34 Diseases of Greenhouse Crops 



ered with heavy burlap to retain the foronaldehyde 

 fumes for a day or two, and then aired for a week 

 before planting. Stirring the soil at frequent inter- 

 vals after uncovering hastens the more rapid escape 

 of the formaldehyde fumes. 



Sterilizing Soils with Steam 



Steam sterilization of soils is by far the best 

 method. There are four ways of steaming soils: 

 (l) Inverted pan method, (2) the perforated pipe 

 system, (3) the steam rake device, (4) the drain 

 tile method. The choice of any one of these methods 

 is a matter of expediency. All four methods have 

 been successfully used on a commercial scale. 



The Inverted Pan Method, This was first devised 

 by A. D. Shamel of the U. S. Department of Agri- 

 culture. To carry it out, the boiler must maintain 

 a pressure of not less than 80 pounds, for at least 

 one and a half hours. In setting a pan, the rim is 

 sunk into the soil of the seed bed or bench, to a 

 depth of two to three inches, to make the inclosed 

 chamber steam tight. In heavy soil, trenching may 

 be necessary. It is also advisable to put a heavy 

 weight on the pan when the steam operates. For 

 one pan, a traction engine or a portable boiler of 

 ten to twelve H. P. will suffice. While the standard 

 of the pan is six by eight feet, the dimensions may 

 be modified to suit the seed beds or greenhouse 

 benches. 



