Soil Disinfection 461 



The seed bed should be covered with several thicknesses of 

 old burlap or blankets to confine the heat to the soil. The 

 steam should be applied at a pressure of from 80 to 150 

 pounds, since at high pressure the soil does not become so 

 wet as when low-pressure steam is used. A treatment of from 

 one to two hours is usually sufficient to disinfect thoroughly 

 the soil to a depth of 18 inches. A few potatoes laid in the 

 surfaces oil will indicate the thoroughness of the treatment 

 by the degree to which they are cooked. The blankets may 

 advantageously be left on for some time. 



While this method offers some advantages for seed beds 

 of limited area, in that the pipes may be left in the ground 

 and used year after year with little extra labor and may 

 also be used for subirrigation, the initial cost of installa- 

 tion, especially on large seed-bed areas, may be prohibitive. 



Inverted-pan method. — The method which has given 

 the best results in practice, and which because of its simplic- 

 ity and small cost recommends itself for use on large or 

 small areas, is the invention of Mr. A. D. Shamel, of the 

 U. S. Dept. Agr. Bur. of Plant Industry, and was devised 

 by him to disinfect nematode-infested soils in Florida. The 

 apparatus consists of a galvanized iron pan, 6' x 10' x 6" 

 which is inverted over the soil and the steam admitted under 

 pressure. The pan is supplied with steam-hose connections, 

 has sharp edges, which are forced into the soil on all sides to 

 prevent the escape of steam, and is fitted with handles for 

 moving it from place to place, the weight of the entire pan 

 bemg not over 400 pounds. 



The soil is prepared as in the greenhouse method, a few 

 potatoes being buried at a depth of a foot to gauge the 

 degree of heat attained, or a soil thermometer may be used if 

 desired. The steam should be kept at as high pressure as 

 possible, from 80 to 150 pounds being best, and the treatment 

 should continue from 30 to 60 minutes, depending upon the 

 pressure maintained and the character of the soil. Wet soils 

 require more time than dry soils. In experiments conducted 

 on tobacco seed beds in the spring of 1907, one hour's steam- 



