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Field capacity, available moisture definitions

Short answer

Field capacity (FC) is moisture after drainage (≈1/3 atm). Available moisture AM = FC – PWP. Readily available moisture RAM is the 50–75% easily extracted fraction. Soil water deficiency is the depth needed to restore root zone to FC.

Define the terms field capacity, available moisture, readily available moisture and soil water deficiency.

  • Field capacity (FC): the amount of water held in the soil after the excess gravity water has drained away (usually 1-3 days after irrigation or rain); soil moisture tension is about 1/3 atm. It is the upper limit for irrigation.
  • Available moisture (AM): the water held between field capacity and permanent wilting point, which plants can use: AM=FC−PWPAM = FC - PWP.
  • Readily available moisture (RAM): the fraction of available moisture (about 50-75%) which crops extract easily without moisture stress.
  • Soil water deficiency (moisture deficiency): the amount of water required to bring the soil moisture of the root zone back to field capacity at any time; =FC−present moisture= FC - \text{present moisture} (expressed as depth, mm). It is the net depth to be applied in irrigation.

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