PHILOSOPHY OF DEEP DRAINAGE. 265 



mass of beautiful trees, shrubs, and plants that they will thrive 

 best, and bear the winter's cold and the summer's heat and drought 

 with least injury, in the most deeply drained soils. If this is true 

 as a general rule, it is plain that for trees which are peculiarly 

 sensitive to either extreme, there is greater need of deep drainage 

 than for any other. 



The airing of the soil, which deep draining secures, acts in two 

 ways for the benefit of all vegetation : first, by equalizing the tem- 

 perature of the soil in consequence of the non-conducting power of 

 air ; secondly, by exposing the deeper soil to the contact of air, it 

 becomes changed in character, and undergoes a constant process of 

 fertilization by the action of air upon it. It is being oxygenized. 

 Any one familiar with farming operations in new countries, knows 

 that when virgin soils are first turned over, there are, usually, only 

 a few inches of dark soil on the surface. If the plow turns a 

 furrow five or six inches deep, it will generally show a much lighter 

 color than the surface which is turned under ; but in a few years of 

 continued culture this lighter-colored soil becomes as dark as the 

 original surface. By the combined action of the sun and air it has 

 all become equally oxygenized. If such ground were repeatedly 

 plowed without growing a crop from it, and so as to permit no 

 growth of vegetation to be turned under, it would still, for a time, 

 gain rapidly in fertility, by the mere chemical changes produced by 

 the sun and air. What plowing effects quickly by the direct ex- 

 posure of the upper soil to these elements, deep draining and the 

 consequent airing of the soil effects slowly, and less thoroughly, in 

 subsoils through which the air is induced to permeate. Imper- 

 ceptibly ', but surely, the earth beneath our feet is being warmed and 

 fertilized by the action of the air upon it, whenever we invite the air in, 

 by drawing the water out. This increased warmth and richness of 

 the subsoil invites the roots of trees deeper and deeper in pro- 

 portion as it approximates in character to the warmth and oxygena- 

 tion of the surface-soil. To have a deeply aired soil, therefore, is 

 to encourage trees to root farther down, and away from the trying 

 changes of winter and spring temperature that weaken or kill semi- 

 tropical trees and shrubs, and often impair the vitality of young 

 trees of hardy species. 



