C 



pared and burnt with advantage; but it is certainly un- 

 profitable as part of a general system in husbandry. 



It has been supposed by some writers, that cer- 

 tain principles necessary to fertility are derived from 

 the atmosphere, which are exhausted by a succession 

 of crops, and that these are again supplied during the 

 repose of the land, and the exposure of the pulverised 

 soil to the influence of the air; but this in truth is not 

 the case. The earths commonly found in soils can- 

 not be combined with more oxygene; none of them 

 unite to azote; and such of them as are capable of at- 

 tracting carbonic acid, are always saturated with it in 

 those soils on which the practice of fallowing is adopt- 

 ed. The vague ancient opinion of the use of nitre, 

 and of nitrous salts in vegetation, seems to have been 

 one of .the principal speculative reasons for the de- 

 fence of summer fallows. Nitrous salts are produced 

 during the exposure of soils containing vegetable and 

 animal remains, and in greatest abundance in hot wea- 

 ther; but it is probably by the combination of azote 

 from these remains with' oxygene in the atmosphere 

 that the acid is formed; and at the expence of an ele- 

 ment, which otherwise would have formed ammonia; 

 the compounds of which, as is evident from what was 

 stated in the last Lecture, are much more efficacious 

 than the nitrous compounds, in assisting vegetation. 



When weeds are buried in the soil, by their gra- 

 dual decomposition they furnish a certain quantity of 

 soluble matter; but it may be doubted whether there 

 is as much useful manure in the land at the end of a 

 clean fallow, as at the time the vegetables clothing the 



