CRUICKSHANK'S PllACTICAL PLANTER. 323 



heights, or heen forced by man^, soil is seldom found 

 to exceed 6 feet in depth, and that only in warm 

 moist situations, propitious to vegetation. In Scot- 

 land we never have seen it exceed 3 or 4 feet in 

 depth where its accumulation had not been aided by 

 the above causes. The most common depth is from 

 6 inches to 2 feet ; but, in many of our sterile dis- 

 tricts, the surface hardly deserves the name of mould, 

 containing very little vegetable matter, or that mat- 

 ter being unavailable from the presence of tannin. 



It is a well known fact, that summer- fallowing al- 

 ways dissipates a portion of the vegetable matter in 

 the soil, although it may, at the same time, tend to 

 fertility, especially in adhesive soils, and where the 

 climate is not very arid and warm, overbalancing the 

 loss from dissipation by the advantage resulting from 

 aeration and absorption of gases and heat, and the 

 sun's rays ; by the mechanical disposition and com- 

 minution from being thoroughly dried and then moist- 

 ened ; and, probably, by the formation of salts, stimu- 



* Vegetable soil is sometimes buried deep under volcanic mud, 

 sand, and ashes, or mixed with the subsoil by earthquakes. In some 

 districts of South America, the country, from being fertile, has 

 been recently reduced to sterility, by the vegetable mould being 

 so much scattered through the subsoil by repeated upheavings and 

 tossings about by earthquakes, as to be out of the reach of plants. 



X 2 



