334 APPLICATION OF PRINCIPLES. 



scarcely bear any manure, and the Peach is often 

 greatly injured by excess of it in a solid state : yet 

 this same plant will bear a very considerable quantity 

 in a liquid form. 



The application of soils-and manures to plants must, 

 therefore, remain at present exclusively within the 

 domain of art. There are, however, some general 

 remarks which it is possible to offer with tolerable 

 confidence. 



Soil, considered without reference to the organisable 

 substances it contains, appears to act upon plants 

 chiefly by its power of absorbing and parting with 

 heat and moisture. When soil is tenacious, or plastic, 

 it absorbs heat slowly, and it parts with its water with 

 great difficulty, as is the case in the London clay ; 

 the number of cultivated plants to which this is suit- 

 able is so small that it is almost expelled from gar- 

 dens, where the object is to expose the cultivated 

 species to conditions more favourable than those 

 afforded them by nature. The small amount of bot- 

 tom heat afforded by clay, and the impossibility of 

 effectually draining it, sufficiently explain the badness 

 of its quality for gardening purposes, even without 

 taking into account the difficulty experienced by 

 plants in rooting in it, from the resistance afforded to 

 the passage of the spongioles by so compact a sub- 

 stance. On the other hand, loose sand, whose par- 

 ticles have no cohesion, although it imbibes water with 

 great facility, parts with it as readily, and, being easily 

 heated by the sun's rays, becomes so soon dried up 

 as to be for that reason as unsuitable to most plants 



