CARBONIC ACID FURNISHED BY HUMUS. 187 



which are necessary for the passage of atmospheric food into the 

 organism of the plant, be deficient, its growth must be impeded. 

 Its proper growth, in dry seasons, stands in exact relation to the 

 quantity of the substances taken up from the soil during the first 

 period of its development. But on a soil poor in mineral food, 

 cultivated plants do not flourish, however abundantly water may 

 be supplied to them. 



The crop of a meadow, or of an equal surface of wood-land, is 

 quite independent of carbonaceous manures, as far as regards 

 its carbon ; it is dependent on the presence of certain ingredients 

 of the soil destitute of carbon, and also on the conditions which 

 enable these to enter into the plants. Now, we are able to in- 

 crease the crop of carbon on our cultivated land, by the use of 

 burnt lime, ashes, or marl, — by substances, therefore, which are 

 entirely free from carbon. This well-ascertained fact indicates 

 that we furnish to the field, in these substances, certain constitu- 

 ents, which enable the cultivated plants to increase in mass, and 

 consequently in carbon — a power which they possessed formerly 

 only in a small degree. 



After these considerations, it cannot be denied that the sterility 

 of a field, or its poverty of produce in carbon, does not arise 

 from a deficiency of carbonic acid, or of humus ; for we have 

 seen that this produce can be increased, to a certain extent, by 

 the supply of matters destitute of carbon. Rut the very same 

 source which supplies the meadow and woodland with carbon, 

 namely the atmosphere, can yield that element to cultivated 

 plants. It therefore becomes especially necessary in agriculture 

 to employ the best, and most convenient means, of enabling the 

 carbon of the atmosphere (carbonic acid) to pass over into the 

 plants growing on our fields. The art of agriculture, in the 

 mineral food which it supplies, furnishes to plants the means of 

 appropriating their carbon from sources offering an inexhaustible 

 provision. But when these constituents of the soil are wanting, 

 the most abundant supply of carbonic acid, or of decaying vege- 

 table matter, cannot increase the crop>s on the field. 



The quantity of carbonic acid that can pass from the air into 

 plants, is limited, in a given time, by the quantity of carbonic 

 ac ;, i entering into contact with the organs destined for its absorp. 



