1864.] CHEMISTRY OF MANURES. 119 



the soil, its true office, as contradistinguished from the imaginary 

 functions assigned it of old, may now be clearly perceived. As 

 living organisms feed on the carbon restored to the air by their 

 defunct predecessors, and as humus is but the debris of previous 

 vegetation in a soil, the carbonic acid developed by its decay must 

 play a proportionate part in nourishing the crop then in course of 

 growth. Hence the necessity of an atmosphere within the soil to 

 oxydize the humus, and thereby to reduce its carbon from the or- 

 ganic to the mineral condition, so as to make it assimilable by 

 plants. The necessity of such an underground atmosphere is an 

 established fact ; air being as essential as warmth and moisture to 

 the germination of seeds, and to the development of plants. One 

 of the main services rendered by ploughing consists in the loosen- 

 ing of the soil, and the multiplication within it of interstitial air- 

 spaces. Of like kind is (in one of its aspects) the benefit render- 

 ed by subsoil-drainage to water-logged soils ; whose interstices of 

 course receive air from above, as fast as the redundant water is 

 drained off from below. Lastly, one principal advantage of the 

 porosity of soils, and of their consequent surface attraction, con- 

 sists in their property, thence derived, of condensing and retain- 

 ing within their pores so much of the underground air. The oxy- 

 gen thus brought into close contact with humus, attacks it and 

 becomes charged with its carbon ; remaining thus charged, within 

 its pores, as carbonic-acid gas, — the appropriate mineral carbonif- 

 erous plant-food, as already explained. This gas, meeting with 

 the moisture also retained in humus by the surface-action of its 

 pores (termed, with reference to fluids, cajyillary attraction), is 

 therein dissolved, and so presented to the ramifying rootlets in the 

 most favorable manner for imbibition by the so-called osmotic ac- 

 tion of their membranous spongioles, and the suction-power devel- 

 oped by the evaporation of their sap from the leaves. 



In this way do decaying organic bodies replenish the atmosphere, 

 whether above ground or below, with gaseous carbon ; which the 

 atmosphere, in its turn, conveys to the plants ; whose leaves appear 

 to inhale it as gas, but to whose roots it is supplied in watery 

 solution. The carbon of the plant and the carbon of the soil have 

 but one primal origin, the atmosphere. From this source the car- 

 bon constantly flows ; to this reservoir it as constantly returns. 

 The hutnus of the soil, and the tissues of plants, are but successive 

 Testing-points for carbon in its circulating course. 



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