Installing soil collars

To install a soil collar, lay a piece of wood across the collar and hit the wood using a hammer or mallet to drive the collar into the soil. Aim to hit the wood near the collar and move the wood to high spots as needed. In hard or compacted soils, you may need to create a channel around the collar with a knife or trowel before installing the collar. Check the collar to ensure it is level.

Note: After installing a soil collar it may take up to 24 hours for the soil gas flux to return to normal, undisturbed levels.

Insertion depth

Optimal collar height will depend upon site conditions and the length of time the collars will be used at a given site. At a minimum, the collar should be inserted into the soil to a depth that gives a solid foundation so the collar does not move when placing the chamber on the collar.

As insertion depth is increased, lateral diffusion of trace gases in the soil column below the chamber will be reduced. The advantage of this is that lateral diffusion can be a source of error in the measurement, but the disadvantage is that as insertion depth increases, the possibility of root shearing increases. Collars may become loose over time and should be moved if this occurs.

Collars should extend 3 cm or more above the soil surface. With greater extension there is increased shading and perturbation of air movement. Over the long term, these perturbations could result in changes of evaporation rate, soil temperature, and soil moisture.

Measuring the collar height

The collar height is used to the determine the volume of air inside the soil collar, which is in turn used to calculate the total system volume. Total system volume is an important parameter of the flux calculation and should be determined as accurately as possible. The collar height is measured by the distance between the soil surface and the upper edge of the chamber base plate. You will enter the collar height measurement into the chamber block on the Configuration page (see Configuring the LI-8250 Multiplexer block).

Figure 4‑1. Collar height is the distance from the soil surface to the top of the chamber base plate.

For the most accurate measurements, it is recommended that you measure collar height in at least 3-4 places and average these numbers. This is especially important when collars are installed in uneven or sloping soil.

Figure 4‑2. Collar height offset in sloped soil should be computed from an average of the offsets.

Making your own soil collars

Although some collars are included with your LI-8250 Multiplexer, you can create your own collars. We recommend making your own collars with materials that are widely available in the United States. In other countries you may need to special order materials.

  • Inside Diameter: 20.1 cm ± 0.03 cm
  • Outside Diameter: 21.34 cm
  • Height: 11.43 cm

LI-COR soil collars are constructed from thick-walled, 8-inch SDR 35 PVC pipe. Begin by cutting a length of pipe to approximately 11.43 cm. Then, use a grinder, coarse file, or large lathe to bevel the bottom edge to a 24° angle. The bevel is optional but does allow for easier insertion of the collar into the soil.