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My work has focused on instruments that can be conveniently read at
a distance from the measurement site, can be easily logged, and whose output
can be readily used to control automated irrigation decisions. Consequently
my discussion of this topic is influenced not only by the crops and environment
with which I work, but also by the philosophy and approach that I have
toward soil moisture measurement equipment. The discussion here will be
rooted in the basics of profitability and instrument operation and proceed
to an example of automated irrigation based on instrumentation. Automation
of subsurface drip irrigation (SDI) or sprinklers can be accomplished using
a combination of soil moisture sensors, data logger, controller, solenoid
switch, valve, and pressurized water source.
The people of Malheur County, Oregon, rely on intensive irrigated agriculture for a large part of their income. Income and human welfare is highly dependent on progressive growers' production of onions, potatoes, sugar beets, seed crops and their associated value added industries. As of 1985, Malheur County growers were not yet using measurements of soil water or estimates of crop evapotranspiration for objective irrigation scheduling. Irrigation water is inexpensive but not always abundant. Growers were not aware of scientific information relating precise irrigation criteria to crop yield or quality. All crops were irrigated by subjective judgment and experience. Tensiometers, capacitance devices, neutron probes, and gypsum blocks had been tried extensively without success and had been set aside. No moisture monitoring instrument had been demonstrated to be a reliable tool for irrigation scheduling in the county.
Potato quality and irrigation criteria. During the 1984 and 1985 seasons, potato tuber quality was inadequate to meet the needs of the potato processors, Ore-Ida Foods Inc. and J. R. Simplot Co. In particular, a condition called "dark-ends" in fried tuber slices resulted from tubers grown on stressed potato plants. Yet, the stresses aggravating the occurrence of "dark-ends" were poorly defined. Growers lost contracted hectares. Economic losses resulting from inadequate product quality provided the incentive for irrigation scheduling research and implementation.
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). 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. Contracted acreage was regained.
Onion irrigation criteria. Growers in the Treasure Valley of eastern Oregon and southwestern Idaho produce long day Sweet Spanish onions on approximately 9000 hectares annually. Income to growers from sales depend on bulb size and storability, both of which were proven to be closely associated with small variations in irrigation management. Onions grown in the Treasure Valley are for medium to long term storage. Onions are marketed starting at harvest in August and then out of storage through April, so quality out of storage is indispensable. Onions in the Treasure Valley are almost exclusively furrow irrigated. The weather during the growing season has high evapotranspiration (annual average of 669 mm of onion Etc for 1992-1994) and low precipitation (annual average of 61 mm during the growingg season for 1992-1994) which makes irrigation essential. Irrigation scheduling using water potential could provide and accurate method of maintaining optimum soil moisture.
Onion variety 'Great Scott' was grown on silt loam soils and submitted to four irrigation thresholds (-25, -50, -75, and -100 kPa) in 1992 and six irrigation thresholds (-12.5, -25, -37.5, -50, -75, and -100 kPa) in 1993 and 1994 (Shock, et al. 1994). Irrigation thresholds consisted of soil water potential measured at 0.2 m depth used as criteria to initiate furrow irrigations. Onions were evaluated for yield and grade after 70 days of storage. In 1992 and 1994 onion total yield, marketable yield, and profit increased up to the highest thresholds of soil water potential utilized: -25 kPa in 1992 and -12.5 kPa in 1994. In 1993 onion total yield was increased up to the highest threshold tested of -12.5 kPa, but marketable yield and profit were maximized by a calculated threshold of -27 kPa due to a substantial increase of decomposition during storage with increasing threshold.
Hybrid poplar is being grown for wood chip production in eastern Oregon. Because of interest in possibilities of saw log production, the hybrid poplar cultivar 'OP-367' was planted in April 1997 on a 4.27 m x 4.27 m spacing at the Malheur Experiment Station in silt loam and submitted to six irrigation regimes (Shock et al., 1998b). Irrigation regimes consisted of a combination of soil water potentials as thresholds for initiating irrigations and water application rates. The irrigation system consisted of microsprinklers installed along the tree row. Wood volume by the end of September 1997 was highest with the wettest treatment (keeping soil water potential at 0.2 m depth wetter than -25 kPa and a total growing season water application of 610 mm). Trees in the wettest treatment averaged 3.02 m in height and produced 0.20 m3/ha of wood volume by the end of September, 1997.
