Why soil moisture monitoring matters in agriculture
Soil moisture content is a key factor in healthy crop development, because of the close relationship within the soil–plant–atmosphere system. Water makes up 75–85% of plant tissue and is essential for all vital processes. The potential difference between leaf and atmosphere makes the plant lose water: this is transpiration. Soil anchors the plant and stores water, air and nutrients, depending on its physical properties.
Key figures
| Indicator | Value |
|---|---|
| Water in plant tissue | 75–85% |
| Tension at field capacity (FC) | 1/3 atm |
| Tension at permanent wilting point (PWP) | 15 atm |
| Available water capacity (AWC) | FC − PWP |
| Available water by soil texture | from 1.0 (coarse sand) to 6.2 cm (clay soils) per 30 cm of soil |
| Wilting point relative to field capacity | PWP = FC × 0.595 |
| Bulk density unit | g/cm³ |
| Measurement methods compared | 3: gravimetric (reference), electrical resistance (point), neutron (area-wide) |
| Neutron measurement features | Area-wide · continuous · contactless |
| Benefits of continuous monitoring | 6: available water, correct irrigation, roots, nutrients, germination, weeds |
Soil hydraulic parameters
- Saturation Point (SP): the maximum amount of water the soil can hold. It depends on the amount and type of clay and on organic matter content.
- Field Capacity (FC): the water the soil retains against gravity (1/3 atm) after it has been fully wetted.
- Permanent Wilting Point (PWP): the soil moisture level at which plants wilt and cannot recover, measured at a tension of 15 atm.
- Available Water Capacity (AWC): the difference between FC and PWP. It depends on clay type, organic matter and soil structure.
- Hydraulic Conductivity (HC): the soil’s ability to move water, and indirectly oxygen, through the profile.
- Bulk Density (BD): the amount of solids and pore space per unit volume (g/cm³). It is best measured in the field so the sample is not disturbed in transport.
Available water by soil texture
| Soil texture class | cm of water per 30 cm of soil depth |
|---|---|
| Coarse sand | 1.0 – 2.0 |
| Sand and loamy sand | 2.0 – 4.0 |
| Silt loam and loam | 4.0 – 5.5 |
| Clay loam, silty clay loam and sandy clay loam | 4.5 – 6.2 |
| Sandy clay, silty clay and clay | 4.0 – 6.2 |
Soil moisture measurement methods
Physical analysis of a uniform soil sample in the lab gives accurate results for irrigation scheduling. The other main methods are three.
Gravimetric method
The direct reference method: a soil sample is taken, weighed before and after drying, and its moisture content is calculated. It cannot be used for continuous monitoring and requires several types of sampling equipment.
- Bulk density: BD = Mds / Vt, where Mds is dry soil mass (g) and Vt is total soil volume (cm³)
- Field capacity: FC = (SP / 1.84) − 0.48
- Permanent wilting point: PWP = FC × 0.595
- Irrigation depth: L = (θFC − θact) × BD × h, where L is irrigation depth (cm), θFC is moisture at field capacity (g/g), θact is current moisture (g/g), BD is bulk density (g/cm³) and h is the soil depth to be wetted (cm)
Electrical resistance method
A porous block of gypsum, nylon or fiberglass with two electrodes reaches moisture balance with the soil and records its changes. It measures a single point, only the soil around the sensor, and is sensitive to salinity, soil type and sensor movement.
Neutron method
Measures the amount of water per unit volume of soil. Neutrons lose energy, and therefore speed, when they collide with light atoms such as the hydrogen in water. Counting the slowed neutrons gives the water content. The measurement is area-wide, continuous and contactless, and gives very useful information for irrigation management.
Benefits of continuous monitoring
- Shows how much water is available to the plant
- Sets the right irrigation depth and volume to avoid deficit or excess
- Improves root penetration and development
- Improves nutrient uptake
- Provides the right temperature for seed germination and root growth
- Reduces and controls weed growth
The Finapp impact
- Contactless soil water measurement based on Cosmic Ray Neutron Sensing
- Less water waste in agriculture
- Lower energy costs
- Higher productivity
- Data on the water available at the plant root zone