Quality Potato Production Dependence on Irrigation Scheduling

    Irrigation Association, 1999 Proceedings of the International Irrigation Show, p 133-140
                   (Reproduced to the web with permission of the Irrigation Association)

Clinton C. Shock, Erik B. G. Feibert, and Lamont D. Saunders
Malheur Experiment Station, Oregon State University,
595 Onion Ave., Ontario, OR 97914, (541) 889-2174, E-mail Clinton.Shock@orst.edu


Potato (Solanum tuberosum) responds negatively to soil moisture deficits so that precise irrigation scheduling is essential. Economic losses due to decreases in tuber yield, grade and internal quality place a premium on careful irrigation management. Potato evapotranspiration varies with production site. Methods to meet the potato crop water needs are discussed, as well as the consequences of deficit irrigation.

Introduction; Benefits to Potato from Precise Irrigation Scheduling

Irrigation scheduling consists of applying the right amount of water at the right time. With a water stress sensitive crop, growers have incentives to make irrigation scheduling work well. Incentives to growers for precise irrigation scheduling include the following:

1. Under-irrigation leads to a loss in market grade, crop quality, yield, and price (Shock, et al., 1998b).

2. Over-irrigation leads to a loss in water, electricity for pumping, leaching of nitrogen (Feibert, et al., 1998), and wastes manpower. Over-irrigation increases crop N needs, fertilizer costs, and nitrogen losses to groundwater. Soil losses in runoff can be aggravated by irrigation induced erosion.

3. In evaluating an unsuccessful crop in a given field, both under-irrigation and over-irrigation can occur during the same season.

Scheduling criteria. Growers irrigate using one of several scheduling criteria:

1. intuition and experience,

2. calendar days since the last rainfall or irrigation,

3. crop evapotranspiration,

4. soil water measurement.

Measurements of soil water and crop evapotranspiration provide objective criteria for irrigation management. These two objective criteria have been used together to minimize irrigation errors.

Use of Evapotranspiration

Potato crop evapotranspiration (ETc) was estimated using AgriMet (U.S. Bureau of Reclamation, Boise, ID) weather stations at various Oregon locations and a modified Penman equation (Wright, 1982). Given the wide variety of microclimates in Oregon, daily and cumulative ETc varied considerably by site (Figures 1 and 2). Potato evapotranspiration varies by planting date, field, local precipitation, and many other conditions.

Figure 1. Daily Potato ET at Three Oregon Sites, 1998


Figure 2. Cumulative Potato ET at Three Oregon Sites, 1998

The Check Book Method for Crop Evapotranspiration. The use of the check book method is straight forward, but the grower has to have access to the following information:

1. A local weather station estimating potato crop ETc based on the crop's coefficients and correct crop development data (dates of emergence, full canopy, and estimated harvest),

2. A rain gauge placed in each production field or group of closer adjacent fields, and

3. A good estimate for the allowable depletion of water for each soil.

Acquiring all three of these needed pieces of information is feasible. The potato plant's water use is well known given local weather data and the stage of plant development. But, someone manually or automatically must calculate the daily potato ETc at each important weather station location. The allowable soil water depletion for potatoes can be calculated by extension agents, crop consultants, and growers. The calculation requires knowledge of the potato plants' effective rooting depth in a given soil and that soil's water retentive characteristics in the range where the potato plant does not suffer water stress as is discussed in soil moisture determination below. Potatoes are very sensitive to water stress, and caution is needed to avoid over estimation of a soil's allowable depletion.

The grower needs to be concerned with keeping a checking account balance of the estimated evapotranspiration and the measured rainfall in the potato fields. The daily evapotranspiration is provided by an automated weather station, extension service, or crop consultants.

An example of matching ETc with irrigation is presented in Figure 3. Here is how it works:

The check book method is operated by keeping a daily record of rainfall, estimated ETc, and accumulated net ETc . Rainfall is subtracted from the accumulated ETc. If there are rainfall events that make the accumulated ETc account negative, the negative balance is dropped. The reason that the negative balance is dropped is the it represents water applied in excess of the soil water holding capacity that will have been lost to leaching. The grower decides to irrigate by not allowing the accumulated ETc to exceed the allowable depletion. The grower decides how much to irrigate by not replacing more than the accumulated ETc. Note that we have come to clear decisions as to when to irrigate and how much to apply; the requirements for successful irrigation scheduling.


Figure 3. Cumulative Et and water applied plus rainfall for potatoes at Ontario, 1994.

Value of Evapotranspiration Guided Irrigation. Research at the Malheur Experiment Station from 1992 through 1994 (Shock, et al., 1998b) demonstrated that irrigating potatoes at less than ETc (Figure 3) resulted in sharply lower potato yield, grade, and profitability (Figures 4 and 5). Nitrogen fertilizer requirements of potato were reduced with careful irrigation scheduling (Feibert, et al., 1998).


Figure 4. Effect of irrigation plus rainfall on potato yield and grade over 3 years and over 4 varieties at Ontario.


Figure 5. Effect of irrigation plus precipitation on potato profit, Ontario, OR.

Soil Moisture Instrumentation

Precise irrigation management for potatoes depends on soil water monitoring. While matching ETc, it is essential to maintain the soil water potential in the root zone within narrow ranges (Figure 6).


Figure. 6 Soil water potential at two depths over time for potatoes irrigated at -60 kPa replacing ET at Ontario

The most commonly used instruments in to monitor soil moisture in Oregon potatoes are neutron probes and granular matrix sensors (GMS; Watermark Soil Moisture Sensors, Model 200 SS, Irrometer Co., Inc.).

