THE SOIL AS A FACTOR 25 



the pipes, and thus interfere with the emission of the steam. If, 

 however, they are on the underside of the pipe, they are not likely 

 to become clogged or packed with soil from the outside, and their 

 efficiency is as great as if they were on the upper side. The length 

 of time that the soil should be left in the boxes will depend upon 

 the character of the soil, the steam pressure, and the tightness of the 



FIG. 3. Shamel soil sterilizer 

 i, inverted metal pan ; 2, steam boiler 



box. The sterilizing box should be fitted with a tight lid so as to 

 hold the steam in the box. A common test used for determining the 

 length of time required for sterilization is to place a potato about 

 three inches below the surface of the soil in the sterilizing box. 

 When the soil has been heated long enough to cook the potato 

 thoroughly, it is considered safe to place it in the greenhouse bench. 

 For use in seed beds, and greenhouses with solid beds, another type 

 of sterilizer, known as the Shamel sterilizer, shown in figure 3, is 



