Malheur Experiment Station Information for Sustainable Agriculture

Watermark Soil Moisture Sensors (Model 200 SS, Irrometer Co. Riverside, CA) are conventionally read with a hand held meter or with a programmed data logger. Both these devices use an AC excitation voltage to the sensor and the meter or data logger infers a sensor resistance. The sensor resistance is converted to water potential through a calibration equation that includes compensation for soil temperature. The Ag Tech Moisture Monitor System (Ag Tech, Othello, WA) was tested to automatically read up to 16 Watermarks using a DC excitation to the sensor, precisely timed readings of the sensors, and a digital output signal sent by radio to a central data logging site. Calibration equations of Ag Tech readings to soil water potential and sensor resistance were developed.

Precision irrigation scheduling based on knowledge of soil moisture levels is very important for horticultural crops, especially those of high value. Precision irrigation scheduling is closely related not only to crop yield and quality but also to conservation of irrigation water and reduction in non-point source pollution from irrigated agriculture.

Watermarks (patented in 1985, 1993, and pending) have been manufactured by Irrometer Co. since 1989. These sensors are normally read using a hand held meter. The meter reads soil water potential in kiloPascals (kPa = cbar or centibars).

Watermark Soil Moisture Sensors have been used to carefully match crop water use with crop needs for onions (Shock et al., 1998b), potatoes (Eldredge, et al., 1992; Shock et al., 1993), and poplars (Shock et al., 1999) among other crops. Yield of these crops is closely related to irrigation management and the maintenance of soil water potential. Onion and potato quality are closely related to avoiding irrigation errors and these crops have been successfully managed using these sensors on silt loam soils (Eldredge, et al., 1996; Shock et al., 1998b and 1993).

Growers usually read sensors with a hand held meter and irrigations are scheduled accordingly. There is a need for automated data logging and low cost data logging for information systems and automatic controls. Some automated reading of soil moisture data requires extensive use of wires. These networks are effective, but can hinder cultivation. There are advantages of radio to extend the usefulness of sensors where hard wired applications or manual readings are cumbersome.

Measuring the soil moisture, collecting the data, and interpreting the data are essential in order to convert soil moisture information into practical irrigation decisions. One new way of collecting the data from sensors in the field is the Ag Tech telemetry system. Telemetry systems are an advantage because they allow the irrigator to read the information from the sensors prior to going to the field.

Ag Tech Moisture Monitoring System consists of a transmitter that sends information to a receiver located outside of the field. The transmitter is a small box that can be connected to up to 16 Watermark sensors. The transmitter uses radio waves to transmit the data to a central receiver where the data is stored in memory. From the receiver, data is periodically downloaded to a computer through a serial cable and saved in a text document.

Twenty four Watermark Soil Moisture Sensors (Model 200 SS) were used to determine the relationship between Ag Tech readings and soil water potential. Twelve sensors were installed in a potato field and twelve were installed in a onion field, both were planted on Owyhee silt loam soil in 1999. Two Ag tech transmitters were used with one Ag Tech receiver to collect the data. The first transmitter was located in the potato field and the second was located in the onion field. The transmitters were mounted on poles above the plant canopy and twelve sensors were connected to each.

Installation in the Potato Field. In a drip-irrigated potato field, twelve sensors were installed 20 cm deep at 23, 46, and 69 cm distance from the drip tape used to irrigate the crop. In addition one thermistor and one soil thermometer were installed at 20 cm depth 46 cm from the drip tape. The thermistor was connected to the Ag Tech transmitter and the soil thermometer was read manually. Irrigations were managed to maintain the soil water potential at -30 kPa at 23 cm distant from the tape and 20 cm deep. A single tape was placed between each two rows of potatoes 92 cm apart. Instruments were inside a 1 ha field. The differing distances from the drip tapes were used to assure that soil moisture readings would vary greatly.

Installation in the Onion Field. In a drip-irrigated onion field, twelve sensors were installed 20 cm deep at 0, 28, and 56 cm distance from the drip tape used to irrigate the crop. In addition one thermistor and one soil thermometer were installed at 20 cm depth 56 cm from the drip tape. The thermistor was connected to the Ag Tech transmitter and the soil thermometer was read manually. Irrigations were managed to maintain the soil water potential at -20 kPa under the onion row (28 cm distant from the tape and 20 cm deep). Instruments were inside a 0.4 ha field.

