CHAPTER XVIII. 



THE PHILOSOPHY OF DEEP DRAINAGE AND CULTR'ATION IN THEIR 

 RELATION TO THE GROWTH OF TREES, AND TILE SUCCESSFUL CUL- 

 TURE OF THOSE WHICH ARE HALF-HARDY; TOGETHER WITH SUGGES- 

 TIONS FOR PROTECTING YOU'NG TREES IN WINTER AND SUMMER. 



A LARGE portion of the gross weight of all soils is water. 

 If we dry any soil perfectly, the residuum of weight will 

 bear a very small proportion to the average weight of 

 the soil in its natural condition. Water, therefore, occu- 

 pies a large part of the texture, of what we call solid earth. When 

 we draw the water from any soil by drains, the space occupied by 

 the water in the earth is supplied by air. Thorough draining, 

 therefore, airs the soil to whatever depth it drains off the water. 

 The air transmits heat and cold less rapidly than water by direct 

 conduction, so that, if air occupies the place of water in the inter- 

 stices of the soil, the latter will feel all changes of temperature 

 more slowly. Deep drainage, therefore, tends to equalize the tem- 

 perature of the earth's surface, and to neutralize the effect of great 

 and sudden changes in the air above. It is impossible to drain a 

 subsoil too thoroughly from beneath, because the capillary attrac- 

 tion of the earth is always sufficient to draw up from below all the 

 moisture that is essential to most forms of vegetable life ; and in 

 addition to the moisture thus drawn from below, the earth, when 

 the air can circulate freely in it, has the power when dry to absorb 

 a vast amount of moisture from the air, as well as to yield it up to 

 the air by evaporation when it holds an excess. To all general 

 observations like these, the reader's intelligence will of course 

 suggest exceptions ; as of trees and plants which thrive best where 

 their roots are immersed in water, and which make water their 

 element rather than earth ; but the fact holds good as to the great 



