32 Diseases of Greenhouse Crops 



are sodium chloride or table salt, calcium sulphate 

 or gypsum, sodium sulphate, magnesium sulphate or 

 epsom salt. In addition to these may be found 

 salts of potassium. 



Methods of Control. Generally speaking the al- 

 kali problem is not serious in greenhouse culture. 

 The alkali soil when mixed at the compost heap 

 generally loses much of its salts due to the action 

 of the manure used. However, in alkali regions 

 alkali soils should be avoided as much as possible. 

 When this is not possible the soil to be used with 

 the compost should be spread out and exposed to 

 the action of winter weather conditions and to the 

 washing by rain. This treatment will result in a 

 loss of the injurious salts through leaching. 



Soil Sterilization 



Damping off, whether induced by Pythium, Rhi- 

 zoctonia or any other parasitic organism, is usually 

 confined to seedlings in the seed bed under cover or 

 in the open. The loss of seedling not only means 

 a waste of seeds, but it also results in poor stands. 

 The disease-producing-organisms are usually brought 

 in with the manure and the compost. Most growers 

 are in the habit of using the same soil in the seed 

 bed or in the greenhouse year in and year out. A 

 number make it a practice to empty their beds and 

 use fresh soil every year. This, however, is too 

 expensive and, moreover, is not always a safe 

 method, for the new soil, too, may be contaminated, 

 or may become infected as soon as it is placed in the 