Other crops: Many crops have been shown
to sensitively respond in yield and or quality to irrigation. These responses
are often of economic importance.
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., 1998c).
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. Under-irrigation and over-irrigation can occur during the same season in a given field.
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 or crop evapotranspiration provide objective criteria for irrigation management in the Pacific Northwest of the US. These two objective criteria can be used together to minimize irrigation errors.
Soil Moisture Instrumentation. Soil water can be measured by the methods that determine the soil water content or the soil water potential. Soil water content is the amount of water per volume of soil or weight of dry soil. Soil water potential is the force necessary to remove the next increment of water from the soil.
Different measurement methods have particular strengths and weaknesses. For example the gravimetric method is very accurate, but it is very slow and many samples are needed for each field and site specific interpretations are necessary.
Methods to determine soil water content:
1. Feel method
2. Gravimetric
3. Neutron attenuation probe
4. Time domain reflectometry (TDR)
5. Capacitance sensors
6. Heat dissipation sensors
7. Velocity differentiation domain (VDD)
Methods to measure soil water potential:
1. Tensiometers
2. Gypsum blocks
3. Granular matrix sensors
4. Psychrometers
5. Pressure plate
There is no perfect sensor that will provide the lowest cost and most accurate estimates of soil moisture conditions for all crops in all soils. An effective soil moisture sensor for a particular application must accurately respond in the range soil moisture critical to the economic response in the crop and soil of interest.
There are many classes of instrumentation for soil moisture monitoring, and these devices serve many research and irrigation management purposes. My work has focused on instruments that can be used for irrigation scheduling, can conveniently be read at a distance from the measurement site and logged, can cost effectively be spread out over a representative area of a commercial field, and can be used to control automated irrigation decisions. For these specifications, one must choose between capacitance sensors, heat dissipation sensors, velocity differentiation domain, tensiometers with pressure transducers, gypsum blocks, or granular matrix sensors.
Psychrometers do not provide good differentiation at the wet end of the soil water potential scale where critical irrigation decisions must be made for many crops and they are not as easy to log remotely. Time domain reflectometry is an accurate but not cost effective way to remotely sample water content on the scale of commercial fields.
Granular matrix sensors (Watermark Soil Moisture Sensor, Irrometer Co., Riverside, CA) reduce the problems inherent in gypsum blocks (i.e., loss of contact with the soil by dissolving, and inconsistent pore size distribution) by use of a granular matrix mostly supported in a metal or plastic screen (Larson, 1985; Hawkins, 1995). Granular matrix sensors operate on the same electrical resistance principle as gypsum blocks and contain a wafer of gypsum imbedded in the granular matrix. The electrodes inside the GMS are imbedded in the granular fill material above the gypsum wafer. The gypsum wafer slowly dissolves, to buffer the effect of salinity of the soil solution on electrical resistance between the electrodes. Particle size of the granular fill material and its compression determines the pore size distribution in GMS and their response characteristics (Larson, 1985).
Because GMS require little maintenance during the growing season, they are suited for sensing soil water potential to automatically control irrigation systems. They have advantages of low unit cost and simple installation procedures, similar to those used for tensiometers. Data acquisition with GMS can be remote from the measurement site by use of electrical wires, so the plants and soil at the measurement site remain relatively undisturbed (Shock and Barnum, 1994). Their operation is less effective in coarse textured soils where the soil fails to draw water out of the sensor as the soil dries.
Granular matrix sensor electrical resistance was calibrated for accurate measurement of soil water potential on silt loam soils in the range of -10 to -75 kPa (Eldredge, et al., 1993; Shock, C.C., 1998). Compensation for GMS resistance change for temperature helps provide accurate estimates of soil water potential. These sensors have been used in automated irrigation systems to control irrigation frequency, maintaining soil water potential, and thereby maximizing yield and quality.
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 one 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 is predominately in the form 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 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 (Model 30KTCD, 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 0.2 m depth and a single sensor is installed at the 0.4 m 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 0.2 m depth and the single reading at 0.4 m depth in each area are plotted. Also the soil water potential of the driest sensor at the 0.2 m depth is plotted. The graphs are designed to help the grower irrigate potatoes at -50 kPa and to avoid the silt loam soil drying beyond -60 kPa. In sprinkler-irrigated fields, information from the 0.4 m depth helps avoid over irrigation which could keep the deeper part of the soil profile saturated and cause tuber decomposition.
