Soil Moisture, Water Movement and Measurement Methods
Water is the most critical component of agricultural production. The plant takes water from the soil; carries it within its body, transforms it and sustains its life cycle. However, the plant needs not just water, but the right amount of water. Too much is as harmful as too little.
To maintain this balance, the physical, chemical and biological mechanisms defining the soil-water relationship must be correctly understood. Soil moisture, water movement and measurement methods are very important in irrigation planning.
In this article, the soil's:
✔ water storage capacity
✔ water movement principles
✔ water forms
✔ pore structure
✔ infiltration–percolation mechanism
✔ moisture measurement methods
✔ dielectric sensor technologies
✔ capillary behavior
✔ laboratory calculations
✔ frequency-based moisture measurements
✔ EC (Electrical Conductivity) relationship
✔ volumetric and gravimetric moisture data
✔ in-field sensor validation tests
are explained with a scientific and applied integrity.
What is Soil? Matter Phases and Internal Structure
Soil consists of three phases:
| Phase | Description |
|---|---|
|
Solid |
Mineral and organic structure |
|
Liquid |
Water and dissolved salts |
|
Gas |
Air voids |
Usually, half of these three phases consists of the solid part (mineral and organic minerals). The other half consists of water-air. The ratio between these three phases determines how the soil:
- holds water
- transmits water
- presents water to the plant
determines. There are two main properties affecting the internal structure of the soil:
Texture (Body)
Sand–silt–clay ratio. In terms of texture, loamy textures are the best.
- Clay soil small pore → high capillary water retention
- Sandy soil large pore → high drainage
Structure (Form)
It is the way soil particles combine. When structure is degraded, water infiltration and root development weaken. Granular (best), blocky, prismatic, platy (worst), massive (no aggregate*).
Aggregate: clod.
Bulk Density and Particle Density of Soil
Bulk Density (BD):
It is the ratio of soil dry weight to total volume. It is performed on oven dry (105°C) soil. Unit: g/cm³.
ρb = Mdry / Vtotal
ρb : Bulk density
Bulk density Sandy>Loamy>Clayey>Organic.
As bulk density increases:
- Pore decreases.
- Available water decreases.
- Wilting point increases.
- Water is held but cannot move.
- Water transmission drops.
Particle Density (PD):
It is the density of only the solid phase. Generally accepted as a constant:
ρk = Mk / Vk
Soil ρk ≈ 2.65 g/cm³
ρk : Particle density.
As organic matter increases, particle density decreases. Generally changes little.
What is Porosity? What is its Formula?
Porosity is the ratio of the total void volume in the soil to the total volume. It is dimensionless and usually expressed in %:
As porosity increases:
✔ infiltration increases
✔ water movement speeds up
✔ air exchange strengthens
As bulk density increases, porosity decreases → infiltration falls.
Water Types in the Soil
Hygroscopic Water
It is the water adsorbed as a very thin layer on the surface of soil particles.
Properties:
- Cannot be taken up by the plant
- Held very tightly
- pF value is high
- Does not evaporate / move
- Exists even if soil looks completely “dry”
This water does not separate even if the soil is dry; only oven drying can remove it.
Capillary Water
It is the water held in soil pores by capillary forces.
Properties:
- The main water that can be taken up by the plant
- Energy requirement is low
- pF is at medium level
- Higher in fine-pored (clay) soils
- Plant roots want water from here
This water is the area where the main moisture sensitivity is measured in the field.
Gravitational Water
It is the water that drains down quickly due to gravity and is not held in the soil.
Properties:
- Not useful for the plant
- Excess water → percolation loss
- Flows through large pores
- More prominent in sandy soils
It is the water where excess water flows away after irrigation.
| Water Type | Can Plant Use It? | Energy Level | Movement Status | Pore Type |
|---|---|---|---|---|
|
Hygroscopic |
Cannot use |
Very high |
None |
Micropore |
|
Capillary |
Can use |
Medium |
Limited |
Small pore |
|
Gravitational |
Cannot use |
Very low |
High |
Large pore |
Soil Moisture Constants
Establishing the correct relationship between soil moisture, plant water stress and irrigation timing is critical for healthy plant development and water efficiency.
Available Water Holding Capacity (AWHC)
Refers to the total amount of water plants can take from the soil.
AWHC = FC – WP
FC = Field Capacity: The highest level at which the soil can hold water.
WP = Wilting Point: The threshold where the plant can no longer take water.
The range from field capacity to wilting point represents the available water for the plant.
Readily Available Moisture (RAM)
Water that the plant can use without entering stress:
RAM = FC – CL
CL = Critical Level: The lower threshold where the plant begins to experience stress.
The range between FC and CL is “readily available water”.
Stress Zone (SZ)
The range from critical level to wilting point:
SZ = CL – WP
When this zone is entered, plant metabolism drops, yield loss begins.
Moisture Percentages – Gravimetric and Volumetric Measurement
Soil moisture is expressed in two fundamental ways:
Weight Percentage (Pw) – Gravimetric Moisture
Calculated based on mass change.
γt = bulk density of dry soil (g/cm³)
D = root depth
Volume Percentage (Pv) – Volumetric Moisture
Expressed according to soil volume.
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