284 GARDEN FARMING 



usually rich garden soil, which can be modified by the use of sand 

 or clay to obtain 'the type desired, and enriched with composted 

 manure and chemicals. Such soils can remain in the houses for 

 many years, and in order to maintain maximum crop production 

 on them it is necessary not only to keep up fertility but to use the 

 utmost care to keep the soil free from harmful organisms. Many 

 of the most destructive greenhouse diseases develop in the soil, 

 and it has been demonstrated that they can be most successfully 

 controlled by sterilizing the soil with steam (see page 24). 



Temperature of greenhouses. Two methods of piping are chiefly 

 employed in large houses. One is to place the heating pipes about 

 1 8 inches above the surface of the beds in such a manner that the 

 posts supporting the roof can carry them. It is essential that the 

 pipes be given a uniform grade. It is customary to space them so 

 that they can be used as supports for boards upon which the work- 

 men lie in planting and cultivating the crop. The other method of 

 heating involves the use of manifolds to carry the required number 

 of pipes to provide the necessary radiation. These coils are sus- 

 pended well up on the side walls or division posts. If the house 

 exceeds 30 feet in width and is located in a region of rigorous 

 winter climate, additional coils are suspended on the posts support- 

 ing the ridge of the house. Because of the great length of these 

 houses, and the difficulty of securing a sufficiently rapid flow of water 

 to maintain the desired temperature with hot water, steam is gener- 

 ally employed in 'heating such structures. The temperature at which 

 lettuce develops best has been found to range from 40 to 50 F. at 

 night, and from 55 to 65 F. during the day. With the solid-bed 

 system of construction these air temperatures will give soil tempera- 

 tures somewhat lower than would be the case with raised benches. 



Watering in the greenhouse. Lettuce, because of its habit of 

 growth, is very susceptible to diseases, and, to overcome those which 

 are most annoying and most destructive, growers resort to special 

 methods of watering. The most satisfactory of these is subirriga- 

 tion, by which it is possible entirely to prevent water from coming 

 in contact with the foliage. When it is not practicable to use sub- 

 irrigation, as is often the case with solid benches, water is applied 

 from the open end of a hose carried close to the surface of the soil 

 and between the plants, so as to avoid wetting the foliage, especially 



