STEAM STERILIZATION OF SEED BEDS. 11 



minimum loss of heat by radiation. The pan is -set on one end of a 

 bed with its inlet nearest the boiler. The hose is attached to the pan 

 and to the pipe leading from the boiler. Soil is banked around the 

 edges of the pan to prevent the escape of the steam. The title-page 

 illustration shows the boiler and apparatus connected for operation. 



It is desirable to maintain a pressure in the boiler of 100 pounds, 

 and if the pressure drops below 70 pounds the steam should be 

 shut off, as this is the minimum for successful sterilization. Steam of 

 high pressure has much greater penetrating power than steam of low 

 pressure, and it is important to realize that the efficiency of the 

 operation is greatly increased by dry high-pressure steam. A little 

 experience in firing the boiler and operating the pan is necessary 

 before one .can maintain high pressure while supplying steam to the 

 pan. Experienced operators can hold 100 to 125 pounds pressure 

 for continuous operation 24 hours per day. The outlet valve should 

 be opened only part way, so that the pressure of the boiler can be 

 kept uniform and unnecessary blowing of the soil in front of the inlet 

 prevented. A great volume of steam is not so essential as great 

 penetrating power. 



After 30 minutes the steam is shut off and the pan moved along 

 the bed to the next area. One end of the pan should slightly over- 

 lap the area just steamed, thus leaving no strip unsterilized. Four 

 attendants, one man at each corner, are usually required to lift the 

 pan, using bars 'or ax handles inserted through the ring bolts. Im- 

 mediately after moving the pan, the steamed area is covered with a 

 canvas or burlap blanket. This is important, because the heat must 

 be conserved in the surface soil to allow it to reach the lower soil by 

 conduction. It is desirable that covers be supplied for the entire bed, 

 so that each area will be covered for several hours after steaming. 



Experienced operators have found it advantageous to use two pans 

 alternately, eliminating the necessity of moving the pan immediately 

 after the steam is shut off. The moving of the pan immediately 

 after operation is quite disagreeable to the workers because of the 

 volume of steam held under the pan. There is also a considerable 

 loss Of heat when the pan is moved before the heat has reached the 

 lower soil layers. 



TEMPERATURES SECURED. 



In sandy soils, after 30 minutes' steaming, the temperatures to be ex- 

 pected in the upper 2 inches of soil directly under the pan are ap- 

 proximately 208 to 212 F., at 3 to 4 inches 170 to 180, and at 6 

 inches 120. Two hours after the removal of the pan the temperature 

 at 6 inches should be about 160 F. If a thermometer is not available, 

 the efficiency of the steaming operation can be easily determined by 

 burying a potato 4 inches under the surface of the soil. The potato 

 should be well cooked when the pan is removed, and this is a common 



