that organic matters 

 in certain conditions of decay do not yield nitric acid 

 under the influence of ozone. 



They caused air highly impregnated with ozone to pass 

 daily for six months through moist mixtures of burned 

 soil with relatively large quantities of saw-dust, starch, 

 and bean meal, with and without lime — in all 10 mixtures 

 — but in no case was any nitric acid produced. 



It would thus appear that ozone can form nitrates in 

 the soil only when organic matters have passed into the 

 comparatively stable condition of humus. 



That nitrogen is oxidized in the soil by ozone is highly 

 probable, and in perfect analogy with what must happen 

 in the atmosphere, and is demonstrated to occur in Schon- 

 bein's experiments with moistened phosphorus (p. 66, 

 also Ann. der Chem. v. Pharm., 89, 287), as well as in 

 .Zabelin's investigations that have been already recounted. 

 (See pp. 75-83.) 



he fact, established by Reichardt and Blumtritt, that 

 humus condenses atmospheric nitrogen in its pores (p. 

 167), doubtless aids the oxidation of this element. 



The third mode of accounting for the oxidation of 



