Capacitance probe (R. McGlynn)

Principle/Description

A capacitance probe is a device for measuring soil water content. It consists of a number of sensors mounted on a vertical probe column which is installed in the soil via a waterproof access tube.

Each sensor consists of an oscillator and a number of electrodes—either two circular rings (see image, left) or an array of parallel metal spikes that form a capacitor. The soil adjacent forms the dielectric of the capacitor, which completes an oscillating circuit.

Principle

A capacitance field is generated between the two rings of each sensor. This field extends into the soil adjacent the sensor, and is affected by the soil's dielectric. The soil dielectric is affected by water content—changes in water content cause a change in the frequency of the oscillating circuit which can be related to actual soil water content via a calibration curve.

Method

Three capacitance probes were installed in the study area—two were located in the cultivated area and one in the pasture. Measurements were taken at 20, 30, 40 and 60 cm depths on each capacitance probe.

Measurements were taken during the afternoon on day 1 of the field trip and in the morning and afternoon on days 2-4 of the field trip.

Analysis

The data set contains values for each depth at the periods specified in the method (above), at each probe. A preliminary observation of the data was undertaken. Time series of relative water content with depth were graphed for each probe. The graphs are displayed below (Click to enlarge).

Results

As can be seen from the graphs above, the pattern of changing moisture content with depth displayed minimal consistency between probes. All soils showed oscillating moisture content at each particular depth throughout the week. We can see here that the range of observed water contents was quite broad across all three probes, though the range of observed water contents at each probe generally did not overlap. The only exception was the overlap of the 60 cm values at the cultivated area "A" probe with those at 60 cm depth at the cultivated area "B" probe.

At probes A and B (cultivated area), the soil tended to be wetter in the morning than in the afternoon, and soil water content tended to increase throughout the week. The soil at probe B was wetter than at probe A. At probe B, the soil was more wet towards the surface, and became dryer at depth. The variation in water content over time was much greater at probe A than at probe B.

In the pasture, the trends were opposite: the soil generally becomes dryer with time, and the soil tends to be wetter in the afternoon than in the morning. The soil surface is dryer than the soil at depth; however, once the soil reaches its maximum water content at about 30 cm, it tends to become dryer with increasing depth. The pasture soil is dryer than the soil in the cultivated area.

The effects of this measurement technique on the agroecosystem are minimal and localised. A bore has to be dug for the probe access tube, but there is no further disruption of the environment once the device is installed.

Problems

• Requires calibration in the field for the most precise results
• Expert knowledge of soil, crop and management at the site required for the optimal set-up of each probe
• Concern about shrink-swell soils creating air gaps next to the access tube, which gives adverse readings. Air-gaps around the access tube need to be eliminated for the most accurate measurement.

References

Agrilink C-Probe Soil Moisture Probe

Atkins, Ronald T.; Pangburn, Timothy; Bates, Roy E.; Brockett, Bruce E., 1998. Soil Moisture Determinations Using Capacitance Probe Methodology. COLD REGIONS RESEARCH AND ENGINEERING LAB, HANOVER NH.

Capacitance sensors (FDR)

Mead, R., 2004. Capacitance probes.