Scheduling irrigation consists of knowing when and how much water to apply in a way that satisfies crop water needs, maintains soil water tension between field capacity and 15 kPa at the 12-inch depth, and prevents nutrient leaching. Because of the low water holding capacity of the sandy soils of Florida (approximately 10%, v:v), proper irrigation management requires an estimate of crop water use, a tool to monitor soil moisture status, and a guideline for splitting irrigation. Splitting irrigation is necessary when the water volume scheduled for one irrigation event is greater than the water holding capacity of the wetted zone. For a sandy soil, a 1-sq foot vertical section of a 100-ft long raised bed can hold approximately 30 gal of water.

In practice, splitting irrigation has to be a compromise between two constraints. On one side, the more frequent the irrigation, the less likely soluble nutrients are to be leached below the root zone. On the other side, frequent and short irrigations may waste water and reduce irrigation uniformity due to a large portion of the irrigation cycle used for system charge and flush. In addition, each irrigation cycle has to deliver enough water to ensure complete wetting between two adjacent emitters to maintain crop uniformity, especially when the plants are small. Based on reports from other states (where soil types are different), it is often believed that the size of the wetted zone can be increased if irrigation is pulsed. In this case, pulsing irrigation refer to the practice of irrigating for a short period (approx. 30 min.), then waiting for another short period, and repeating this on-off cycle until the entire irrigation water is applied.

Testing, whether or not splitting irrigation increases the size of the wetted zone, may be achieved by visualizing water movement in the soil using soluble dye.

A dye test was conducted at the North Florida Research and Education Center (NFREC) - Suwannee Valley, near Live Oak, FL on a 15-foot deep Lakeland fine sand on 3 Dec. 2003. Before the day of the test, the field was rototilled, 28-inch-wide raised beds were formed, and drip tape and polyethylene mulch were laid. The dye test consisted of injecting a soluble blue dye (Terramark SPI High Concentrate, ProSource One, Memphis, TN), irrigating according to treatments, digging longitudinal and transverse sections of the raised beds, and taking measurements. All treatments received a total of 4 hours of irrigation using Roberts Ro-Drip drip tape (24 gal/100ft/hr flow rate; 12-inch emitter spacing) applied in different ‘splits’ (Table 1). The drip irrigation system consisted of a well, a pump, a back-flow prevention device, a fertilizer injector (model DI16-11, Dosatron, Clearwater, FL), a 150-mesh screen filter, a 10-psi pressure regulator, and drip tape.  After pressurizing the irrigation system, the dye was injected at the 1:49 (v:v) dye:water dilution rate for approximately 30 min.  After dye injection, clear water was injected according to treatments.

At the end of the test, a 4-ft-long longitudinal and two transverse sections were dug shortly after completion of each irrigation treatment, which allowed to take measurements on 4 and 2 emitters, respectively. Vertical depth (D), width (W) and length (L, emitter-to-emitter coverage) of the wetted zone under each emitter were measured as the longest vertical distance from the drip tape to the bottom of the blue ring, the horizontal length perpendicular to the bed axis at the widest point of the wetted zone, and the horizontal length parallel to the bed axis at the widest point of the wetted zone, respectively.

Because the dye was injected first followed by clear water, the dye patterns in the soil appeared as a 1-inch-thick blue ring surrounding an uncolored section of soil. If the dye were continuously injected with all the irrigation water, the entire wetted section would appear colored. The dye was easily distinguishable in the soil, but the contrast between the soil color and the blue ring was improved by allowing a 1 to 2 hour drying period after digging.

In this test, all treatments received the same total amount of water (96 gal/100ft) but initial amounts were different. The initial amount of water applied was 24, 48, 72 or 96 gal/100ft (Table 1). The width, depth, and length of the wetted zone were not affected by the split treatments (Figs. 1, 2, 3, 4). These results do not support the practice of splitting irrigation as an attempt to increase the size of the wetted zone. Splitting irrigation should be made as an attempt to keep the irrigation water (and soluble nutrients) within the root zone. In addition, splitting irrigation during crop production (in fields with actively growing vegetable crops) allows a partial depletion of soil water between splits, thereby further reducing the risk of water movement below the root zone.