PR2 Multi-Depth Soil Moisture Profile Probes

The PR2 Series Profile Probe measures volumetric soil moisture content at multiple depths using patented electromagnetic field sensing technology. The probe operates by generating a 100 MHz signal applied to pairs of stainless steel rings, creating an electromagnetic field that extends approximately 100 mm into the surrounding soil. The field response is dominated by soil permittivity, which is directly related to water content.

In commercial agriculture, the PR2 enables precision irrigation management by providing continuous or spot-check data on water depletion across the root zone. Growers can schedule irrigation based on actual soil moisture status rather than calendar days or visual crop cues, reducing over-watering and under-watering. The multi-depth measurement reveals water extraction patterns at different root zone layers, allowing growers to identify deep percolation losses, shallow rooting issues, or compacted layers restricting water movement. Typical applications include center pivot and drip irrigation management for row crops, orchards, vineyards, and high-value vegetable production.

Construction and Design

The probe consists of a sealed polycarbonate rod with pairs of stainless steel sensor rings spaced at fixed intervals along its length. The rod diameter is 25.4 mm. The probe is inserted into a permanently installed access tube and achieves IP67 environmental protection when installed. The sensor rings are recessed into the probe body, and the electronics are potted to prevent moisture ingress.

The access tube is constructed of fiberglass and sealed with a tapered plug and rubber cap. Correct installation is essential for accurate measurement. The augering kits are designed to produce a straight, smooth-sided hole of 27 mm diameter, slightly smaller than the 28 mm inner diameter of the access tube, ensuring optimal soil-to-tube contact. A stabilisation plate keeps the auger vertical, preventing conical enlargement (funnelling) that can cause poor contact in the top 30 cm of soil. An insertion rod drives the access tube into the augered hole by applying force to the bottom of the tube via hammering, reducing flexing that can produce air gaps.

Measurement Specifications

Sensor Depth Configurations

Access Tube Specifications

Augering Starter Kits

The short augering starter kit includes 24 mm pilot auger, stabilisation plate, and access tube insertion rod. It is suitable for access tube installation up to 40 cm depth in normal soils. For dry, sandy soils, a 25 mm spiral auger should be ordered in addition.

The long augering starter kit includes all items in the short kit, plus finishing auger and mallet. The finishing auger is adjustable and expands pilot holes to the exact diameter required for an access tube, producing straight, smooth-sided holes in most soil types. The long kit is required for installation up to 100 cm depth. For dry, sandy soils, a 25 mm spiral auger should be ordered in addition.

Power Requirements

Output and Interface

The SDI-12 variant provides digital serial output compliant with SDI-12 protocol version 1.3. Power consumption in SDI-12 mode is less than 60 mA at 12 V DC during measurement. The probe includes generalized calibration curves for mineral and organic soil types. Individual sensors are interchangeable, with recalibration advised every three years depending on use.

Multiple probes can be connected to a compatible data logger via a single cable, enabling low-cost sensor networks. Up to 50 six-depth probes or 62 four-depth probes can be connected to a single GP2 SDI-12 data logger, subject to cable length and power requirements. Analog and SDI-12 sensors cannot be mixed on the same cable system. The probe uses an M12 x 5-way stainless steel IP67 connector. The M12 x 5-way cables are also compatible with ML3, SM150T, and EQ3 sensors.

Handheld Moisture Meter

The HH2 Moisture Meter is a versatile readout unit that displays and stores readings from PR2 SDI-12 Profile Probes. It enables portable use of a single probe moved across multiple access tube locations, allowing large amounts of soil moisture data to be collected at multiple sites without installing loggers at each location. The meter stores up to 1500 time-stamped readings. It achieves IP54 environmental protection and includes RS232 output. Dimensions are 125 x 80 x 45 mm.

Connection Cable

The 0.9 m connection cable links the SDI-12 Profile Probe to the HH2 handheld moisture meter. The cable is shielded and terminated with 5-pin connectors on both ends. For probe-to-logger connections, separate cables are available including 1.5 m cable with M12 to 25-way D-socket (to HH2), 5 m cable with M12 to bare wire (to third-party loggers), and extension cables in 5 m, 10 m, and 25 m lengths that can be connected together to create custom lengths.

Field Applications in Commercial Agriculture

The PR2 supports both portable readings across multiple access tube locations and long-term continuous monitoring at a single installation. The system is suitable for:

• Irrigation scheduling: Measure soil water depletion before and after irrigation events to determine application depth and timing

• Variable rate irrigation: Map moisture variability across fields to inform prescription maps

• Crop water use monitoring: Track extraction patterns at different root zone depths throughout the growing season

• Leaching assessment: Detect moisture movement below the root zone indicating deep percolation

• Soil compaction identification: Observe restricted water movement between depth increments

• Salinity management: Function in sites with pore water conductivity up to 400 mS·m⁻¹ (4 dS·m⁻¹)

Installation Notes

Access tubes require an installation hole of 27 mm diameter, allowing easy installation with minimal soil disturbance. Access tubes are manufactured to strict tolerances and are durable in soil. The augering starter kits are designed for normal soils. Installation may not be possible in very stony soils, or where a site features compaction, voids, foreign bodies, or soil instability. For dry, sandy soils, a 25 mm spiral auger should be ordered as a supplement. An optional extraction kit is available for removing installed access tubes from the ground.