Soil Moisture Sensor Interpreter
Your soil moisture sensor gives you a number - but what does it actually mean? This free tool translates your volumetric water content (VWC) reading into actionable information. Select your soil type, enter the reading from your capacitance, TDR, or FDR sensor, and instantly see whether your soil is too dry, optimal, or overwatered. You'll get a color-coded status, matric potential estimate, days until plant stress, and specific irrigation recommendations based on van Genuchten soil physics used by researchers worldwide.
Your Sensor Reading
Optimal Moisture
Your soil moisture is in the ideal range. No irrigation needed.
Soil Properties: Loam
Understanding Your Reading
What is VWC?
Volumetric Water Content (VWC) measures the percentage of soil volume that is water. A reading of 25% means 25% of the soil volume is water.
- Field Capacity: The moisture level after drainage stops (maximum usable water)
- Wilting Point: The moisture level where plants can no longer extract water
- Plant Available Water (PAW): The difference between field capacity and wilting point
- Management Allowed Depletion (MAD): Typically 50% of PAW before irrigation is needed
Reference: Field capacity and wilting point thresholds are derived from the van Genuchten (1980) soil water retention model using USDA soil texture parameters. See our Van Genuchten Curve Explorer for the underlying physics.
Sensor Calibration Note
Different sensors measure moisture differently. For best results:
- Capacitance sensors (most common) may read higher in clay soils
- TDR sensors are more accurate but more expensive
- Install sensors at root zone depth (typically 6-12" for turf)
- Consider multiple sensors to account for spatial variability
Related Tools
- ET Calculator - How fast is your soil drying? Calculate evapotranspiration from weather conditions.
- Van Genuchten Curve Explorer - Understand the physics behind soil water retention.
- Water Savings Calculator - See how much smart irrigation could save your property.
Learn more: Why Smart Irrigation Beats Timers
Soil Moisture Levels by Soil Type
Reference chart showing key soil moisture thresholds for all 7 USDA soil texture classes. These values help you interpret sensor readings and determine when to irrigate.
| Soil Type | Saturation | Field Capacity | Wilting Point | Plant Avail. Water | Irrigate Below |
|---|---|---|---|---|---|
| Sand | 43% | 10% | 5% | 5% | ~7.5% |
| Loamy Sand | 41% | 12% | 6% | 6% | ~9% |
| Sandy Loam | 41% | 18% | 8% | 10% | ~13% |
| Loam | 43% | 27% | 12% | 15% | ~19% |
| Silt Loam | 45% | 33% | 13% | 20% | ~23% |
| Clay Loam | 41% | 34% | 19% | 15% | ~26% |
| Clay | 38% | 36% | 21% | 15% | ~28% |
"Irrigate Below" is the 50% management allowed depletion (MAD) threshold - the recommended VWC at which irrigation should begin for most turfgrass and landscape plants. Values derived from van Genuchten soil water retention curves using USDA parameters.
Frequently Asked Questions
What is a good soil moisture percentage?
The ideal soil moisture percentage depends entirely on your soil type. For loam soil, the optimal range is 19-27% VWC (between the irrigation threshold and field capacity). For sandy soil, the optimal range is much lower at 7.5-10% VWC because sand holds less water. For clay, optimal moisture is 28-36% VWC. A reading that seems "dry" for clay might actually be waterlogged for sand. Always interpret your sensor reading in context of your soil type using the table above.
How wet is too wet for grass?
Soil moisture above field capacity is generally "too wet" for sustained periods. For loam, this means above 27% VWC; for clay, above 36%. Short-term saturation after rain or irrigation is normal and harmless - excess water drains within 1-2 days in well-structured soil. However, persistently saturated conditions (above 90% of saturation for more than 48 hours) can cause root rot, fungal disease, and reduced oxygen availability. If your sensor consistently reads above field capacity, you are overwatering or have drainage problems.
What type of soil moisture sensor should I use?
The three main types are: (1) Capacitance sensors (most common, $30-150) - measure the dielectric constant of soil, which correlates with water content. Good for relative readings but may need calibration for clay soils. (2) TDR sensors (Time Domain Reflectometry, $150-400) - more accurate, measure the travel time of an electromagnetic pulse. Best for research and precision irrigation. (3) Tensiometers ($50-200) - measure matric potential directly in kPa rather than VWC. All sensor types should be installed at root zone depth, typically 4-6 inches for turf and 6-12 inches for trees and shrubs.
Why does the same VWC reading mean different things for different soils?
Different soils have different pore structures. Sand has large pores that drain easily, so even 15% VWC represents near-saturation. Clay has tiny pores that hold water tightly, so 15% VWC in clay is actually very dry (near wilting point). The van Genuchten model captures this relationship mathematically. The key metric is not the absolute VWC number, but where it falls relative to that soil's field capacity and wilting point - which defines the plant available water range.