88 STEAMING THE SOIL 



soil would be worthless. What actually is done is to 

 partially sterilize the soil by killing off some of the micro- 

 organisms that are in it, but it has been found that certain 

 forms of protozoa that are inimical to the nitrifying bac- 

 teria are killed off in far greater numbers than these latter, 

 which increase again with great rapidity after the sterilizing 

 process, and make the soil rich again quickly. There 

 are two chief methods: (i), by the use of steam under 

 pressure; (2) by watering the soil with the volatile hquid 

 called formaldehyde (formalin). 



The perforated pipe method of steaming consists of a 

 system or set of perforated pipes, connected and buried in 

 the soil of the bed, the surface then being covered with 

 canvas or other covering and the steam passed into the 

 system for such a period as is required to heat the soil to 

 the necessary temperature. This temperature for best 

 results is 180 deg. to 212 deg. Fahr., maintained for a 

 period of an hour or more. The time required to reach 

 this temperature will vary with the boiler area, the pressure 

 and other steam and soil factors. The length of pipes of 

 the system will be adapted to the beds, being one-half or 

 one-third the total length of large beds. Generally, i in. 

 pipe is used with 3^ in. holes bored in a straight line about i 

 ft. apart. These pipes are buried in the beds 12 in. to 16 

 in. apart. The crossheads and main to boiler should be 

 2 in. pipe or larger. 



In burying the pipes it is well to see that they lie level 

 in the bed so that condensed steam does not accumulate 

 at any one point; the pipes should be buried to a depth 

 of 4 in. to 6 in., and evenly covered over with the soil. 

 Canvas, burlap or carpet may be spread over the surface 

 to be sterilized, thus preventing the ready escape of steam. 



In practice it is found that to make the best use of 



