How to Calculate the Field Capacity of Soils

Knowing how to calculate the field capacity of your soil in the right way is important for determining what kinds of crops you can grow, how much you will have to irrigate and how often. Field capacity is the amount of water that is left in the soil after all of the free water has been drained. There are different ways to calculate field capacity with varying degrees of accuracy. The simplest method is by hand.

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Instructions

- 1
  Take four to eight random samples from the top 12 inches of the soil.
- 2
  Determine the texture of your soil by working a small moist ball of the soil between your thumb and forefinger. If the soil feels gritty it has a moderately coarse texture. If the soil feels like wet dough it has a medium texture (more silt than sand). If it feels mostly sticky it is a fine textured soil and has lots of clay.
- 3
  Squeeze a ball of the soil in your hand. Depending on the texture you determine how much water is in the soil. For any textured soil when you squeeze it and no water drips out of the ball but your hand is wet, then the soil is at field capacity. This means that it will require no additional water to reach field capacity. Repeat for each sample.
- 4
  Weigh out about a pound of one of your samples that was at field capacity.
- 5
  Dry the soil in an oven on low heat.
- 6
  Weigh the dry soil and subtract its weight from the wet weight to calculate the percentage of water in the soil when it is at field capacity. With this information you can estimate how much water to add to your soil when it is dry to get it to field capacity.

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