364 GLASS STRUCTURES AND GREENHOUSE CROPS 



iron type. Such greenhouses are built with concrete 

 walls and with the interior braces and supports of iron pipes. 

 The roof bars and other wooden parts, if made of cypress 

 and painted every year, will last for some time and may 

 then be replaced without a large expense. The construction 

 should be such as to provide a maximum amount of light. 



A greenhouse should have ample provisions for thorough 

 ventilation. Ventilators are placed at one or both sides 

 of the ridge and frequently extra ventilators are placed 

 along the sides of the house. It is the aim to place the 

 ventilators in such a manner as to prevent cold drafts from 

 striking the plants and to provide ample ventilation dur- 

 ing all kinds of weather. 



Greenhouses are heated by steam or hot water. The 

 larger greenhouses are generally heated by steam. Hot 

 water is preferred for heating small houses, as the pipes re- 

 tain heat for a greater length of time and the boiler may be 

 left longer without attention. 



Soils. A well-drained soil of good texture containing 

 an ample supply of plant food is required for greenhouse 

 culture. Usually a sandy loam is preferred, although muck 

 soils are frequently used. Where large quantities of soil 

 are required, a suitable garden area is selected in the spring 

 and given a heavy coat of stable manure. The land is 

 then plowed and afterwards harrowed repeatedly during 

 the summer. By fall, it is in excellent condition for use. 

 If a smaller quantity of soil is required, it may be pre- 

 pared by composting. A good blue grass sod is selected 

 and stacked in alternate layers with manure. It is then 

 left to decompose. After standing for about a year, the 

 pile is shoveled over two or three times at intervals of a 

 few weeks, and is then ready to use. 



