Field Capacity
Also called: FC, water holding capacity
Field capacity is the amount of soil moisture held in a soil after excess water has drained by gravity — typically 24–72 hours after saturation. It is the upper limit of plant-available water. Sandy soils reach field capacity at roughly 10% volumetric water content; clay soils at 35–45%. Field capacity is the baseline from which irrigation scheduling and drought risk are measured.
How Field Capacity Works
Soil water exists on a continuum from saturation (all pores filled with water) to permanent wilting point (water bound so tightly that roots cannot extract it). Field capacity sits near the top of this range: the soil is holding all the water it can against gravity, but air has returned to the large pores so roots can breathe.
Numerically, field capacity corresponds to a soil-water tension of about 10–33 kPa (-0.1 to -0.33 bar). Water content at field capacity depends on soil texture and organic matter: sandy soil (loose, large pores) holds 8–15% water at FC; loamy soil holds 18–28%; clay holds 35–45%. Plant-available water is the difference between field capacity and the permanent wilting point (tension -15 bar).
Irrigation scheduling uses field capacity as the target. A typical rule is to irrigate when soil moisture drops to 50–70% of plant-available water — waiting longer induces stress, irrigating earlier wastes water. Modern irrigation controllers combine soil moisture sensors, field capacity reference values, and crop-specific depletion thresholds to deliver water only when needed. This practice (deficit irrigation scheduling) can reduce water use 20–40% while maintaining yield. WiseYield's irrigation scheduler accepts field-capacity inputs (from soil tests or regional datasets) and computes optimal irrigation windows automatically.
Sources
- USDA NRCS National Engineering Handbook — irrigation guide, chapter 1.
- FAO-56 Irrigation and Drainage Paper — allowable depletion guidance.