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How to Calculate Moisture Content

By Denise Bertacchi ; Updated September 21, 2017

The amount of moisture found in garden soil can vary depending on several factors. The type of soil, the climate and amount of organic material (humus) will all play a part in the moisture content of garden dirt. A gardener should be aware of the moisture content of the soil in order to take better care of his or her plants. There are tests and tools that gardeners can use to calculate the moisture content of the soil.

Hot plate test

Take a sample of garden soil and place it in a coffee filter. Weight the sample on an accurate kitchen scale. Make a note of the weight. You may wish to add more soil to the sample until the weight reaches a number easy to work with, like five ounces.

Place the sample on a hot plate and set to 225 degrees. Allow to dry undisturbed for 24 hours.

After 24 hours, weigh the dried sample of soil on the kitchen scale. Compare the new dry weight to the old damp weight. Calculate the moisture content by dividing the dry weight by the damp weight to find the percentage weight of the soil. The percent missing is the moisture. For example: if the damp soil weighed 5oz and the dry soil weighed 3oz, divide 3 by 5, which equals 60 percent. This is the percent of weight of the SOIL. Subtract 60 percent from 100 to find the weight of the missing water, which is 40 percent.

 

Things You Will Need

  • Sample of garden soil
  • Coffee filter
  • Kitchen scale that measures in ounces
  • Hot plate
  • Calculator

Tips

  • Moisture meters can be purchased at your local garden center. These inexpensive devices can give you a fairly accurate reading of moisture content.
  • Simply placing a finger about an inch into the plant's soil can help you gauge the moisture content. If the soil feels dry at a depth of one inch, the plant needs watering.
  • Plants in clay pots will need to be watered more often because the porous nature of the clay, which draws out moisture. Save on watering by using non-porous pots, like enameled clay, metal or plastic.
  • Small pots can be spot checked for moisture level by estimating their weight. Pick up the pot, if it feels lighter than normal, the soil is dry.