LECT. V.] THE ROOT. SOILS. 233 



soil was first pointed out by Jethro Tull, in 1733 ; 

 but although his ideas on this subject extended to 

 an absurd degree, and led him to form a theory of 

 vegetation altogether mechanical, yet the direction 

 of the agriculturist to the importance of pulve- 

 rization, has been productive of the most beneficial 

 results. It allows of the easy extension of the 

 roots of plants, admits a necessary supply of air 

 during the process of germination, and assists 

 those decompositions which are requisite for ren- 

 dering manure useful. 



The first place among the substances fit to 

 answer the purposes already specified, is certainly 

 due to lime. This substance acts upon soils either 

 mechanically or chymically; and on the plants 

 it acts physiologically. When in the state of a car- 

 bonate, or united with carbonic acid, it is added to 

 clayey soils, it acts mechanically by rendering 

 them more free, loose, and pervious both to air, 

 moisture, and the roots of plants : it acts chymi- 

 cally when it is deprived of carbonic acid or is in 

 the caustic state by destroying worms, and other 

 insects hurtful to young vegetables; and, by quick- 

 ening the decomposition of their dead bodies, ren- 

 ders them useful to vegetation. In either state 

 it neutralizes acids, and decomposes salts of iron 

 and other injurious saline matters often contained 

 in soils ; and by the healthy stimulus which it af- 

 fords when in the state of quicklime it invigorates 



