How do I find my soil field capacity and wilting point?

Soil testing labs can measure these values directly. For estimates, use USDA soil texture classes: sandy soils have FC around 10-15% and WP around 5-8%, while clay soils have FC around 35-45% and WP around 20-25%. Loam soils fall between these ranges.

What MAD percentage should I use?

Use 50% MAD for most field crops (corn, wheat, cotton). Use 30-40% for vegetables and shallow-rooted crops that are sensitive to water stress. Fruit trees can often tolerate 50-65% MAD depending on growth stage. During flowering and fruit set, reduce MAD to avoid yield loss.

Why does root zone depth matter so much?

Deeper root zones access more stored water, which extends the interval between irrigations. A crop with 0.3m roots may need irrigation every 2 days, while the same crop with 0.9m roots might go 6 days. Encouraging deep root growth through proper irrigation management reduces irrigation frequency.

Soil Moisture Depletion Calculator — Irrigation Scheduling from Soil Properties

Calculate when to irrigate and how much water to apply based on your soil field capacity, wilting point, root depth, and daily crop water use. This calculator determines total available water (TAW), readily available water (RAW), and the optimal irrigation interval so you can prevent crop stress without over-watering.

Inputs Explained

Field Capacity (FC): Soil moisture content after drainage stops, expressed as a volume percentage. Sandy soils: 10–15%, loam: 25–35%, clay: 35–45%.
Permanent Wilting Point (WP): Moisture level at which plants can no longer extract water. Sandy: 5–8%, loam: 12–18%, clay: 20–25%.
Root Zone Depth: Effective rooting depth of your crop in meters or feet. Deeper roots access more stored water and extend irrigation intervals.
Management Allowable Depletion (MAD): Percentage of TAW you allow to deplete before irrigating. 50% for most field crops; 30% for stress-sensitive vegetables.
Daily Crop ET: Daily crop water use in mm/day — from ET₀ × Kc. Drives the rate at which soil moisture declines between irrigations.

How This Calculator Works

Based on: TAW = (FC − WP) × root depth; RAW = TAW × MAD; Interval = RAW / daily ET

Best for: Determining irrigation frequency and application depth for any soil-crop combination

Check locally: Get FC and WP from a soil testing lab or use USDA soil texture class estimates as a starting point.

Units supported: Metric (mm, m, %vol), Imperial (in, ft, %vol)

Worked Example

Scheduling irrigation for mid-season corn on a silt loam soil

1. Enter soil properties
FC = 30%, WP = 15% — typical silt loam values.
2. Set root depth
0.9 m for mid-season corn with established root system.
3. Calculate TAW
TAW = (30 − 15) / 100 × 0.9 × 1000 = 135 mm.
4. Set MAD and calculate RAW
MAD = 50%. RAW = 135 × 0.50 = 67.5 mm.
5. Determine interval
Daily ET = 7 mm/day. Interval = 67.5 / 7 = 9.6 days.

Irrigate every 9–10 days with approximately 68 mm of water to refill the root zone to field capacity.

How to Interpret Your Results

Condition	What It Means
Irrigation interval < 3 days	Very frequent irrigation needed — shallow roots, sandy soil, or extreme ET. Consider drip irrigation.
Irrigation interval 3–7 days	Moderate frequency — standard for sprinkler systems on most soils.
Irrigation interval 7–14 days	Infrequent irrigation — deep roots on water-retentive soil. Ensure system can deliver the larger application depth.
RAW < 20 mm	Very limited soil water buffer. Soil moisture sensors are strongly recommended to avoid stress between irrigations.

Common Mistakes to Avoid

Using textbook FC/WP without verifying for your actual soil

Request a lab analysis of FC and WP, or at minimum confirm soil texture class with a hand-texturing test.

Using mature root depth for young crops

Seedlings may only have 0.15–0.30 m roots. Increase root depth gradually through the season as the crop grows.

Setting MAD too high for sensitive crops

Vegetables and shallow-rooted crops can be stressed at 50% depletion. Use 30–40% MAD during flowering and fruit set.

Related Calculators

ET₀ Calculator

Calculate reference evapotranspiration using the FAO-56 Penman-Monteith equation for irrigation planning

Irrigation Water Requirement Calculator

Calculate crop water needs from ET₀, crop coefficient, and irrigation efficiency

Effective Rainfall Calculator

Calculate effective rainfall using the USDA-SCS method to determine irrigation reduction

Related Guides

Irrigation Scheduling: When and How Much to Water

Master irrigation timing using soil moisture, MAD thresholds, and crop water use to prevent both over- and under-watering.