Switching between manual and automated readings. Switches were inserted on every sensor wire so that each sensor could be isolated from the automated system and read manually with a Watermark Soil Moisture meter (Model 30 KTCD-NL, Irrometer Co. Inc.) as well as automatically. After all the sensors and switches were installed readings were taken two times a day, once in the morning and once in the afternoon, Monday through Friday. Manual readings were taken using a manual reader manufactured specifically for watermarks by Irrometer. The Ag Tech readings were taken using a laptop and a serial cable. The information was downloaded, saved in a notepad file, and inserted into an Excel spreadsheet. Data from both the manual and automatic readings were imported into NCSS (Number Cruncher Statistical System, Kaysville, UT) to determine functional relationships. Ag Tech readings above 180 (on the dry end of soil moisture) and those readings that translated to negative sensor resistance (on the extreme wet end of soil moisture) were excluded from the interpretations.

The relationship of soil water potential S in kPa can be calculated for sensor model 200 SS with resistance R in kOhms at soil temperature T in C as measured by Irrometer Watermark Soil Moisture Meter (Model 30 KTCD-NL) as provided by Shock et al. (1998a).

S = -(4.093+3.213*R)/(1-0.009733*R-0.01205*T) r² = 0.872, n=710 (1)

Given the meter readings, S, and soil temperature, T, it is possible to infer sensor resistace as seen by the meter by the use of equation (1) and solving for R:

R= ((0.01205*T*S) - S - 4.093)/(3.213 - 0.009733*S) (2)

Ag Tech readings were compared with both soil water potential readings and calculated resistance values.

During the conduct of these field trials, at the times that comparison between reading methods was tested, the soil temperature ranged from 19 to 25 C. No attempt was made to correct Ag Tech readings for temperature.

Instrument performance in the potato field. Soil water potential was closely related to Ag Tech readings in the range of -10 to -80 kPa (Figure 1). The relationship between soil water potential, S in kPa, and Ag Tech readings, X, is given by the relationship

S = -8.262199+((.133301)*X)-(0.003518543*(X²)) r² = 0.62, n = 175 (3)

Figure 1. The relationship between soil water potential and Ag Tech readings in a potato field

Sensor resistance was closely related to Ag Tech reading in the range of Ag Tech readings below 181 (Figure 2). The relationship between estimated sensor resistance, R in kOhms, and Ag Tech reading, X, is given by

R = 16.8753/(1+(74.73901)*exp((-0.0340171)*X)) r² = 0.67, n = 177 (4)

Figure 2. The relationship between estimated sensor resistance and Ag Tech reading in a potato field.

Instrument performance in the onion field. Once again the soil water potential was closely related to Ag Tech readings in the range of -10 to -80 kPa (Figure 3). In the onion field the relationship between soil water potential in the onion field, S in kPa, and Ag Tech readings, X, is given by the relationship

S = -20160.24/(1+(6910.505)*exp((-0.0216288)*X)) r² = 0.84, n = 196 (5)

Figure
 
 
 
 
 
 
 
 
 
 
 
 

3. The relationship between soil water potential and Ag Tech reading in an onion field.

In the onion field, sensor resistance was closely related to Ag tech readings in the range of Ag Tech readings below 180 (Figure 4), similar to what was found for the sensors in the potato field. In the onion field the relationship between estimated sensor resistance, R in kOhms, and Ag Tech reading, X, is given by

R = 62.3945/(1+(187.6677)*exp((-0.02567363)*X)) r² = 0.85, n = 196 (6)
 
 
 
 
 
 
 
 
 
 
 
 
 

Figure 4. The relationship between estimated sensor resistance and Ag Tech reading in an onion field.

The cost effectiveness of this data reading, transmission, and logging system will depend upon cost, reliability, and convenience in controlling irrigations and providing useful information to the irrigation manager. Additional studies will be required to determine the temperature effects on the Ag Tech system.

1. These results demonstrated that the Ag Tech system is capable of reading Watermark Soil Moisture Sensors.

2. At the ambient soil temperatures 19 to 25 C, Ag Tech readings below 180 and comparable to soil water potential in the range of -10 to -80 cbars were closely related to Watermark Soil Moisture Sensors readings of soil water potential in the same range.

3. At the ambient soil temperatures 19 to 25 C, Ag Tech readings below 180 were closely related to estimated Watermark Soil Moisture Sensors resistance in the range of 600 to 23,000 ohms.

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

Eldredge, E.P., C.C. Shock, and T.D. Stieber. 1992. Plot sprinklers for irrigation research. Agron. J. 84:1081-1984.

Shock, C. C., E. B. G. Feibert, and L. D. Saunders. 1999. Irrigation Management for hybrid poplar production, 1997-1998. Oregon State University Agricultural Experiment Station, Special Report 1005. pp. 90-103.

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. Onion yield and quality affected by soil water potential as irrigation threshold. HortSci. 33:1188-1191.

Shock, C. C., E. B. G. Feibert, and L. D. Saunders. 1998c. 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. Zhang. 1993. The effect of timed water stress on quality, total solids and reducing sugar content of potatoes. Am Potato J. 70:227-241.

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Malheur Agricultural Experiment Station

Last updated  Tuesday July 30, 2002 .