STERILIZING 263 



Rotation of crops. — One of the most effective and practical means of 

 heading off fungous diseases. Especially applicable to diseases oi 

 roots or root-crops, but also to many other diseases of annual 

 plants. 



Sterilizing by steam. — An effective fungicidal practice for several soil-in- 

 habiting organisms which attack roots and stems. This includes 

 nematode worms. It is especially applicable in the greenhouse, 

 where it may be applied (a) through sub-irrigation tile or through 

 specially laid perforated steam pipes in the bottom of the 

 bed. Cover the beds with blankets, introduce steam under pres- 

 sure of 40 to 80 pounds for two hours. Insert thermometers at 

 various places to see that the soil is being uniformly heated. (6) 

 A large galvanized iron tight box may be constructed with finely 

 perforated trays 4 to 6 inches in depth. Soil placed in these 

 trays and steamed for two hours as above will be freed from par- 

 asitic organisms. In this case the frames should be sprayed with 

 a solution of formalin, 1 pint in 10 gallons of water. 



Steam sterilization of soil may be used on intensively cultivated 

 areas or extensive seed-beds, A portable boiler is necessary. 

 The beds are sterilized after they have been prepared for seed, 

 and just before the seed is sown. A galvanized pan 10 bj' 6 feet 

 and 6 inches deep is inverted, and the edges are pushed down 

 into the soil one or two inches. The pan is connected with the 

 steam boiler by means of a steam hose and live steam is run into 

 the pan for about forty minutes under a pressure of 100 pounds 

 and up. The higher the pressure the more thoroughly the soil 

 will be sterilized. 



The cost of sterilizing is approximately three-fourths of a cent 

 the square foot. It should be noted that soil sterilization has an 

 invigorating effect on the plants, and it will be necessary to run 

 greenhouses at a lower temperature (5°-10°) both night and day. 

 Field sterilization also kills weed seeds, and with the reduction of 

 the cost of weeding makes the process practicable. 



Substances 



Bordeaux mixture. — A bluish-green copper compound that settles 

 out when freshly slaked lime and a solution of copper sulfate (blue- 



