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Soil sensors for irrigation scheduling

The use of soil sensors for irrigation scheduling is a common practice in irrigated agriculture. Soil sensors provide data of the soil water status. This is used to determine the irrigation amount and timing for crops. Because soil sensors provide important information to support crop management, it is extremely important to follow specific steps for soil sensor installation to guarantee good quality data. If a sensor is not well installed, the sensor will provide misleading information that will lead to inaccurate irrigation prescriptions.

Commercial Soil Sensors

There are many commercial soil water-sensors available. Most of the sensor companies also provide tools that facilitate soil sensor data interpretation to support irrigation decisions, such as smartphone apps and websites. Some examples of soil sensors are soil matric potential (Watermarks®) sensors (Figure 1a), capacitance sensors (Figure 1b), and time reflectometry-based (TDR) sensors (Figure 1c).

Each sensor has its advantages and limitations, and therefore a farmer or consultant should decide which type of sensor better fits their needs. For example, TDR sensors are well known to provide accurate volumetric water content data, but the installation process is more complex than soil matric potential or capacitance sensors.

Soil Matric Potential-Based Sensor

Figure 1a. Soil matric potential-based sensor. Commercial sensor: Watermark®

Capacitance-based sensor

Figure 1b. Capacitance-based sensor. Commercial sensor: AquaSpy

Time domain reflectometry-based sensor.

Figure 1c. Time domain reflectometry-based sensor. Commercial sensor: Acclima True TDR

Basic Soil Sensors Installation Considerations

Several factors should be considered when installing soil sensors.

Soil Depth

Because soil sensor data is used to support irrigation decisions, the soil depth selected to be monitored should reflect the type of crop that will be irrigated and where the majority of the root system resides.

Root length growth and biomass increases with growth stage. Because of this, having soil sensors at various soil depths could provide data of soil water changes and plant water uptake changes throughout the growing season.

Early in the season, soil sensors should monitor one-third the depth of the root zone. However, at the peak of crop water use, soil sensors should monitor at least two-thirds of the crop root zone.

Location

The selection of the location where the sensor will be installed is critical. Within- field variability in soil type and/or terrain elevation is common in many crop fields. If significant within-field variability exists, the farmer or consultant should install more than one sensor in the field.

If the farmer decides to use only one sensor per field, then the sensor should be installed on a location that represents the field conditions of the majority or most of the crop planted area. For example, some farmers tend to install a sensor at the highest point on the field because it dries out faster. Does this high terrain elevation location represent the majority of the field conditions? It is important to answer that question because a soil sensor installed in the wrong place could result in over or under watering.

Hills, valleys, or high slopes on a field are extreme field growing conditions that do not always represent the majority of the conditions present on the field. If a farmer has rolling terrain fields, they should avoid installing the sensor on a water-way because either surface or sub-surface flow on those locations will impact your soil sensor readings.

Selecting a homogeneous spot in the field where plants are not stunted is important. Soil sensors should be installed between two plants within a crop row. During the sensor installation process, it is important to prevent damage to plants that will be next to the sensor because this will certainly impact soil sensor readings.

Key Steps

Soil irrigation sensor in a corn field..The following are some key steps growers should follow when installing a sensor:

  • Select a location that represents the majority of the crop growing conditions on the field.
  • Install the sensor in between two plants within the crop row.
  • During sensor installation, avoid damaging the plants that will be in close proximity to the sensor.
  • Make sure there is a good soil to sensor contact.
  • Avoid air pockets between the soil and the sensor.
  • Avoid installing the sensor on a water-way.
  • Check the sensor’s manufacture instructions for installation. Some sensors, especially soil matric potential sensors, require that they be placed in water for 24 hours prior to installation.

Installation Steps and Tips

Although the method of installation changes depending of the soil sensor type, below are installation steps and tips that can be used to improve the soil sensors installation process for the most common sensors used in Alabama irrigated fields.

Watermark and Capacitance Sensors Installation Guidelines

The installation process for these two types of sensors are similar. The steps below can be used for these two sensors. The main difference is the probe size (24 inches for the Watermark sensor and 48 inches for the Capacitance sensor).

Step 1

In between two healthy plants, dig a hole with a diameter slightly bigger than the sensor diameter (2.5 inches for AquaSpy) and the depth of the deepest target depth for monitoring (48 inches for AquaSpy). The hole can be opened using a manual auger or engine powered auger. Be careful opening an oversized hole. Air gaps will result in incorrect data.

Step 1 and 2 Sensor installation

Step 2

Fill a bucket with the soil extracted from the hole and mix it with water to create a slurry.

  • Tip 1. Avoid particles such as small rocks and plant residues.
  • Tip 2. A think slurry might create air gaps close to the sensor. A thin slurry might crack more easily if the soil dries out, creating air gaps and preferential water flow.

Step 3

Pour the slurry up to one third of the hole depth.

Step 3 Sensor installation

  • Tip 1. If the slurry is too thick, the bottom of the hole will be compacted by the probe.
  • Tip 2. If the slurry is too thing, the slurry around the sensor will certainly crack as the soil dries out, creating air gaps.
  • Tip 3. For light soil with high sand content, pour the slurry after inserting the sensor.

Step 4

Insert the probe into the hole

Step 4 Sensor installation

  • Tip 1. Make sure there is excess slurry coming from the hole while the probe is inserted.
  • Tip 2. Shake the probe firmly while inserting it into the hole. This helps remove small air gaps, especially for heavy soils.
  • Tip 3. Insert probe fully into the ground.

Step 5

Remove excess slurry.

Step 5 Sensor installation

  • Tip 1. If necessary, insert a thin object through the slurry to remove air gaps.
  • Tip 2. After removing the excess slurry, put dry soil on the topsoil close to the hole. It will prevent soil cracking and preferential flow or water through the hole.

Time Reflectometry-Based Sensor Installation Guidelines

Step 1

Open an 8 inch diameter pit using an engine-powered auger.

Step 1: Open a 8 inch in diameter pit using an engine-powered auger.

  • Tip 1. The wall of the pit should be 2 to 3 inches apart from the the plants to avoid damaging the root systems.
  • Tip 2. A lot of soil could come out from the pit when digging the hole. This can easily damage the plants. Cover the plants with 5 gallon buckets to prevent them from breaking.
  • Tip 3. Prevent compacting the loosened soil coming out of the pit. It will facilitate the soil reconstruction process.

Step 2

Insert the sensor horizontally at different depths with respect to the soil surface and parallel to the crop row.

Step 2. Insert TDR sensors horizontally at different depths with respect to the soil surface and parallel to the crop row.

  • Tip 1. Use an installation guide provided by the manufacturer to prevent the sensor rods from bending and to ensure the rods are evenly separated during installation. Sensor reading will be affected if the rods get bent.
  • Tip 2. Sensors should be installed from the bottom to the top.
  • Tip 3. Repack the soil using the same soil order/type extracted from the hole.
  • Tip 4. To prevent from preferential water flow that could interfere with sensor readings, position the cables on the opposite side of the pit from where the sensor was installed. This can be done while repacking the soil.

Note. Sometimes TDR sensors are installed at deeper depths than what is shown in the images below (30 and 36 inches). If that is the case, it is recommended to use a post hole digger or a manual auger to reach the target depth. Sensors at deeper depths should be installed vertically.

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