Soil moisture and how to monitor it
Soil moisture is vital for healthy plant growth and has a direct impact on yield: without the water they need, crops cannot grow at their best. The share of water the soil holds that plants can actually use is called available water, and it varies with soil type and farming practices.
- Moisture content: how much water is in the soil.
- Moisture potential: how tightly soil particles hold the water, that is, how hard roots must work to take it up.
Together, these values tell you how often to irrigate each crop and how much water to apply. Soil moisture can be estimated with simulation models or satellite data, but measuring it directly in the field of interest is always preferable.
Key figures
| Indicator | Value |
|---|---|
| Benefits of in-situ monitoring | 7 |
| Sensor technologies compared | 3: point moisture sensors, tensiometers, area-wide CRNS sensors |
| Satellite estimates: depth measured | about 5 cm (surface layer) |
| Satellite estimates: resolution | 1 km |
| Point moisture sensors: cost | €100 – €1,500 |
| Tensiometers: cost | €50 – €300 |
| Tensiometers: accuracy limit | inaccurate above 70–80 kPa |
| Tensiometers: preparation | at least 24 hours before use |
| CRNS sensor: area measured | radius of about 120 m |
| CRNS sensor: depth measured | about 50 cm (maximum 80–100 cm) |
| CRNS sensor: cost | about €700/year (10-year amortisation) – €1,500/year (rental) |
| CRNS sensor: maintenance | none, with a very long service life |
Benefits of in-situ monitoring
- Match the water applied to the crop’s actual needs
- Avoid water losses through deep percolation or runoff
- Save energy
- Prevent erosion and soil loss
- Optimise production
- Reduce environmental impact
- Get real-time data to assess the water balance correctly and react immediately
Direct and indirect methods
- Direct method: water is extracted from a soil sample and calculated from its weight before and after drying. It is useful for getting to know your soil, but can only be done occasionally and is often never done at all.
- Indirect method: soil properties linked to moisture content are measured.
The three measurement technologies
Point soil moisture sensors
Probes installed in the field measure electrical conductivity, impedance, permittivity or frequency, and derive moisture from these. They can be used in different soil types. Limitations:
- Spatial coverage: they measure a single point and miss variation within the field
- Interference: salinity and placement distort readings
- Installation: there is no installation standard
- Soil movement: creates air gaps around the sensor and requires recalibration
- Field operations: they cannot be installed where machinery passes
- Lifespan: installation in the soil shortens their life
Cost: €100 to €1,500.
Point soil water tension sensors (tensiometers)
Installed in the soil, they measure actual water tension, the effort roots make to take up water. A meaningful reading requires several devices, chosen according to soil type. Limitations:
- Accuracy: narrow measurement range, inaccurate above 70–80 kPa
- Preparation: must be prepared at least 24 hours before use
- Spatial coverage: they measure a single point
- Quantity: several are needed for usable data
Cost: €50 to €300.
Area-wide soil water content sensors (CRNS)
CRNS (Cosmic Ray Neutron Sensing) is the state of the art in soil moisture measurement:
- Passive devices, with no contact with the soil
- Self-powered and independent
- Very long service life, maintenance-free
- Measure moisture over an area of about 120 m radius, down to about 50 cm depth
Limitations: they do not take point measurements (below 10 m) and do not measure deeper than 80–100 cm.
Cost: about €700 per year with 10-year amortisation, or €1,500 per year to rent.
Technology comparison
| Point moisture sensors | Tensiometers | CRNS | |
|---|---|---|---|
| Area measured | One point | One point | Radius of about 120 m |
| Soil contact | Yes | Yes | No |
| Maintenance | Frequent recalibration | Preparation before use | None |
| Interferes with field operations | Yes | Yes | No |
| Cost | €100 – €1,500 | €50 – €300 (several needed) | €700 – €1,500/year |
What about satellite data?
Satellite estimates provide volumetric soil moisture maps (m³/m³) of the surface layer only (about 5 cm), at 1 km resolution. They are useful at large scale, but not enough to manage irrigation in a single field.
The Finapp impact
- Contactless measurement of water content in soil and snow
- Precise, digital water management
- Less water waste and lower energy costs
- Higher productivity
- Data on the water available at the plant root zone