t 529 ] 



the soil in a non-mineralised and non-available form. Peat 

 contains i to 4 per cent, (usually about 2 per cent.) of N. 

 The usual proportion of N in soils is from '01 to '5 per cent, 

 when a soil contains more than *5 per cent, of N it should be 

 considered very rich in this important constituent. Humus 

 boiled with alkalies gives off N in the form of NH 3 . The 

 nitrogen in humus exists in various unavailable combinations, 

 and it is only slowly rendered available by the action of 

 alkalies, by fermentation. Part of the N is lost as free N in 

 course of 'fermentation, but the greater portion enters into 

 organic combination which are more or less* insoluble and 

 undecomposeable. The albuminoids of the vegetable refuse 

 are decomposed into two amides (Leusine and Tyrosine) 

 which are allied to gelatine. In fact gelatine is an amide 

 carbohydrate. If sugar or dextrine is heated at a temperature 

 of 35oC in contact with NHs vapour, a substance containing 

 about 16 per cent, of N is formed, which resembles natural 

 gelatine and gives off NHs when heated with alkalies. 

 Albuminoids, amides, and other nitrogenous organic com- 

 pounds occurring in the humus of soils are probably incapable 

 of affording nourishment to plants. Boussingault performed 

 some experiments to show that if N is supplied to soils in 

 organic forms only, plants do not grow well; but sup- 

 plied in mineral forms and especially as nitrates, they 

 do well. He also showed that nitrates are slowly formed 

 out of the organic compounds by natural oxidation in 

 the soil. Nitrates are undoubtedly the most valuable of all 

 plant-foods. 



849. Ammonia, urea, uric acid, hippuric . acid and guanin 

 (which occur in urine) are also assimilated directly by plants. 

 Urea is of equal value to salts of ammonia applied in equivalent 

 quantities. Though the constituentsof urine just mentioned are 

 capable of direct assimilation by plants and are so taken up 

 in part by plants, they are actually readily transformed first 

 into ammonia and then into nitrates. It is curious, that urea 

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