ABSORPTION OF OXYGEN BY SOILS. 221 



factorily ascertained, at the same period, that earth taken from the 

 galleries of these mines, only became fertile after having been ex- 

 posed to the atmosphere for a considerable length of time. I have 

 quoted these curious observations because they are, so far as I know, 

 the first udiich established the necessity of the presence of oxygen 

 in the interstices of the soil, or, as M. Humboldt then said and, in- 

 deed, as may still be maintained, the utility of a previous oxidation 

 of the soil. 



All our agricultural facts, indeed, confirm this view of the necessity 

 of air in the interstices of the soil that is destined for the growth of 

 vegetables. When, by ploughing very deeply, for example, we 

 bring up a portion of the subsoil into the arable layer, in order to in- 

 crease its thickness, we always lessen the fertility of the ground for 

 a time ; in spite of the action of manures, and of any treatment we 

 may adopt, a certain time must elapse before the subsoil can pro- 

 duce an advantageous effect ; it is absolutely necessary that it have 

 been exposed to the atmospheric influences, and it is then only that 

 deep ploughing, which gives the arable layer a greater thickness, 

 pays completely for the expense it has occasioned. 



I am disposed to ascribe the absorption of oxygen gas by clayey 

 soils, to the oxide of iron, which they almost always contain, and 

 which is in the minimum state of oxidation, when the clay lies at a 

 certain depth. In the performance of some soundings in a tertiary 

 soil of the department of the lower Rhine, which I performed in 

 1822, I had occasion to observe that the clays brought up white by 

 the borer, very speedily became blue by exposure to the air; and 

 that in gaining color they condensed oxygen. I propose returning 

 upon this fact to shovi^ the important part which this simple super- 

 oxidation probably plays in the amelioration of soils.* 



M. Schiibler, again, has studied the action of oxygen gas upon the 

 component parts of arable soils, and, according to him, the absorp- 

 tion of this gas cannot be doubted ; it is very trifling in connection 

 with sand and gypsum, very decided as regards clay, loam, and 

 humus. As M. de Humboldt and M. de Saussure had already done, 

 M. Schiibler observed humus to change a portion of the oxygen 

 which it fixed, into carbonic acid ; but, in general, the other sub- 

 stances, or soils, or elements of soils, upon which he experimented, 

 appeared to absorb the oxygen by the intermedium of the protoxide 

 of iron, from which they are never altogether free. Besides this 

 cause, due to the superoxidation of a metal, M. Schiibler thinks that 

 a certain portion of the oxygen disappears by condensation within 

 the pores of some soils ; and in support of his opinion he appeals to 

 the admirable observations of M. de Saussure, on the condensation 

 of the gases by porous bodies. Starting from the fact that the roots 

 of plants require the presence of oxygen in order to thrive, he 



* Austin proved, that during the oxidation of metallic iron under water, there is a 

 constant production of ammonia. Certain experiments commenced some time ago, and 

 wliich I still continue, will establish in the most precise manner, as I hope, the fact 

 liiat this formation of ammonia likewise takes place during the passage of the protox- 

 ide of iron to the state of hydrated peroxide. The theoretical conclusions deducibl« 

 from this fact, and the economic applications which may tlow from it, mXist be obvious. 



19* 



