Subirrigation has been investigated at a number of the experiment stations, but has not become generally popular among market gardeners. The details of installment may vary greatly. The pipes are usually ordinary drain tile, which range in size from 2 1/2 to 4 inches. They may be only a few inches under the surface or 2 or 3 feet, depending upon climate and soil conditions and crops grown. Eight to 12 inches are proper distances for most garden soils, and the lines should be 10 to 15 feet apart. The tile are placed end to end as close together as possible and the end of the line farthest from the water main closed. A 3/4-inch stream of water should cause water to flow to the end of a 200-foot line of tile.
Theoretically, this is a fairly good system of watering. There is smaller loss by evaporation from the surface than with any other system; there is less baking of the soil and the least amount of tillage is required, and in addition, tillage is never delayed because of a wet surface. On the other hand, considerable outlay is required for tile, which must be laid, lifted and stored every year, unless placed below the frost line. Percolation is too rapid and capillary action too slow in most soils. It is doubtful if this system will ever be generally used.
The most extensive system of subirrigation in the United States is at Sanford, Fla. Large tracts of waste land have been brought under cultivation and utilized in growing lettuce, celery and other crops. The tile serve both for irrigation and for drainage. An impervious bed of hardpan lies from 2 to 4 feet below the surface and the sandy or loamy top soil is about 18 inches deep. The 4-inch water mains are made of sewer tile, cemented at the joints, the 3-inch laterals of drain tile are 20 to 25 feet apart and 12 to 15 inches below the surface. Sawdust, cinders or moss are placed over the joints to prevent filling with sand. A fall of at least 1 inch in 100 feet of tile is provided for drainage. The laterals run into open ditches dug for drainage purposes. Subirrigation is an ideal system for conditions as they exist at Sanford.
Fig. 15. Drilling Machine For The Installation Of The Skinner System Of Irrigation.