Soil moisture tension curves

Principle:

Measuring the soil water content at several known water potentials allows you to construct a curve showing the ability of the soil to hold plant-available water.

Equipment:

  • Core rings of the type for measuring bulk density, one per sample
  • Rubber mallet
  • Filter paper
  • Rubber bands
  • Balance (Note: balance in sample prep room maxes out at 300 g.  Old balance in stream table room can handle more.)
  • Field notebook
  • Pans with rims
  • Soil moisture cans or small widemouth mason jars 1/3 full of sand (you can find bags of sand in the sedimentation laboratory)

Procedure:

Before you sample:
  1. Be sure each brass core ring has a unique label (e.g. "A", "B", etc.)
  2. For each brass core ring, assemble two pieces of filter paper (or you can use a paper towel - be careful not to tear it) and two rubber bands to secure the paper to the ring.  
  3. Weigh the ring-filter paper assemblies and record their labels and masses in your notebook.
In the field:
  1. At the field site, remove the O horizon.
  2. Carefully hammer the cylinder straight into the soil with the rubber mallet until the soil is flush with the top of the soil (Fig. 1).  Carefully collect the ring with the soil intact.
  3. Cover the bottom and top of each core ring with the filter paper and secure it with the rubber bands.
  4. Carefully transport the rings back to the lab for analysis.
Back in the lab:
  1. Weigh the brass ring assemblies full of soil.  This will give you the water content at the time you sampled. 
  2. Put the ring assembly with the soil inside into a pan.  Pour water into the pan (not the ring) until the water level in the pan is about 1 cm below the height of the top of the soil inside the ring.  Let the sample sit in the water for 15 minutes while the water moves upward through the soil, filling the pore space.
  3. Weigh a soil moisture can with sand and record the weight.  Quickly take a ring assembly and put it into the can, and record the weight.  This will let you calculate the water content of the soil at saturation.
  4. Put the cans containing the ring assemblies somewhere out of the way and allow them to drain.  After two days, return to the lab and reweigh the cans, recoring the results.  This will allow you to calculate water content at field capacity.
  5. Put the ring assemblies on a tray and put them in a drying oven at 105 degrees C.  After two days, return to the lab and take the tray out of the oven.  Immediately, before the samples absorb water from the air, weigh the ring assemblies and record the weight.  This will allow you to calculate the dry mass of the soil.
  6. Allow the soil to dry sit in the lab for three days, absorbing water from the air.  This will let you calculate the water content of the soil at the hygroscopic coefficient.
  7. At the end of the experiment, put your sand into the stream table.
Figure 1: Collecting a reasonably-undisturbed sample with a rubber mallet and a brass ring.


Figure 2: Safety first!


Figure 3: Samples ready for analysis
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