Gravimetric soil water

The gravimetric water content is usually defined as the mass of water divided by the mass of dry soil. It is experimentally measured by oven-drying the soil and comparing the beginning and ending mass.

Equipment

  • Soil samples, at least 10 g or so per sample
  • Drying oven
  • Analytical balance (1 mg precision)
  • Aluminum weigh tins
  • Metal tray

Procedure

  1. Work in the sedimentation laboratory. Set the drying oven to 105 degrees C.
  2. If the aluminum weigh tins are unlabeled, put a unique label (the "tin number") on each with a sharpie.
  3. For each soil sample:
    1. Weigh an aluminum tin. Record the tin number and the tare weight of the tin.
    2. Add about 10 g of soil to the tin. Record the exact mass of the tin plus soil, then remove the sample to a tray.
  4. Put the tray of samples in the oven. Dry overnight, at least 24 hours.
  5. Take out the samples and weigh again, recording the mass of the tray plus dry soil.
  6. Return the tray to the oven for a few more hours. Weigh them again.
  7. If the mass is unchanged from the previous step, you are finished. If they have dried more, repeat this step until the soil does not dry out any further.