Sub-irrigation, also known as subsurface irrigation, delivers moisture directly to the root zone from beneath the soil surface. This technique shifts from traditional surface watering, where water is applied from above and moves downward by gravity. Sub-irrigation allows the growing medium to draw moisture upwards, providing a regulated and consistent water supply. It is used across various scales, from container gardens to large commercial fields.
How Water Reaches the Roots
Sub-irrigation functions using capillary action, or capillarity. This physical phenomenon moves water upward, working against gravity. Capillary action relies on two forces: adhesion, the attraction between water molecules and soil particles, and cohesion, the attraction between the water molecules themselves.
Soil or growing media is composed of a network of tiny pores, which act as miniature tubes. When water is introduced below, adhesive forces cause water molecules to cling to the inner walls of these pores. Cohesive forces then pull neighboring water molecules along, causing the water column to rise into the drier soil layers above the source.
The height water is lifted depends on soil texture; finer soils with smaller pores exhibit a greater capillary rise than coarse, sandy soils. Plant roots absorb water from this continuously moist layer created by the upward movement. This passive delivery replenishes the water supply as the plant uses it, maintaining optimal moisture without the saturation or drought periods common with top-down watering.
Common Sub Irrigation Systems
Sub-irrigation is implemented through various structures that place the water source below the growth medium. A common example is the wicking bed or self-watering container, popular in home gardening. These systems use an integrated water reservoir at the base of the planter, separated from the soil by a porous layer or wicking material that facilitates capillary movement.
For large-scale agriculture, subsurface drip irrigation (SDI) is frequently employed. SDI involves burying plastic drip lines or tapes, fitted with emitters, a few centimeters below the soil surface. Water is delivered directly to the root zone, where it moves outward and slightly upward, drastically reducing surface evaporation losses.
In regions with naturally high water tables, sub-irrigation is achieved through water table control, sometimes called seepage irrigation. This large-scale method uses an extensive network of buried perforated pipes or drainage tiles. By regulating the water level within these pipes, farmers can raise or lower the groundwater table, controlling the upward capillary movement of water into the crop’s root zone.
Practical Outcomes of Using Sub Irrigation
A primary advantage of sub-irrigation is substantial water conservation achieved by eliminating surface evaporation. Since the water source is below the soil, less moisture is lost to the air compared to overhead sprinklers, which can lose up to 40% of applied water. This method also allows for efficient nutrient delivery, as fertilizers can be dissolved directly into the water source in a process called fertigation.
Another benefit is the reduction of foliage diseases, such as powdery mildew, because plant leaves remain dry throughout the watering process. Delivering water directly to the roots minimizes the risk of fungal and bacterial pathogens that thrive on wet leaves. Furthermore, the relatively dry soil surface helps suppress the germination of surface weeds.
However, this method is not without challenges. The consistent upward movement of water, followed by evaporation from the soil surface, can lead to the unique drawback of accumulating soluble salts and minerals. Since salts are not regularly flushed downward by top-watering, they migrate with the water flow and concentrate on the surface layer. This salt buildup, especially pronounced in arid environments, may require corrective action, such as occasional heavy top-watering or flushing to push the salts below the root zone.