This has become far more general than outdoor sub-irrigation. The trenches are made water-tight by spreading cement over the slate bottoms and sides. In the bottom drain-tile are laid in which water circulates and slowly escapes at the joints and through the pores of the tiles. Some lay the tiles two feet apart and others much prefer laying them quite close together. Where the tiles do not fit well some cement is used to tighten the joints to prevent too much water escaping at one point. The water is raised by capillary attraction through the soil covering over the tiles. The benefits of the system arise from the regularity of moisture supply, and the surface does not harden or bake as in surface-watering.
Practical growers often start plants in shallow boxes and secure irrigation from below by setting the boxes in a shallow pan or vat with water in the bottom until moisture appears on the surface.