Introduction. Previous research with furrow irrigated onions at the Malheur Experiment Station has demonstrated the sensitivity of onions to small water deficits and the need to maintain small negative soil water potentials for optimum yield (Shock et al., 1994). The superior water application efficiency with subsurface drip irrigation (SDI) allows for more precise irrigation management than with furrow irrigation. With subsurface drip irrigation, onions can be irrigated at different soil water potentials and the soil water potential can be maintained nearly constant, avoiding the oscillations in soil water common with furrow or sprinkler irrigation. The objective of this trial was to evaluate the effects of different soil water potentials on onion yield and quality when grown with SDI.
Methods. The trial was conducted on an Owyhee silt loam previously planted to wheat at the Malheur Experiment Station. A soil sample taken from the top foot on March 8, 1995, showed a pH of 7, 1.5% organic matter, 5 µg/g nitrate-N, and 7 µg/g ammonium-N, 20 µg/g P, and 1.8 µg/g Zn.
The field and crop were prepared and treated by normal commercial practices. Onions were planted at 346,000 seeds/ha. Drip tape (Nelson Irrigation Corp., Walla Walla, WA) was laid at the same time as planting at 0.15 m depth between two double onion rows. The drip tape had emitters spaced 0.30 m apart and a flow rate of 6.1 l/minute/100 m. The trial was irrigated on April 23, April 25, and May 2 with a microsprinkler system (R10 Turbo Rotator, Nelson Irrigation Corp., Walla Walla, WA) in order to enhance uniform onion emergence. Risers were spaced 7.6 m apart and connected to 3 flexible polyethylene hoses spaced 30 feet apart. Onions started emerging on May 1.
Irrigation treatments consisted of five soil water potential levels (-10, -20, -30, -50, and -70 kPa), maintained nearly constant during the entire season, and three treatments where the soil water potential was maintained at -20 kPa until July 15 and then decreased to -30, -50, or -70 kPa for the remainder of the season. The soil water potential (0.2 m depth) was maintained constant by applying 1.6 mm of water up to 8 times a day based on soil water potential readings every 3 hours. The irrigation treatments were started on June 13, 1997. The 8 irrigation treatments were replicated five times and arranged in a randomized complete block design. Plots were 2 beds wide and 15.2 m long.
Soil water potential was monitored in each plot by five GMS. In each plot, four GMS were installed at 0.2 m depth from the surface of the soil, and one GMS was installed at the 0.45 m depth. All GMS were installed below one of the two onion double rows in the plot center. The 200 GMS and two soil temperature sensors installed 0.2 and 0.45 m were connected via five multiplexers (AM 410 multiplexer, Campbell Scientific, Logan, UT) to a datalogger (CR 10 datalogger, Campbell Scientific, Logan, UT). The datalogger was programmed to read the GMS resistance using an alternating current signal, read the temperature sensors, calculate the soil water potential at the 0.2 m depth in each plot and, if necessary, irrigate the plots individually, according to the plot's irrigation criteria. GMS were calibrated to soil water potential (Shock, 1998). The irrigations were controlled automatically by the datalogger using a controller (SDM CD16AC controller, Campbell Scientific, Logan, UT) connected to solenoid valves in each plot. The water pressure in the drip lines was maintained at 67 kPa during irrigations by pressure regulators in each plot. The amount of water applied to each plot was recorded daily from water meters installed between the solenoid valve and the drip tape in each plot. Irrigations were terminated on August 29.
Onion evapotranspiration (Etc) was estimated using an AgriMet (U.S. Bureau of Reclamation, Boise, ID) weather station at the Malheur Experiment Station and a modified Penman equation (Wright, 1982). Onion Etc was estimated and recorded from crop emergence until the final irrigation.
Fertilizer solutions were applied through the drip lines via a venturi injector (Mazzei injector Model 1087). Urea ammonium nitrate solution at 22 kg N/ha was applied on June 2, June 17, June 23, June 26, and July 11, for a season-long total of 112 kg N/ha. A plant sample was taken from the field for nutrient analyses on July 14. The plants were washed, the roots were analyzed for nitrate-N, phosphate-P, K, sulfate-S, and the leaves were analyzed for micronutrients. Analysis of onion leaves and roots showed all nutrients analyzed to be within the established sufficiency ranges.
