2 2 CULTURE METHODS AND TECHNIQUE 



this should be repeated if possible the next day. This method is 

 available when there is only a small number of pots to be handled. 

 When, however, the work must be conducted on a larger scale 

 an effective apparatus is the Britton soil sterilizer. Britton has 

 described 1 a steam sterilizer for soils which he has devised for use 

 in the station greenhouses. This apparatus is simple and seems to 

 be wholly practicable. It is described as follows : 



It consists of a square box made of heavy galvanized sheet-iron connected 

 with the steam-heating system in the potting room of the forcing-house (or 

 elsewhere convenient). This box is cubical in form, each of its three dimen- 

 sions being thirty inches ; six inches of the top is in the shape of a removable 

 cover. Steam enters through a hole in the center of one side, to which side is 

 soldered a coupling. A three-fourths inch pipe, fitted with a valve, connects 

 the apparatus with the steam-heating system. A few strips of wood placed 

 under the box raise it a half inch from the cement floor to prevent rusting. 

 Inside the metal box are similar strips upon which the trays rest. Two small 

 holes in the bottom allow the condensed water to escape. 



The soil is placed in the trays which are made of wooden frames having 

 bottoms of galvanized wire netting. The frames are made of strips of wood 

 three and one-half inches wide and seven-eighths of an inch thick ; after fasten- 

 ing the netting, a half-inch strip is nailed on to hold the netting firmly and to 

 cover its jagged edges. The dimensions of the trays in inches are 27 x 13x4 

 over all, and inside are 25 1 x 11J x 3! inches. 



The wire netting has six meshes to the inch. Soil is spread loosely and 

 evenly in the trays to the depth of about three inches and the trays packed 

 inside the metal box in cob-house fashion. . . . 



There is a space of one and one-half inches all around the trays inside the 

 box, and a space of an inch between the two trays. The half-inch strips on the 

 bottom edges of the trays allow the steam to enter above and below the coil 

 in each of the trays. As the steam comes in contact with the soil, both above 

 and below it, much less time is required to heat it than when in a solid mass. 

 The sterilizer contains fourteen trays, which, when filled to the depth of three 

 inches, hold 6.9 cubic feet of soil. . . . Steam entering through a three-fourths 

 inch pipe at a pressure of five pounds per square inch, heats the soil to the 

 boiling point of water in about fifty minutes — a great deal depending, of 

 course, on the density of the soil, as a loose soil heats through much more 

 rapidly than if packed closely. The box is not steam tight, but nearly so for a 

 low pressure ; considerable expense would be necessary to make it perfectly 

 steam tight and at the same time permit of convenience in opening the box. 



Soil was kept in the apparatus one hour for the purpose of killing the nem- 

 atodes. This also doubtless destroyed many fungous germs, but where absolute 



1 Britton W. E. A Steam Sterilizer for Soils. Conn. (New Haven) Agl. Exp. 

 Sta. Report (1897): 310-312. 



