SYSTEMS OF CROPPING 383 



siderable extent. The fundamental principles involved 

 are practically the same as for double cropping in the 

 open ground, but attention should be called to a few addi- 

 tional factors, as follows : 



(1) Land values under glass are much higher than out 

 of doors, and this is an argument for intensifying opera- 

 tions by intercropping. 



(2) Steam sterilization destroys weed seeds and the 

 soils used are usually light, so that cultivation, hoeing 

 and weeding are not so necessary, and close planting, 

 therefore, is less objectionable from this standpoint than 

 in the open ground. 



(3) A higher standard of quality is required for green- 

 house products, and companion cropping may be the 

 means of lowering the quality, as when radishes and 

 lettuce are crowded and shaded by larger plants. 



(4) Soil adaptation must be carefully considered. It 

 is a mistake to attempt to grow two crops together 

 in the greenhouse unless both are well adapted to the 

 soil. 



(5) The question of temperature is the most important 

 factor, and often the most serious handicap. A common 

 practice is to plant cucumbers in beds of lettuce. Al- 

 though the plan is successful to a greater or lesser extent, 

 one or both crops usually suffer because of unsuitable 

 temperatures. This condition is so serious in the opin- 

 ion of some growers that they never attempt to grow 

 crops together which have different temperature require- 

 ments. 



(6) The manurial requirements of each crop should also 

 receive consideration. This problem, however, is not 

 serious, because a soil that contains sufficient manure to 

 produce a good crop of lettuce will also be satisfactory 

 for tomatoes and cucumbers. 



(7) The size of each kind of plants at the time of set- 

 ting should be given the closest attention. For example, 



