144 REPEATED TRENCHING. 



the elements of nature more subservient to the pur- 

 poses of vegetation, is permanent, and cannot wear 

 out or lose its effect, as that of manuring, at what- 

 ever expence, must certainly do. 



But though permanent in this respect, it is not 

 to be inferred that there is no further need of sub- 

 sequent trenching. A repetition of this work, at 

 any future period, gives the great benefit of rest to 

 that part of the soil which has been exhausted by 

 continual bearing. We are aware that some theo- 

 rists decry the notion of exhaustion, and contend 

 that nothing more is needful to a vigorous growth 

 than the proper supplies of heat and moisture 

 inferring, at the same time, that all manures are 

 serviceable only in so far as they give the land an 

 aptitude for the retaining of moisture and heat. 

 But whilst they bury thermometers and hygro- 

 meters at various depths, for the purpose of expe- 

 riments, they overlook those phenomena which 

 take place above ground, and which are sufficient 

 to establish the fact, that by repose the soil is 

 strengthened for the labour of future production. 

 Hence the well ascertained benefit of a succession 

 of crops; hence the law, that when an old forest 

 dies out, and nature is left to herself, trees of the 

 same kind do not spring up in room of the decayed; 

 and hence the fact now becomes appalling to the 

 husbandman, that in many places where it has been 

 too often sown on the same ground, though heat 

 and moisture be in all respects the same as in for- 

 mer times, red clover almost refuses to grow. 



