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For the lower soil profiles the interpretation is often not so clear, because the rate of
infiltration from the soil profile directly above may not be great enough to cause field
capacity to be exceeded. In that case, use the maximum soil moisture level
encountered as an estimate of the Field Capacity.
Laboratory Methods
Take an undisturbed soil core from each soil profile, and saturate. Allow to drain, and
when drainage has stopped, weigh. Measure the volume of each sample, and then
oven dry. Finally weigh again. The field capacity is equal to the difference in weights
of each sample divided by its volume.
In Europe “when drainage has stopped” is determined by placing the saturated
sample on dry soil or a wad of dry filter papers, and allowing it to drain for 2 hours. In
America, it is taken to be the soil moisture content when the soil water potential, φ
w
,
has fallen to –33kPa (- 1/3 bar).
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