BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE. 



least 80 pounds into the inverted pan. While the pressure in the 

 pan itself may be considerably less than a pound, the advantage in 

 high pressure lies in the fact that dry steam is preferable to wet 

 steam and in that through a small connecting hose more steam can be 

 supplied to the pan per minute. The ordinary steam traction en- 

 gine, when obtainable, is especially suitable for this work. 



The pan is preferably made of Xo. 16 galvanized iron reinforced 

 with angle iron, although other materials ma} T be used. Its size, 

 shape, and construction may be made to fit the conditions at hand. 

 The pan may ordinarily vary from 50 to 100 square feet in area, 6 

 by 12 feet by 8 inches in height being common dimensions, although 

 pans as long as 14 and 16 feet are used. Handles, usually of iron 

 pipe, are ordinarily attached. The lower edge of the pan should be 

 sufficiently sharp so that it may be pressed readily into loose soil. 

 The inlet for the. steam may be made through the top or side of the 



Fig. 



-Steaming tobacco seed beds. This method is now widely used to prevent 

 plant-bed diseases, and it is also beneficial in other ways. 



pan, but it should be so placed that the steam is not thrown directly 

 into the soil. 



Soil steaming is usually done in the spring, some time before sow- 

 ing the seed, although fall steaming is sometimes practiced with 

 good results, especially where some protection can be given to the 

 beds to prevent reinfestation with parasites and weed seeds. 



The seed beds should be made practically ready for sowing before 

 sterilization. The soil should be fairly loose, not too dry nor too 

 wet, when steamed. The length of time required to sterilize the soil 

 will depend largely upon its condition, especially with reference to 

 moisture content and compactness, but ordinarily 30 minutes of 

 steaming suffices. The cost of steaming is usually between 1 and 1J 

 cciils per square foot. 



Surface firing consists simply in burning brush or other material 

 on the surface of the seed bed for a period of 30 to 60 minutes. The 



