CHAPTER IX 



DRAINAGE 



It has been shown in the preceding pages, that rain, apart 

 from its action in dissolving the soluble substances of the 

 soil, has a markedly beneficial effect in other ways. In the 



The effect of first place it washes down the carbonic acid, ammonia and 

 nitric acid of the atmosphere, and brings them to the roots 

 of the plants. In the second place, the passage of the rain 

 through the soil causes air to follow it, so as to fill up the 

 vacant spaces left by the water as it descends to the sub-soil. 

 A porous soil contains httle spaces and channels between 



The soil is the earthy particles, for the soil is not properly solid, but 



not sohd. consists of innumerable little pieces of wasted rock which do 

 not fit into each other, but only touch at certain parts, 

 thereby leaving open spaces in all directions. Now, when 

 the soil is dry, these spaces are filled with air ; and, when 

 much rain falls, they become filled with water. But, as the 



Porous soil, water drains down into the lower layers of the soil, fresh air 

 is drawn in again — for the spaces cannot exist with nothing 

 in them. This explains what is meant by Xhe porosity of a 

 soil, and it shows how advantageous it is for the planter to 

 change a heavy, compact, and sodden clay soil into a 

 porous one. 



Another physical property of soil is that known as 

 capillarity, and which has been considered in Chapter III. 

 Capillarity, or capillary attraction, is due to the same cause 

 as porosity ; but, in the case of capillarity, the particles of 



