WATERING, HEATING, VENTILATING AND SHADING 157 
(9) According to growers who have had considerable 
experience with this system, over-watering is impossible 
when water is applied through tiles laid in beds that are 
not water-tight. This unquestionably is one of the great- 
est advantages of sub-irrigation. 
(10) Larger yields are often obtained with sub-irriga- 
tion. 
This method of watering, as seen in Fig. 53, may be 
used on raised benches as well as in beds on the ground. 
It is necessary, of course, for the benches to be water- 
Fig. 53.—Tile laid in bed for sub-irrigation. 
tight, which involves an additional expense that must be 
charged to the cost of installing the system. Inasmuch 
as benches are not generally used in extensive vegetable- 
forcing establishments, it is seldom that we find benches 
constructed for sub-irrigation. Aside from the expense, 
it is a simple matter to make reinforced benches with 
concrete bottoms and sides which will be entirely water- 
tight. Such beds should be not less than 6 inches deep. 
B. H. Thorne, who had 12 years of experience in the 
use of this system, claimed that it is not desirable to have 
the ground beds water-tight, because there is then no 
danger of over-watering. There are two main points to 
