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pointed out. By following sriich guides, we may often learn 

 so to alter the constituent parts of soils, as to increase their 

 fertility, by the addition of ingredients, in which they are 

 deficient, and in some cases, by the subtraction of such as 

 too copiously abound in them ; or else, by effecting chemical 

 changes of some constituent part by incineration, or by the 

 application of mineral manures. Soils, considered as agents 

 of vegetable culture, are subjected to operations, which effect 

 changes on them, either mechanically or chemically. Of 

 the former description there are none so important for the 

 use of trees, as deepening and pulverizing. Deepening can 

 be executed with eflfect, only by trenching or double-digging 

 (for the plough can do little in such a business), and pulver- 

 izing is naturally combined with that process. The depth 

 of pulverization, as Sir H. Davy well observes, must depend 

 on the nature of the soil and subsoil. In rich clayey soils, 

 it can scarcely be too deep ; and even in sands, unless the 

 subsoil contain some principle noxious to vegetables, deep 

 comminution should be practised. When the roots are deep, 

 they are less liable to be injured by excessive rain, or exces- 

 sive drought, and the radicles are shot forth into every part 

 of the soil.* In a word, nothing but water stagnating under 

 the trench, in consequence of a clayey bottom, and the risk 

 of the roots being thereby chilled, should prevent trenching 

 from being always executed as deep as possible. 



The surprising changes worked on all soils, in consequence 

 of a minute comminution of their parts, and the various 

 ways in which it increases fertility, have only of late years 

 been communicated to agriculture, by chemical analysis and 

 investigation ; so that there is the less wonder that they 

 should, in a great measure, have escaped the planter's notice. 

 If the process be important in general to woody plants, it 

 must be greatly more important to subjects meant for removal ; 



* Elements of Agricul. Chemist. 



