192 



The evidence gathered by these earlier investigators shows a distinct, though not 

 always uniform, relation between the productive power of soils and their content of 

 carbon dioxid, of ammonia, or of nitrates. We note that the air of fertile soil usually 

 contains more carbon dioxid than the air of unproductive soil. Similarly, the fertile 

 soils contain, as a rule, more ammonia and more nitrate than unproductive soils. But 

 even admitting this, it seems hardly practicable to draw definite conclusions as to the 

 future behavior of a soil from its content of the substances in question. The amount 

 of carbon dioxid in the soil air is an indication of oxidation changes already accom- 

 plished, but not necessarily a guide to future oxidation intensity. The organic con- 

 stituents of the humus, as well as the character of the microorganisms, may have been 

 modified to preclude rapid oxidation. In the same way the quantity of ammonia in 

 the soil is only a measure of past performance, and a very inadequate measure at that. 

 As a transition product ammonia may be speedily oxidized to nitrates, or it may be 

 transformed into protein substances by plants or fungi. Hence the quantity of am- 

 monia present at any time in cultivated soil can not even serve to indicate past intensity 

 of ammonia formation. As to nitrates, they, too, are not stable in the soil. Like am- 

 monia, they may be utilized by higher plants, or by bacteria, yeasts, and molds for the 

 production of new protein compounds. They may likewise be destroyed by denitri- 

 fying bacteria, or they may be leached out of the soil by excessive rainfall. In a word 

 then, the amounts of carbon dioxid, ammonia, and nitrates in field soil are but an 

 incomplete measure of past performance and a very inadequate guide as to future 

 efficiency. 



The better understanding of the functions of humus, which has gradually come in 

 the wake of bacteriological investigations, has suggested new methods for the study of 

 organic matter and its transformation in the soil. In experiments like those of 

 Wollny, or in the more recent experiments of Stoklasa and Ernest, & the evolution of 

 carbon dioxid from soils kept under definite experimental conditions has been em- 

 ployed as a measure of the activities of the soil bacteria and of the susceptibility of the 

 humus to decay. The same purpose has been accomplished in the experiments of 

 Russell, c and of Darbishire and Russell,** by measuring the absorption of oxygen 

 instead of the evolution of carbon dioxid. These methods enable us, therefore, to 

 study the possible future behavior of the humus compounds under given conditions. 

 In other words, we are enabled to secure some information concerning the relative 

 availability of the constituents in the soil humus. For instance, it was found by 

 Stoklasa and Ernest in a comparison of several soils that the average daily production 

 of carbon dioxid in 1,000 grams of soil ranged from 17.5 to nearly 60 milligrams. More 

 carbon dioxid was produced by the soil than by the subsoil, the aerobic activities being 

 more prominent in the former, the anaerobic activities in the latter. Similarly, at 

 35 C. about twice as much carbon dioxid was produced as at 20 C. Darbishire and 

 Russell found that in a number of untreated soils examined the absorption of oxygen 

 in nine days ranged from 6 to 27 millimeters. 



Analogous attempts at measuring the rate of decay of soil humus and of other 

 organic materials have been made, not by determining the oxidation products of 

 the carbon but of the nitrogen in the soil humus. It was well known that ammonia 

 almost invariably appears as one of the products in the oxidation of nitrogenous 

 materials of organic origin. It was likewise recognized after the convincing experi- 

 ments of Miintz and Coudon that ammonia formation in the soil is a biological 



J. Landwirtsch., 1886, 34 :222. 



&Centrbl. Bakt. Para., 1905, pt. II, 14:723; also Zts. Zuckerind., Bohmen, 1907, 

 57:291. 



cj. Agr. Sci., 1905, 1:260. 



dlbid., 1907,2:305. 



< Compt. rend. acad. sci. Paris, 116 : 395. 



