Legumes
Soybeans Irrigation Guide
Soybeans runs a 135-day season with water demand peaking at a crop coefficient of 1.15. On a hot mid-summer day (ETo of 6 mm), that is about 6.9 mm per day, or 48 mm (1.9 inches) per week. Every number on this page comes from the FAO-56 standard and is exactly what RainRule schedules with.
Season length
135 days
20 + 30 + 60 + 25 by stage
Root depth
0.9 m
≈ 3.0 ft at maturity
Peak daily use
6.9 mm
at mid-season, ETo 6 mm/day
Depletion threshold
50%
of available water before stress
Crop coefficient (Kc) through the season
Multiply any day's reference evapotranspiration (ETo) by the Kc below to get Soybeans's actual water use that day.
| Stage | Length | Kc | What's happening |
|---|---|---|---|
| Initial | 20 days | 0.4 | Establishment: bare soil dominates, low water use |
| Development | 30 days | 0.4 → 1.15 | Canopy closes; demand ramps day by day |
| Mid-season | 60 days | 1.15 | Full canopy: peak water use, most stress-sensitive |
| Late season | 25 days | 1.15 → 0.5 | Maturation: demand tapers toward harvest |
Refill points by soil type
Total available water (TAW) is what your soil holds across Soybeans's 0.9 m root zone. Irrigate when the running deficit crosses the refill point (RAW), 50% of TAW for Soybeans, so the crop never enters stress.
| Soil | Holds (TAW) | Irrigate at (RAW) | Days of peak use per refill |
|---|---|---|---|
| Sand | 63 mm | 31.5 mm | ~4 days |
| Sandy Loam | 108 mm | 54 mm | ~7 days |
| Loam | 135 mm | 67.5 mm | ~9 days |
| Silt Loam | 153 mm | 76.5 mm | ~11 days |
| Clay Loam | 144 mm | 72 mm | ~10 days |
| Clay | 126 mm | 63 mm | ~9 days |
Sandy soils hold the least water, so Soybeans on sand needs small, frequent irrigations; clay holds the most and supports longer intervals with bigger applications.
Worked example: a peak-summer week
Take Soybeans at mid-season on a week averaging an ETo of 6 mm/day:
- Daily crop use: 6 mm × Kc 1.15 = 6.9 mm/day
- Weekly demand: 6.9 × 7 = 48.3 mm (1.9 inches)
- On loam (67.5 mm refill point), a full profile lasts about 9 days with no rain before irrigation is due.
Real weeks aren't averages: ETo swings with heat waves and rain changes everything. That's the gap between a rule of thumb and a live schedule.
Related legumes guides
Get a live schedule for your soybeans field
The free calculator combines these coefficients with your field's actual 7-day forecast. Or start a trial and let RainRule track your real water balance every hour.