Neutron probes. Neutron probes are used extensively on coarse textured soils in the Columbia Basin. Measurements are provided by independent crop consultant services to irrigation managers to recommend whether or not central pivot irrigation systems are keeping up with crop water demand. Neutron access tubes are placed in representative places in each field. Neutron probe readings are made once or twice a week at multiple depths. Data is taken back to the consultant firm's office, entered into computer programs, and interpreted. Interpreted reports are sent to irrigation managers. Land tenure in the Columbia Basin is predominately in the form of large parcels with highly centralized management.

Granular matrix sensors. Daily soil water potential readings of GMS are made in growers' fields by the grower or hired help and the readings are used to schedule irrigations. In Malheur County some growers use GMS in their own fields and the Soil Water Conservation District and Malheur County Extension Service monitors GMS in onion and potato fields, providing data to the growers. The cost is paid for by the growers with assistance from the Oregon Department of Agriculture.

Actual readings are made by student summer labor using a hand held digital meter, Watermark Soil Moisture Meter (Model 30 KTCD-NL, Irrometer Co., Riverside, CA). Six GMS are used to characterize the soil water potential in each field. Typically one area of a 16 ha field is chosen by the grower based on irrigation experience in prior years. Field size varies. Sometimes both a typical area and a difficult (usually drier) area are chosen for GMS installation. Five of the six GMS are distributed widely across each area and each GMS is connected by up to 150 ft of 18 gauge insulated wire to a terminal strip. All sensors in a given area are wired to a single location for rapid reading. For each area, all but one of the sensors are installed at 8-inch depth and a single sensor is installed at the 16-inch depth. Responsive GMS placement has been determined. Sensors are read daily and the soil water potential data is plotted daily. The data is plotted for immediate interpretation and use by the grower. Copies of the day-to-day graph stay both in a

newspaper box at the site and with the person making the readings. The average readings at 8-inch depth and the single reading at 16-inch depth in each area are plotted. Also the soil water potential of the driest sensor at the 8-inch depth is plotted.

The graphs are designed to help the grower irrigate potatoes at -50 kPa (cbar) and to avoid the silt loam soil drying beyond -60 kPa. In sprinkler-irrigated fields, information from the 16-inch depth helps avoid over irrigation which could keep the deeper part of the

soil profile saturated and cause tuber decomposition.

Importance of Soil Water Potential. Research at the Malheur Experiment Station from 1986 through 1989 determined the soil water requirements for quality potato production (Eldredge, et al., 1992 and 1996) and how to carefully monitor soil water status using granular matrix sensors, GMS (Eldredge, et al., 1993; Shock et al., 1998a). Granular matrix sensors were useful because they responded closely to the wetting and drying of the silt loam soils, required little maintenance, were low cost, and could be read at a distance from the site of measurement. Potato plants are fragile, subject to damage with repeated visits to fixed soil moisture measurement sites.

Potato tuber grade is reduced with a single irrigation scheduling error drier than -50 kPa during early tuber bulking (Eldredge, et al., 1992). Increased dark-ends, reducing sugars in the tuber stem-ends, and jelly end rot were associated with a single irrigation scheduling error drier than -68 to -80 kPa during early tuber bulking (Eldredge, et al., 1996). Increased tuber stem-end reducing sugars in stored potatoes and corresponding dark-end fry strips were associated with a single irrigation scheduling error for Malheur County Russet Burbank potatoes anytime during tuber bulking (Shock et al., 1993).

Growers modified their irrigation and other cultural practices so as to minimize water stress to the potato plants during tuber development.


Future human welfare depends on the efficient use of agricultural inputs to produce abundant, high quality food and fiber. Environmental sustainability of agriculture depends on careful balance of inputs and outputs. In spite of vast expanses of rain fed cropland, much of the world's food supply comes from irrigated agriculture.

The productivity, profitability, and environmental sustainability of irrigated systems are tied to careful management of water so that the land is not subjected to undue irrigation induced leaching and irrigation induced erosion. Careful irrigation management for potatoes is dependent on instrumentation to determine both evapotranspiration and soil moisture. From a scientific perspective, the use of instrumentation for soil water determination can satisfy an academic interest in the dynamics of the flow of water and solutes. From a practical engineering perspective,

instrumentation of soil moisture measurement can provide guidance for irrigation management, helping to assure maximum economic yield in the short term and long term environmental protection.

Literature Cited

Eldredge, E. P., Z. A. Holmes, A. R. Mosley, C. C. Shock and T. D. Stieber. 1996. Effects of transitory stress on potato tuber stem-end reducing sugars and fry color. Am Potato J 73:517-530.

Eldredge, E.P., C.C. Shock, and T.D. Stieber. 1993. Calibration of granular matrix sensors for irrigation management. Agron. J. 85:1228-1232.

Eldredge, E. P., C. C. Shock and T. D. Stieber. 1992. Plot sprinklers for irrigation research. Agron. J 84:1081-1084.

Feibert, E. B. G., C. C. Shock and L. D. Saunders. 1998. Nitrogen fertilizer requirements of potatoes using carefully scheduled sprinkler irrigation. HortSci. 32:262-265.

Shock, C. C., J. Barnum, and M. Seddigh. 1998a. Calibration of Watermark soil moisture sensors for irrigation management. Irrigation Association. Proceedings of the International Irrigation Show pp. 139-146. San Diego, CA.

Shock, C. C., E. B. G. Feibert, and L. D. Saunders. 1998b. Potato yield and quality response to deficit irrigation. HortSci. 33: 655-659.

Shock, C. C., Z. A. Holmes, T. D. Stieber, E. P. Eldredge, and P. Zang. 1993. The effects of timed water stress on quality, total solids and reducing sugar content of potatoes. Am Potato J 70:227-241.

Wright, J.L. 1982. New evapotranspiration crop coefficients. J. Irrig. Drain. Div., ASCE 108 (1): 57-74.