132 EXPERIMENTS WITH PLANTS 



Compare the water -holding capacity of sand, clay and 

 humus (leaf-mold). 



Air reaches the roots of potted plants not only 

 from above but also through the sides of the pot. 

 How permeable the latter is to air may be tested by 

 cementing a piece of it to a glass tube and proceed- 

 ing as in the experiment shown in Fig. 28, or by sim- 

 ply placing a piece in an air-pump (see page 187) and 

 exhausting. The outside of a pot should be washed 

 occasionally to keep it clean and porous, so that air 

 may enter it freely. 



To retain in good condition the soil in which potted 

 plants are grown, they should not be watered too often, 

 but, when water is given, it should be applied copi- 

 ously (preferably by submerging the entire pot in a 

 pail of water) . A good rule is to water about once a 

 week in this way and to delay watering in any case 

 until the pot sounds hollow on being tapped. The 

 frequent application of a little water merely wets the 

 top layer of soil, leaving the bottom dry. On the 

 other hand, too much water deprives the roots of 

 air and causes them to decay. Devise an experi- 

 ment to determine in the case of some house plants 

 how much water should be given. 



How may the soil moisture be regulated ? We may 

 answer this question by reviewing what has already 

 been said. (1) Moisture may be decreased by under- 

 ground drains or by plowing land without subsequently 



