72 OTHER IMMEDIATE SOURCES OF NITROGEN. 



imal manures upon vegetation, justify us in believing that ammonia 

 actually enters into the roots, and perhaps into the leaves, of plants — we 

 ought not hastily to conclude that all the nitrogen which plants are ca- 

 pable of deriving from decaying animal matter must enter into liieir cir- 

 culation in the form of ammonia. Other soluble compounds containing 

 nitrogen arc formed during the decay of animal substances — they ac- 

 tually exist largely in the liquid manures of the stable and fold-yard, 

 and they can scarcely fail, when applied to the soil, to be to a certain 

 extent absorbed by the roots of plants. This urea is a substance con- 

 taining much nitrogen, which exists in the urine or excrements of most 

 animals, and by its decomposition produces carbonate of ammonia. 

 But being very soluble, tliis substance may enter directly into the roots, 

 and may be there decomposed, and made to give up its nitrogen to the 

 living plant. To other compound substances of animal origin the same 

 observation may apply,* — so that while the fact, that animal manure in 

 a state of fermentation Is very beneficial to vegetation, may be consid- 

 ered as rendering it highly probable that the ammonia which such 

 manure contains, enters directly and supplies much nitrogen to the 

 growing plants, it must not be entirely left out of view that, in nature, a 

 portion of the nitrogen, derived from animal substances, may be ob- 

 tained immediately from other compounds in which ammonia does not 

 exist. 



To what amount ammonia actually enters into the circulation of 

 plants, or how much of the nitrogen they contain it actually supplies, 

 we have no means of ascertaining. Were it abundantly present in the 

 soil, its great solubility would enable it to enter, with the water absorbed 

 by the roots, in almost unlimited quantity. In a subsequent section we 

 shall consider the conditions under which ammonia is produced in nature, 

 the comparative abundance in which it exists on the earth's surface, 

 and the extent of the influence it may be supposed to exercise on the 

 general vegetation of the globe. 



§ 8. Absorption of nitric acid hy plants. 

 1°. That ammonia is actually present in the juices of many living 

 vegetables has been adduced, as a kind of presumptive evidence, that 

 this compound is directly absorbed by plants. A similar presumption 

 is offered in favour of the direct entrance of nitric acid, by its invariable 

 presence in combination with potash, soda, lime, or magnesia, in the 

 juices of certain common and well known plants. Thus it is said to be 

 always contained in the juices of the tobacco plant, of the sunflower, of 

 the goosefoot,f and of common borage. The nettle is also said to con- 

 tain it, and it has been detected in the grain of barley. t It exists pro- 

 bably in the juices of many other plants in which it has not hitherto 



• Thus it may be applied more strongly to the hippuric acid., which exists in the urine of 

 the horse, and other herbivorous animals. This acid decomposes naturally into benzoic 

 and and ammonia. The sweet-scented vernal-grass (Anthoxanthum Odoratum) by which 

 hay is perfumed, owes its agreeable odour to the presence of this benzoic acid. It may, 

 therefore, be supposed that, where cattle and horses graze, the grasses actually absorb the 

 hippuric acid contained in the urine, which reaches their roots, decompose it as it ascends 

 with the sap, appropriate its nitrogen, and exhale the odoriferous benzoic acid. 



t Chenopodium, probably in all the species of this genus.— See Liebig, p. 82. 



t Grisenthwaite (New Theory of Agriadture, p. 105) says, it is always present in barley in 

 Ihe form of uilrate of soda.— (See Appendix. 



