88 VEGETABLE FORCING 



at a temperature of 212 degrees, that there is considerable difference 

 in the yield of the crop. It has been found that the same quantity 

 of soil, after the action of heat, yields a crop many times in excess 

 of the former or untreated soil. In other words, by heating we con- 

 vert the humus compounds in the soil into a more available form 

 for the utilization of the plant. That heating of the soil gives rise 

 to some changes is shown by its darker color and more porous con- 

 dition, and it is undoubtedly due to these changes which have taken 

 place in the humus compounds, which account for the accelerated 

 and vigorous growth of the plants. 



"Another feature which is characteristic of sterilized soils is the 

 unusual occurrence of humus-loving plants, or saprophytes, that grow 

 upon it, which is a good indication that the organic matter contained 

 in the soil has undergone changes through the action of the heat. 

 We have ourselves observed more than once certain species of 

 saprophytic fungi growing upon our steamed beds which have never 

 shown any tendency to grow in unheated soil, although with the 

 exception of being steamed the soil was exactly the same as that 

 upon which they never appeared." 



Evil results sometimes follow the use of steam, prob- 

 ably because of injurious effects upon the physical 

 properties of the soil, especially when the soil has not 

 been properly handled after sterilization. All things 

 considered, steaming is the most complete, effectual 

 and practical method of soil sterilization. 



Formalin, however, has a useful place in the manage- 

 ment of many greenhouses. While the usual strengths 

 have little effect upon the animal life of the soil and do 

 not destroy nematode eggs, many of the diseases may be 

 controlled by the use of this disinfectant. Small areas 

 of soil sometimes show infestation at midwinter, and 

 they may be drenched with formalin when it would not 

 be practicable to use steam. Again, there are hundreds 

 of small houses heated by flues or hot water where steam 

 is not available and formalin can be used to advantage. 

 Its use is not so harmful to silty and clay soils, the 

 structure of which is often injured by steaming. 



