The relationship between water and soil in agriculture
Water is vital for plant growth: it drives many physiological processes and is the main component of plants, about 80% in herbaceous plants and 50% in woody plants. Solar energy controls how much water plants use through transpiration.
Key figures
| Indicator | Value |
|---|---|
| Water in herbaceous plants | about 80% |
| Water in woody plants | about 50% |
| Water content of seeds | 5–15% |
| Pores that stay water-filled after irrigation or rain | 5–75 μm |
| Forms of soil water | 3: gravitational, capillary, hygroscopic |
| Soil–water relationship factors | 5: porosity, permeability, texture, effective depth, drainage |
| Soil moisture states | 3: maximum capacity, field capacity, wilting point |
| Area measured by a CRNS sensor | radius of about 120 m |
| Depth measured by a CRNS sensor | 50 cm |
| CRNS sensor features | Passive · contactless · self-powered · maintenance-free · long service life |
| International recognition | Technology recommended by the FAO for monitoring water in agricultural soils |
Properties of water
- High specific heat
- Helps stabilise temperature
- Good heat conductor
- High surface tension
- Near-universal solvent
The role of soil
Soil anchors plants and creates the conditions for them to grow, supplying the nutrients they need. It is also home to many organisms: bacteria, fungi, algae, mites, earthworms, nematodes, ants and more. It is made up of organic matter, minerals, water and air. Soil and water are therefore interconnected resources: soil stores water, and water dissolves the minerals that plants take up through their roots.
Forms of water in soil
- Gravitational (percolating) water: not held by soil particles, it moves freely through the pores under gravity until it drains away. It drains faster from large pores and more slowly from smaller ones.
- Capillary water: held by soil particles through surface tension. Plants take up the water in larger capillaries, where the retention force is lower than root suction. All water plants can use is capillary, but not all capillary water is usable.
- Hygroscopic water: bound so tightly to solid particles that plants cannot absorb it. It is a very small fraction of the total.
These forms match three thresholds: maximum capacity (saturated soil, excess water percolates), field capacity (capillary water available to plants remains) and the wilting point (only hygroscopic water remains).
Soil–water relationship factors
- Porosity: the spaces between soil particles, where water circulates
- Permeability: how easily water and air move through the soil
- Texture: the proportion of sand, silt and clay
- Effective depth: how deep roots can reach for water and nutrients
- Drainage: how fast water moves through the soil and the soil dries after wetting
Water, germination and roots
Seeds, with a water content of 5–15%, barely respire in dry soil. As water increases, respiration rises and germination begins. Root growth also depends heavily on water: pressure inside the cells provides the force that drives cell elongation.
Water enters the production system through irrigation or rainfall. When the supply stops, air replaces water in the larger pores, while the smallest pores (5–75 μm) stay filled with water held by soil particles.
Measuring soil water: CRNS sensors
Using technology to monitor soil water is now essential in farming. CRNS (Cosmic Ray Neutron Sensor) sensors are the state of the art for area-wide 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 50 cm depth
The FAO recommends this technology for monitoring water in agricultural soils and managing water resources more efficiently.
The Finapp impact
- Contactless measurement of water content in soil and snow
- Less water waste in agriculture
- Lower energy costs
- Higher productivity
- Data on the water available at the plant root zone