Subsurface drip irrigation (SDI) has been utilized in farming for decades (Coolong, 2013). There has been more than 30 years of ongoing research projects using SDI in corn in the Midwest, and it is a viable irrigation method that can save water resources while maintaining yields on the loamy soils in the region (Lamm, 2023). Many permanent or semipermanent SDI systems are used in agronomic crops, such as corn, and are buried at soil depths of 16–20 in.
In contrast, shallow subsurface drip irrigation (SSDI) is a system that is typically buried at depths of 3–5 in. and is removed annually or after a cropping cycle is completed. Because of the shallow roots of many vegetable crops, SSDI systems are often preferred. This publication highlights some of the ongoing research into using SSDI in organic vegetable crop production. SSDI facilitates cultivation without harming drip tubing and limits surface soil wetting, which may also reduce weed pressure. Prior research has shown that SSDI offers some advantages over surface-placed drip tubing in conventionally grown crops (Coolong, 2016).
In organic systems, the movement of surface-placed drip tubing may be required every time the crop is cultivated, making it somewhat impractical on a larger scale. It is important to note that this research has been conducted in the Piedmont region of Georgia, which has clay and loam soils. These soils hold more water and allow for more lateral water movement—and thus may be better suited to SSDI—than the sandy soils found in the Coastal Plain region. At this time, SSDI has not been thoroughly researched on the sandy soils of southern Georgia and it may not be a suitable alternative for farms in the region.
Shallow subsurface drip irrigation can be placed using a variety of implements. We utilize a purpose-built toolbar with drip reels and tubes modified from a bed shaper/plastic layer on our research site, but growers can use a variety of methods to install SSDI on their farms (Figure 1).
Figure 1. Two examples of SSDI tools. On the left is a sled-type device, while the implement on the right uses small tires to maintain a constant depth with the soil surface.
In research trials in Watkinsville, GA, the SSDI was placed between 4 and 5 in. deep (Figure 2). In spring we grew a large-seeded crop, sweet corn, and in the fall we grew transplanted broccoli. In both crops, a line of drip tubing was buried approximately 3–4 in. from the planted row.
Video showing one method of burying drip tubing from the edge of a field.
Figure 2. Overhead irrigation (right/bottom area of image) compared to SSDI (left side of image). Notice the small circles of surface wetting in the SSDI portion of the field. While soil moisture was adequately maintained below the surface, very little surface wetting was observed, resulting in less weed pressure.
In 2021 and 2022 we were able to successfully germinate sweet corn in spring and establish broccoli in the late summer using SSDI. Plots were compared to overhead irrigation as a control. In addition to comparing SSDI and overhead irrigation, two cultivation regimes reflective of tools available to organic growers in the region were compared (low input and high input), as well as non-weeded and fully hand-weeded control plots. The low-input cultivation regime featured a three-point hitch-mounted tine weeder that was used approximately once per week for 3–4 weeks after establishment. The high-input cultivation regime featured the tine weeder (Figure 3) in addition to a finger weeder mounted on a tractor that was specifically designed for cultivation. Cultivation ceased when plants were too tall to drive over (in the case of corn) or had formed complete canopies and could no longer be cultivated without damaging the crop (in the case of broccoli).
Figure 3. A tine weeder (left) and finger weeder (right) used in cultivation research. Drip irrigation lines are buried to a depth of roughly 4 in. so they do not interfere with cultivation and are not damaged by the equipment.
Soil moisture levels were measured with a probe that was inserted in the soil at a depth of approximately 6 in. equidistant between two planted rows (and drip lines) in both SSDI and overhead-irrigated plots (Figure 4). Average soil moisture results suggest that the SSDI system maintained slightly greater moisture at a depth of 6 in. compared to overhead watering when applying the equivalent amount of irrigation water (Figure 5). While one might expect SSDI to maintain greater subsurface soil moisture than overhead watering, it is important to note that sensors were placed between the drip lines, which suggests that there is sufficient movement of water laterally in the Cecil loam soils where the tr
