SEEDLING DISEASES AND INJURIES 7 



The active substance in the formahn solution is Hberated as a 

 gas (formaldehyde). The cover is necessary in order to retain 

 this gas in the soil for a period sufficient to kill the pathogenes. 



Three or four days after the cover is removed, the soil should 

 be thoroughly forked and allowed to stand in a loosened con- 

 dition for another day or two, after which the bed may be 

 prepared for sowing the seed. It is important to time the 

 application of the formalin so that the seed may be sown as 

 soon as the operations above described are completed, since 

 the soil may become contaminated again from surrounding 

 soil. Experience shows that beds contaminated after dis- 

 infection may exhibit greater loss than those not disinfected. 

 The increased virulence of damping-ofi fungi in disinfected 

 beds is thought to be due to the lack of competition with 

 other soil organisms which have been killed by the disin- 

 fectant. With ordinary care, however, under most condi- 

 tions a clean crop of seedlings is assured if the disinfection 

 is thorough. Even after carefully disinfecting the soil, all 

 the measures advised above under Protection should be ob- 

 served (see page 5). 



The application of stilfuric acid to the soil at the time of 

 seeding has given good results in controlling damping-off in 

 coniferous seed-beds. Sulfuric acid should never be used on 

 deciduous seedlings. The amount of acid used with safety 

 will necessarily vary with the natural acidity or alkalinity of 

 the soil. A too heavy application of sulfuric acid will cause 

 injury to the seedlings. This method is more difficult to handle 

 than the formalin treatment because in loose sandy soils the 

 capillary movement of the water will bring the acid to the 

 surface and produce there a concentrated solution, which must 

 be counteracted by daily watering. With heavier soils, no wa- 

 tering seems necessary from the experiments so far reported. 

 With the two uncertain factors in mind, the natural acidity 

 or alkalinity and the physical nature of the soil to be treated, 