The onions were lifted on September 22 and allowed to cure in the field. On September 25 the onions in the central 12.2 m of the middle two double rows in each plot were topped, bagged, and placed into storage. The onions were graded on December 16 based on size according to market class, decomposition, and other visual defects. All bulbs from each plot were counted during grading to determine the actual plant population. The average plant population was 261,700 plants/ha.
Results and Discussion. The automated drip irrigation system maintained the soil water potential at 0.2 m depth relatively constant for the -10 kPa and -20 kPa treatments (Shock, et al., 1998a). The soil water potential at 0.2 m depth for the -30 kPa, -50 kPa, and -70 kPa treatments oscillated more and the oscillations increased with decreasing soil water potential. The soil water potential at 0.45 m depth generally was close to the soil water potential at 0.2 m depth. A failure in the datalogger program in early July, 1997 resulted in a brief cessation of all irrigations, and a brief decrease in the soil water potential for all treatments. The soil water potential decreased rapidly upon the termination of irrigation on August 29.
The total amount of rainfall plus water applied with the drip irrigation system from May 14 to August 29 was 914, 686, 559, 457, and 356 mm for the -10, -20, -30, -50, and -70 kPa treatments, respectively. Onion Etc in 1997 totaled 686 mm, indicating the possibility of deep percolation and nitrate leaching with the -10 kPa treatment. Water applications to the -20 kPa treatment closely tracked Etc during the season.
Reducing the soil water potential below -20 kPa after July 15 did not decrease storage rot but reduced the bulb yield of the largest market classes. Decomposition in storage was low for all treatments in this trial, averaging 1.6%.
Summary. Onions were submitted to 8 soil water potential treatments using subsurface drip irrigation. Soil water potential was maintained nearly constant at 5 levels by automated, high frequency irrigations based on soil water potential measurements at 0.2 m depth. The highest total yield, marketable yield, and yields of the largest size bulbs were achieved with the wettest soil water potential, -10 kPa. The highest economic return was achieved with the wettest soil water potential, -10 kPa, but leaching occurred. Maintenance of soil water potential at -10 and -20 kPa required 914 and 686 mm of rainfall plus applied water, respectively. Onion evapotranspiration for 1997 totaled 686 mm from emergence to the last irrigation. The wettest irrigation criteria consistent with just barely applying water to satisfy crop evapotranspiration was -20 kPa, which produced slightly less than the -10 kPa treatment.
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.
Hawkins, A. J. 1985. Electrical sensor for sensing moisture in soils. U.S. Patent 5,179,347. Date issued: 12 January.
Larson, G. F. 1993. Electrical sensor for measuring moisture in landscape and agricultural soils. U.S. Patent 4,531,087. Date issued: 23 July.
Shock, C.C., E.B.G. Feibert, and L.D. Saunders. 1998a. IRRIGATION management FOR DRIP-IRRIGATED ONIONS. Oregon State University Agricultural Experiment Station, Special Report 988: 42-48.
Shock, C.C., E.B.G. Feibert, and L.D. Saunders. 1998b. IRRIGATION MANAGEMENT FOR HYBRID POPLAR PRODUCTION. Oregon State University Agricultural Experiment Station, Special Report 988: 61-68.
Shock, C. C., E. B. G. Feibert, and L. D. Saunders. 1998c. Potato yield and quality response to deficit irrigation. HortSci. July, in press.
Shock, C.C. and J.M. Barnum. 1994. Integration of granular matrix sensors for soil water monitoring into AgriMet and HydroMet. Oregon State University Agricultural Experiment Station, Special Report 936:169-186.
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.
Shock, C.C. 1998. Granular matrix sensor calibration for irrigation scheduling. Agronomy Abstracts.
Shock, C.C., E.B.G. Feibert, and L.D. Saunders. 1994. Soil water potential criteria for onion irrigation, 1994 trial. Oregon State University Agricultural Experiment Station, Special Report 947: 68-78.
Wright, J.L. 1982. New evapotranspiration crop coefficients. J. Irrig. Drain. Div., ASCE 108 (1): 57-74.
Clinton C. Shock
Malheur Experiment Station
595 Onion Ave.
Ontario, OR 97914
(541) 889-2174, Fax (541) 889-7831
Clinton.Shock@orst.edu
June 18, 1998
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