Soil Type & Available Water Capacity: Choose the Right Soil in Your Irrigation Plan
Understanding Available Water Capacity (AWC)
Available Water Capacity (AWC) refers to the volume of water stored in soil that plants can actually extract. It is calculated as the difference between Field Capacity (FC), when soil holds the maximum water after drainage, and the Permanent Wilting Point (PWP) below which plants cannot recover.
AWC = FC - PWP
This water is typically expressed in inches per foot of soil or as a fraction per inch. Knowing AWC helps you determine how much water soil can hold and when you need to irrigate.
AWC Ranges by Soil Texture
Here are typical AWC values (inches per foot) by soil type, data based on USDA NRCS and extension services.
| Soil Texture | Typical AWC (inches/ft) |
|---|---|
| Coarse Sand | 0.1–0.4 in (≈ 0.25) |
| Sandy Loam / Fine Sand | 0.6–0.8 in (≈ 0.75) |
| Loam | 1.9–2.2 in (≈ 2.0) |
| Silt Loam / Clay Loam | 2.3–2.5 in (≈ 2.4) |
| Clay | 1.7–1.9 in (≈ 1.8) |
This table helps clarify how soil type affects how much water is held.
Why AWC Matters in Irrigation
Water Retention Varies Widely: Clay loam stores significantly more, over twice as much, as sandy soil.
Irrigation Frequency: Soils with low AWC dry out faster and need more frequent watering. High-AWC soils allow longer intervals.
Avoid Waste: Knowing AWC prevents overwatering in high-retention soils or under-watering in sandy soils.
How Irrigation Calculator Pro Uses AWC for Smart Watering
The Irrigation Calculator Pro app uses AWC as follows:
Select Your Soil Type: Choose from options like sandy loam, loam, clay loam, etc.; the app auto-loads the appropriate AWC value.
Input Rooting Depth: Deeper-rooted plants extract water from a larger volume of soil.
Calculate Total Available Water: Total AWC = AWC (inches/ft) × Rooting Depth (ft)
Estimate When to Irrigate: Using water use (ETc) data and rainfall, the app tells you how long that water will last and when to irrigate.
Visual Output & Logs: You get clear volume recommendations, irrigation intervals, and a history log. All done automatically and offline
Example Walk-Through
Suppose you have:
Soil: Loam (AWC ≈ 2.0 in/ft)
Rooting Depth: 2 feet
Total AWC: 2.0 × 2 = 4.0 inches
With this data, the app projects how many days of watering the soil can sustain at the current crop water use. It adapts as you add rainfall and evapotranspiration numbers.
Final Thoughts
Soil texture dramatically affects how much water your plants can access. With Irrigation Calculator Pro, you simply choose your soil, and the app handles AWC calculations, helping you water smarter, not harder.