ON SOILS. 



the construction oi their leaves, blossoms, seeds, and the proximate 

 constituents contained in them. 



_ "Carbonic acid is generated in t'he soil wherever plants are produced. 

 Fallen leaves, roots remaining in the ground, and the worms and in- 

 sects which feed thereon, all become as soon as life has left them the 

 T>re> ol corruption and decay, and by tliis means the carbon they con- 

 tain is converted into carbonic acid. \Ye call such decaying organic 

 matter humus when, as very speedily happens, it has assumed a dark- 

 color Humus, when air and iuoisture can act upon it, is slowly but 

 unceasingly decomposed still further, and therefore continually 'fur- 

 nishes fresh supplies of carbonic acid to the roots of plants as nutri- 

 ment. At the same time also the nitrogenous and mineral substances 

 which it contains become soluble and capable of being received as 

 iood by plants, and can thus in like manner be appropriated to their 

 nourishment. The farmer js therefore quite correct in attributing to 

 humus an especial beneficial influence upon the growth of plants, and 

 consequently is laboring with all his energies to render his land rich 

 in humus, especially as at the same time he makes the soil at once 

 looser, w r armer, and better suited to the absorption and diffusion of 

 moisture, as well as stronger in the power of attracting the nutritive 

 .materials existing in the air. 



" The farmer, however, must not suppose that this enrichment of the 

 land in humus can be achieved only by directly introducing into the 

 ground large quantities of such substances, for example, straw manure, 

 as have especially the power to produce much humus. This end can 

 also be indirectly attained, and frequently with far greater pecuniary 

 advantage, by a judicious rotation of crops and the application of very 

 strong manures guano, bonedust, &c. Though these in themselves 

 furnish but little humus, they produce roots, &c. 



'Plants receive nitrogen chiefly through the ammonia which is pro- 

 duced in the putrefaction and decay of vegetable, and more particularly 

 of animal substances. Plants are always 1 surrounded by air, and the 

 air consists mainly of nitrogen. Hence it might be concluded that 

 they could never suffer .from deficiency of this element for the develop- 

 ment of their structure, inasmuch as they have the opportunity of ab 

 sorbing it in any quantity from the atmosphere. And yet they are 

 without it in many, perhaps the greater number, of fields. From the 

 circumstance, therefore, that plants do not take up tlie nitrogen of 

 the air as nourishment, we must in-ier that they cannot, and that pure 

 nitrogen is not digestible and suitable as food for them. And this is 

 a fact. 



" The chemist undertakes to explain this indigestibility of nitrogen 

 from its natural constitution. One of the distinctive characters ai 

 nitrogen is its disinclination to combine with other bodies ; if this is 

 to be accomplished it must be brought about by compulsion, for which 

 the chemist has frequently to employ very circuitous methods. This 

 unwillingness to give up its natural freedom seems so strong that the 

 plant does not possess sufficient power to overcome it. Except in the 

 atmosphere we find nitrogenous combinations only in the structure 

 of plants and animals ; and this is the nitrogen which benefits plants 

 when after the death of these living bodies it has undergone an altera- 

 tion by putrefaction and decay. The nitrogen is thereby carried over 

 from the more complex quartcrnary compounds in which they exist 

 in animal ;um plants, into a simpler combination. Withdrawing itself 

 from two of these elements carbon and oxygen it remains in com- 

 bination with the third hydrogen and then forms the most important 

 and valuable: i.u*iitnl of plants namely, ammonia. By combining 

 with acids, such as sulphuric or muriatic, or with humus, it is termed 

 ammoniacal salts. 



" There is also a second combination of nitrogen, which must be 

 regarded as a means of nourishment of plants. This is produced when 



307 



