Growers need easy and convenient ways to monitor soil moisture status to improve their irrigation scheduling. We examined the AM400 Soil Moisture Data Logger with Graphic Display (M.K. Hansen Co., East Wenatchee, WA) to see if it would aid irrigation scheduling using data from Watermark Soil Moisture Sensors (Irrometer Co., Inc., Riverside, CA). For simplicity, we refer to the AM400 Soil Moisture Data Logger with Graphic Display, as "Hansen unit". Each Hansen unit was wired to six Watermark Soil Moisture Sensors and one temperature probe. Hansen units were installed in 14 crop fields as aids to irrigation scheduling during the 2000 season. The practical usefulness of the loggers and their data are presented.

Crop yields and quality in Malheur County Oregon are directly related to the quality of irrigation management. Watermark Soil Moisture Sensors have been used for managing soil water in potatoes and irrigation scheduling by growers in Malheur County since the late 1980's and that use has expanded to onions and other crops. We have shown that onion yield and grade and the growth of poplar trees are closely related to irrigation scheduling and maintenance of soil water potential (SWP) within narrow bounds.

Watermark readings have been recorded manually with a 30 KTCD meter (Irrometer Co., Inc., Riverside, CA) , transferred to computer files, and graphed manually or by computer to demonstrate whether the SWP was wetter or drier than the irrigation criteria for the particular crop. It is easier for the grower to see the SWP in graphical form, because the relative position (wet or dry) is clearer and the rate of drying over time makes more sense as a graph. Hansen units were tested for ease of interpretation for irrigation scheduling at the Malheur Experiment Station and in growers' fields.

Set up in 2000

Six granular matrix sensors (GMS, Watermark Soil Moisture Sensors, (Irrometer Co., Inc., Riverside, CA) were installed at 10- or 12-inch depth (depth to the bottom of the sensor) in the crop rows in 14 fields (Table 1). The GMS were installed directly in the center of the crop rows for furrow-irrigated and drip-irrigated onions and sugar beets. For potatoes, the sensors were located at 10-inch depth, and between two plants, 4 inches off of the center of the hill. Sensors were installed with the aid of a 7/8-inch-diameter soil probe. The sensor was pressed to the bottom of the soil probe hole with an insertion wire, 2 oz of water were poured into the hole, soil was gently packed above the sensor, and the soil left level with little trace of installation except the wires coming out of the soil.

An additional 50 to 125 ft of wire was added to each of the GMS before installation and attachment to the Hansen units. This extra wire allowed the grower to spread the sensors over a wider area of the field. Insulation was stripped off of the GMS wire and the GMS was connected to an 18-gauge wire using a butt connector adapter (4*260-5,3M Highland) and shrink tubing, (3KH56-7, W.W. Grainger). The other end of the wire was connected to the Hansen unit. Six GMS and one temperature probe were connected to the Hansen unit starting at the double portal reading no. 1 and finally the temperature probe was connected to portal no. 7.

The Hansen units were mounted on 4- by 6-inch posts, and set facing to the north. The posts themselves were placed in an area that was judged to be representative of the entire field.

The SWP from sample fields is presented below. The SWP irrigation criteria for alfalfa forage on silt loam is in the range of -60 kPa. Regular use of sensor readings allowed the average SWP to remain within the ideal range most of the time (Fig. 1). The frequency of irrigation depended on the weather and the stage of growth of the alfalfa.

The SWP criteria for drip-irrigated onions on silt loam is in the vicinity of -20 kPa. This criteria was rather carefully followed in a grower's drip-irrigated onion field (Fig. 2). The crop was maintained too wet at the beginning and end of the season.

The irrigation criteria for furrow- or sprinkler-irrigated potatoes on Malheur County silt loam is -50 to -60 kPa. This criteria can be closely followed (Fig. 3), but notice that at the end of the season the crop was irrigated before it reached its criteria. This systematic irrigation error resulted in tuber decomposition before harvest.

The use of drip irrigation for sugar beets is entirely experimental. No irrigation criteria has been established (Fig. 4). If the criteria is in the range of -30 to -40 kPa, the crop was maintained too wet.

Reading for Irrigation Scheduling

Since the Watermarks are already wired to the Hansen unit, the reading of the sensors is very rapid. The outside cover was removed, and the red button, located in the center of the unit, was pressed. After the button is pressed, the screen will show the data for Watermark sensor no. 1, including the temperature and the SWP in centibars or kilo Pascals (1 cbar = 1 kPa). Data from the last 5 weeks is displayed on the screen. The most recent logged data is on the right side of the screen. The lower the point is on the screen, the drier the soil.

Take careful note of the left hand side of the screen. The magnitude of the scale changes with the range of the data.

To read the graphs and instantaneous data for the other five Watermark sensors, continue to press the red button; each sensor is read in turn. When all the sensor have been scrolled through, replace the cover. The unit will turn itself off.

If you wish to collect all the data for a season, the data can be collected using a laptop computer or palm pilot. For instructions for use of a palm pilot please see

http://www.cropinfo.net/downloads/soilwater.html.

Table 1. Hansen units were installed in 14 crop fields during the 2000 crop season. Malheur Experiment Station, Oregon State University, Ontario, OR.

Crop	rrigation system	Location	Depth to the bottom of the 5 or 6 shallow sensors ---inches---	Depth to the bottom of the single deep sensor ---inches---
1. Alfalfa	Sprinkler	MES Field B2	12	NA
2. Alfalfa	Sprinkler	MES Field A2	12	NA
3. Potatoes	Furrow	MES Field D-1a	10	NA
4. Onions	Drip	Skyline Farms	10	14
5. Potatoes	Sprinkler	Gressley Farms	10	14
6. Onions	Furrow	MES B-8b	10	14
7. Wheat	Drip	Ontario Farms	10	NA
8. Sugar beets	Furrow	MES B-8a	10	14
9. Potatoes	Furrow	MES D-1a	10	14
10. Onions	Drip	Ontario Farms	10	14
11. Sugar beets	Drip	Ontario Farms	10	14
12. Potatoes	Sprinkler	Teramura Farms	10	14
13. Onions	Drip	Komoto Farms	10	14
14. Onions	Drip	DeBoer Farms	10	14

Figure 1. Soil water potential at 12-inch depth in sprinkler-irrigated alfalfa as measured by 6 GMS and recorded by a Hansen unit, Malheur Experiment Station, Oregon State University, Ontario, OR 2000.

Figure 2. Soil water potential at 10-inch depth in drip-irrigated onions as measured by 6 GMS and recorded by a Hansen unit in a grower's field, Malheur Experiment Station, Oregon State University, Ontario, OR 2000.

Figure 3. Soil water potential at 10-inch depth in a furrow-irrigated potato field as measured by 6 GMS and recorded by a Hansen unit, Malheur Experiment Station, Oregon State University, Ontario, OR 2000.

Figure 4. Soil water potential at 10-inch depth in drip-irrigated sugar beets as measured by 6 GMS and recorded by a Hansen unit in a grower's field, Malheur Experiment Station, Oregon State University, Ontario, OR 2000.

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Last updated  Friday August 13, 2004 .