IRRIGATION WATER MANAGEMENT IN SUGARBEET PRODUCTION

W. Howard Neibling and John J. Gallian

Proper timing of irrigation and application of the appropriate amount of water can maximize crop yield while minimizing disease, fertilizer and water use. Excess crop water stress, resulting from inadequate irrigation, can reduce crop yield. Over-irrigation can also reduce crop yield and create more favorable conditions for disease development.

UNDER-IRRIGATION EFFECTS

Because sugarbeets have a deep root system which can efficiently extract water to depth of 3-3.5 feet in a deep soil with no restrictive horizons, effects of temporary under-irrigation can be minimal if adequate water is available somewhere in the crop root zone. On high water use days, beets may wilt because water cannot be supplied to the crop at a sufficiently high rate, even though adequate water is present in the soil. During the evening and night, the plants will recover if sufficient water is available. If plants remain wilted the next morning, it is a strong indication that irrigation is needed.

Tensiometers or watermark granular matrix sensors are one set of tools that can be used to monitor soil water status and schedule irrigation. These instruments both measure the amount of work required to extract water from the soil. Immediately following irrigation, readings will be about 0 and will rise as the soil dries. To avoid crop water stress, beets should be irrigated when sensors in the 12-18 inch range read 30-40 centi-bars (cbars) on a sandy soil and 60-80 cbars on a silt loam or similar soil. Irrigating at these levels avoids excessive crop water stress, although several studies in Southern Idaho have indicated that beets can be moderately stressed with only a minor yield reduction.

For example, on one study this last summer, soil moisture status was monitored with watermark granular matrix sensors at 8, 12, and 24 inches on plots irrigated by buried drip and center pivot. Due to the lack of availability of water, the beets under the center pivot entered the peak use portion of the season with only a partially full soil profile.

Because center pivot systems in this area are typically not designed to meet peak use, the system was never able to “catch up” and the soil profile continued to dry so that by mid-August, sensors at 1 and 2 feet were at 150 cbars or higher (high water stress). The pivot was keeping the surface 8 inches at 100 cbars or less during this period. Crop water stress was significant and beets were wilting daily. Soil moisture measurements indicated water use from below 24 inches, which allowed the beets to continue to grow with relatively severe water stress in the top 2 feet of the soil profile.

On an adjacent subsurface drip irrigated area, system capacity was such that water could be added at greater than peak use rates and the watermark readings at all depths never exceeded 50 cbars. Plants never showed any sign of wilting.

When final yields were measured, the never-stressed beets on the subsurface drip area yielded 28.5 tons/acre while the water-stressed beets under the center pivot yielded 25.8 tons/acre. Based on the appearance of the beets and the level of crop water stress, we would have expected a greater yield reduction under water stressed conditions.

OVER-IRRIGATION EFFECTS

Rhizomania and Rhizoctonia root rot, two serious diseases of sugarbeets, both require moist soil conditions for disease development. The ideal soil moisture environment for disease formation is that found in most soils during the first 2-3 days following irrigation. For Rhizomania, disease development is maximum at saturation and decreases as the soil dries as shown in Figure 1. Although these data are for Chinese cabbage, they provide an excellent model to explain Rhizomania disease development in sugarbeets. At soil moistures of 30 cbars, incidence of disease is about 1/3 that at saturation, while at 60-80 cbars, incidence is less than 1/10 that at saturation. These data would suggest that on soils prone to diseases favored by high soil moisture, disease development could possibly be slowed by keeping the soil a little on the dry side of optimum and lengthening the time between irrigations as much as possible.

An irrigation management study was conducted on a Rhizomania-infested sandy loam soil near Rupert, Idaho, in 1996. Results, shown in Figures 2, 3, and 4, suggest that irrigation to maintain slightly drier than optimum soil moisture did provide higher beet yield. In an extreme comparison, beet yields were about 2.3 times greater for a Rhizomania-resistant variety grown in soil conditions maintained at slightly drier than optimum (treatment 1) than for the same variety irrigated excessively to maintain optimum disease conditions (treatment 6).

Under a more normal range of irrigation conditions, yield was about 10% greater under drier than optimum [matric potential kept at 50-70 cbars on a sandy soil (treatment 1)] relative to optimum soil moisture [matric potential of 30-40 cbars (treatment 2)] and about 15% greater for wetter than optimum conditions [matric potential of 20-30 cbars (treatment 3)]. Treatments 1-3 were irrigated every 2 to 3 days with sufficient water to keep soil moisture in the desired range. Treatments 4 and 5 were irrigated on a 5-day interval to simulate set-move sprinkler irrigation. Irrigation differences had little effect on these treatments. Because of moist early season soil conditions, yield differences in a “normal” year could be larger.

In summary, sugarbeets can tolerate some water stress, so to minimize disease problems from moisture-loving diseases, stretch out the interval between irrigations and keep the soil moisture a little on the dry side.

TIPS FOR IRRIGATION WATER MANAGEMENT ON SUGARBEETS:

1. If water is available, have the soil profile full before planting to minimize irrigations (and potential crusting) due to irrigation just after planting.

2. On soils that crust, sprinkler irrigate beets only when necessary for germination and emergence. Apply light, frequent irrigations.

3. After germination, keep soil moisture in the expanding beet root zone adequate, but not excessive for disease control. Root zone depth will start at about 6 inches and expand to 3-3.5 feet by mid season.

4. After emergence and for the remainder of the season, soil should dry to 50% available soil moisture before irrigation on surface-irrigated or set-move (hand or wheel line) systems. Tensiometer or watermark moisture sensor readings at 12-18 inches should rise to 30-40 centi-bars on sandy soils or 50-70 centi-bars on silt or loamy soils before irrigation is required.

5. After emergence and up to nearly the peak use period, center pivots should be managed to keep the soil moisture at 50% available or more moist. As the peak use period approaches, the pivot should be managed to keep the entire root zone nearly full in anticipation of soil moisture mining by the crop during the peak use period when system capacity cannot meet peak crop water use.

Presented at Sugarbeet Schools on January 27-31, 1997