LAND-DRAINAGE 215 



the water from the watering-can will not flow 

 through, nor will there be any air admitted from 

 below, and for lack of this the roots will decay. On 

 the other hand, if the water, after saturating the 

 earth, runs out freely by the hole in the bottom of 

 the pot, the damp soil will become a sort of sponge 

 to which the air will have access from all sides, 

 and the plant will thrive. 



"This reasoning applies to the most extensive 

 agricultural operations as well as to the care of a 

 potted plant. After water has soaked into the 

 ground it should find some channel to carry it off; 

 otherwise the roots will decay for want of air. That 

 is why clayey soils, which retain water when they 

 are once saturated, are unsuited to agriculture, while 

 light soils, having sand mixed with the clay and 

 thus readily allowing the water to drain off, are well 

 adapted to it. For the same- reason, again, a sandy 

 subsoil accelerates vegetation, and a clayey subsoil 

 retards it. A sandy subsoil offers the same advan- 

 tage as a flower-pot open at the bottom, whereas a 

 clayey subsoil is like a flower-pot closed at the bot- 

 tom. In the first case the surplus of water drains 

 off and the air has free access; in the second the 

 superabundant moisture finds no outlet and the air 

 cannot reach the roots. 



"Now let us suppose we have a marshy soil to 

 deal with. Because of the stagnant water either on 

 the surface or a little below it nothing can grow on 

 this piece of ground except rushes or other hardy 

 plants designed by nature for this kind of soil. Ac- 



