412 HOW THESE CHEMICAL CHANGES BENEFIT VEGETATION. 



same instant the nitrogen of the atmosphere, which is everywhere 

 present, seizes a portion of the hydrogen and forms ammonia. Thus 

 a variable, and in any one Hmited spot a minute, but over the entire 

 surface of the globe, a large quantity of ammonia is produced during 

 the oxidation even of the purely carbonaceous portion of the organic 

 matter of the soil. 



Now in proportion as the presence of lime promotes this decay of 

 vegetable and other organic matter in the soil — in the same propor- 

 tion does it promote the production of ammonia and nitric acifl, at the 

 expense of the free nitrogen of the atmosphere, and this may be re- 

 garded as one of the valuable and constant purposes served by the 

 presence of calcareous matter in the soil. 



§ 30. How these chemical changes directly benefit vegetation. 



You will scarcely, I think, inquire how all these interesting chemical 

 changes which attend upon the presence of lime in the soil are di- 

 rectly useful to vegetation, and yet it may be useful shortly to answer 

 the question. 



1°. Lime combines with the acid substances already existing in the 

 soil, and thus promotes the decomposition of vegetable matter which 

 those acid substances arrest. The further decompositions which en- 

 sue are attended at every step by the production either of gaseous 

 compounds — such as carbonic acid and light carburetted hydrogen — 

 which are more or less abundantly absorbed by the roots and leaves 

 of plants, and thus help to feed them — or of acid and other compounds, 

 soluble in water, which, entering by the roots, bear into the circula- 

 tion of the plant not only organic food, but that supply of hme also 

 which healthy plants require. 



2°. The changes it induces upon substances in which nitrogen is 

 present are still more obviously useful to vegetation. It eliminates am • 

 monia from the compounds in which it exists already formed, and pro- 

 motes its slow conversion into nitric acid, by which the nitrogen is 

 rendered more fixed in the soil. It disposes the nitrogen of more or 

 less inert organic matter to assume the form of ammonia and nitric 

 acid, in which state experience has long shown that this element is 

 directly favorable to the growth of plants, 



3°. It influences in an unknown degree, the nitrogen of the atmos- 

 phere to become fixed in larger proportion in the soil, in the form of nitric 

 acid and ammonia, than would ott^rwise be the case, and this it does 

 both by the greater amount of decay or oxidation which it brings 

 about in a given time, and by the kind of compounds which, under its 

 influence, the organic matter is persuaded to form. The amount of 

 nitrogenoua food placed within reach of plants by this agency of lime 

 will vary with the climate, with the nature of the soil, with its con- 

 dition as to drainage, and with the more or less liberal and skilful 

 manner in which it is farmed. 



§ 31. Why lime must he kept near the surface. 



Nor will you fail to see the important reasons why lime ought to 

 be kept near the surface of the soil — since 

 P. The action of hme upon organic matter is almost nothing m 



