Moisture and Water Requirements 85 



necessary. If the subsoil is a stiff clay it may be de- 

 sirable to spread an inch of gravel. After thor- 

 oughly ramming it is covered with a thin layer of 

 the cement as described above. 



Overhead Irrigation 



This method is in greater use than any other prac- 

 tice of greenhouse watering. The advantages claimed 

 for it are the cheapness of installation and the more 

 uniform way in which the water is applied. By this 

 method, too, the dry atmosphere of the house can be 

 quickly changed. This is especially desirable during 

 the hot summer days. The disadvantages of this 

 system are the packing of the surface soil and the en- 

 couragement of disease through the excessive mois- 

 ture applied to the plant. 



Ventilation 



Next to watering, ventilation is of utmost im- 

 portance from the health viewpoint. Many of the 

 plant diseases which are confined to the greenhouse 

 are encouraged by improper ventilation. The lack 

 of it is as harmful as an excess. The practical grower 

 will give this careful thought and consideration. 

 As a rule, plenty of ventilation should be given 

 whenever weather conditions permit it, avoiding, 

 however, draft and strong air currents. 



