46 Diseases of Greenhouse Crops 



case in steamed soils deficient in lime. The inves- 

 tigations of Schreiner and Lathrop* have shown 

 that as a result of heating, dehydroxystearic acid is 

 produced, and that this is harmful to plant growth. 

 Heating soil produces both beneficial and harmful 

 substances. The fertility is raised or lowered, de- 

 pending on which of these predominates. The re- 

 sult, however, is influenced by the crop, the fer- 

 tilizer used, and the amount of lime applied. Cole- 

 man t has found that intermittent sterilization by 

 means of dry heat at 82 degrees C. for five suc- 

 cessive days in moist soil produced but very slight 

 chemical changes. But this slow method is not very 

 popular with the grower. Since, however, sterilized 

 soils lose their harmful substances by standing, the 

 treatment of the soil during the summer months, 

 when there is no crop in the greenhouse, will obvi- 

 ate the main difficulty. 



Other Methods of Controlling Damping Off, 

 Damping off may be largely controlled by careful 

 cultural conditions. Unless the soil of the seed bed 

 has been sterilized, it is unwise to use the same 

 soil in the beds where damping off has occurred 

 previously. Thick sowing, too, should be avoided. 

 In Table 4, Johnson + presents some interesting data, 

 showing the effect of thick sowing on damping off. 



*Schreiner, O., and Lathrop, E. C, U. S. Dept. of Agr., Bur. 

 of Soils, Bui. 89: 7-37, 1912. 



t Coleman, D. A., et al., Soil Science: 259-274. 1916. 



t Johnson, James, V^isconsin Agr. Expt. Sta., Research Bui. 31: 

 31-61, 1914. 



