Laboratory/Classroom
Gypsum Block Calibration
Question
We have a problem calibrating the soil moisture meter. The soil
moisture meter readings are showing very little variation over
a period of 2-3 weeks (the time we have been taking soil samples
from depths of 10 cm and 30 cm's for gravimetric analysis).
The gravimetric moisture percentages do show some variation;
apparently this is just an indication of experimental error.
With these results it is difficult for us to construct a
calibration curve. What should we do?
Solution
Construct a laboratory/classroom calibration curve
Materials Needed:
- Med. size clay or plastic pot
- Scale - .1 g accuracy, 5 kg capacity
- 105 degC Drying oven
- one gypsum block and soil moisture meter
Procedure:
- Fill pot with soil from near your site
- Plant gypsum block in center of soil mass
- Try to maintain the same soil density or degree of compaction as the field soil
- You might need to saturate soil; or just start with natually damp soil
- Find "wet weight" of pot, etc.
- Put aside to dry or place in an oven
- Reweigh and measure soil moisture periodically (every 2 hrs??; use hotpads)
- Continue to dry for at least 24 hours or until no weight change, this is
the "dry weight"
- Do 2-3 times to check consistancy
Analysis:
- Calculate water lost in each time period and plot against soil mositure
meter.
- Ask yourself if it makes sense to use this lab calibration in the field:
- Do your field calibration points (assumed wet) fall near this curve?
- Link to example (under development)
- Share this data with me (jwash@hwr.arizona.edu) for my opinion
Last updated: 1/3/97
Comments? globe@hwr.arizona.edu