Library · Substrate & soil
Soil Moisture Tension.
Soil Moisture Tension Reference
Read your tensiometer / Watermark sensor. When to irrigate by crop and soil type. The single best $30 investment in irrigation efficiency.
Reading scale
| Reading (cb / kPa) | Soil condition |
|---|---|
| 0-10 | Saturated; possibly waterlogged. Roots can lack oxygen. |
| 10-30 | Field capacity to mid-range. Optimal for most crops. |
| 30-50 | Slightly dry. Most crops happy here. |
| 50-80 | Dry. Many crops show stress; sandier soils need irrigation by 60. |
| 80-200 | Permanent wilting point approached. Most crops severely stressed; growth stops. |
| > 200 | Sensor max; soil is bone dry. Plants likely dying. |
Sensor types
| Sensor | Range | Cost | Best for |
|---|---|---|---|
| Tensiometer (water-filled tube) | 0-80 cb | $50-100 | Real-time field reading; needs maintenance; limited range |
| Watermark gypsum block | 0-200 cb | $30-50 sensor + meter | Wide range; durable; low maintenance; standard agricultural choice |
| TDR (capacitance) probe | Volumetric water content (%) | $200-1500 | Accurate; reads % directly; expensive |
| Soil moisture sensor (FDR) | Volumetric water content | $30-300 | Hobby home gardens; consumer apps |
| Wireless / IoT logger | Variable | $200-2000 | Cellular / LoRa data logging; commercial |
| Squeeze test (qualitative) | Low / mid / high moisture | $0 | Cheapest; works for hand-tend gardens |
Squeeze test (no equipment)
Pull soil from the root zone (4-8" deep). Squeeze in a fist:
- Sand: Should form a weak ball when wet. If grains fall through fingers and won't stick = irrigate.
- Loam: Should ribbon out 1-2" when squeezed at field capacity. If brittle and crumbly = irrigate. If pasty/squelches water = too wet.
- Clay: Should ribbon 2"+ at field capacity. If brittle = irrigate. If sticks tightly to hand and gleams = too wet.
Irrigation thresholds by crop
| Crop | Sandy soil (irrigate at) | Loamy soil | Clay soil |
|---|---|---|---|
| Lettuce / leafy | 20-30 cb | 30-40 cb | 40-50 cb |
| Tomato (vegetative) | 30-40 cb | 40-50 cb | 50-60 cb |
| Tomato (fruit set / sizing) | 20-30 cb | 30-40 cb | 40-50 cb |
| Pepper | 20-30 cb | 30-40 cb | 50-60 cb |
| Cucumber | 30-40 cb | 40-50 cb | 50-60 cb |
| Potato | 30-40 cb | 40-50 cb | 50-65 cb |
| Corn | 40-50 cb | 60-70 cb | 70-80 cb |
| Beans | 30-50 cb | 50-70 cb | 60-80 cb |
| Strawberry | 20-30 cb | 30-40 cb | 40-50 cb |
| Cannabis (veg) | 20-30 cb | 30-40 cb | 40-50 cb |
| Cannabis (flower) | 40-50 cb | 50-60 cb | 60-70 cb |
| Grape | 50-70 cb | 70-90 cb | 80-100 cb |
| Apple | 50-70 cb | 70-90 cb | 80-100 cb |
| Alfalfa | 60-80 cb | 80-100 cb | 100-150 cb |
| Lawn / turf | 30-50 cb | 50-70 cb | 60-80 cb |
Free under CC BY 4.0. Cite as "OAT Soil Moisture Tension Reference (openagriculturetechnology.com)". Tension thresholds from Watermark / Irrometer publications and university extension references.