Methods of Determination of Soil Moisture

Definition

Soil moisture determination is the process of quantifying the amount of water held in the spaces between soil particles. It is typically expressed as a percentage by weight (gravimetric) or by volume (volumetric), representing the water content critical for plant growth, irrigation scheduling, and geotechnical stability.

Main Content

1. Direct Method (Gravimetric Method)

• This is the most accurate and fundamental method, often used to calibrate other electronic sensors.
• It involves taking a physical sample of soil, weighing it, drying it in an oven, and weighing it again to find the water loss.

2. Electrical Resistance Methods

• This method utilizes the physical property that wet soil conducts electricity better than dry soil.
• Porous blocks (gypsum or nylon) are buried in the soil; as moisture changes, the electrical resistance between electrodes inside the block changes, which is then measured by a meter.

3. Neutron Scattering Method

• This method relies on the interaction between high-energy neutrons and hydrogen atoms found in soil water.
• A probe containing a radioactive source is lowered into an access tube; the number of "slowed down" (thermalized) neutrons reflects the total moisture content in the surrounding soil volume.

Working / Process

1. Sampling and Weighing

• A soil auger is used to extract a known volume or mass of soil from a specific depth.
• The fresh sample is immediately placed in an airtight container to prevent evaporation before the initial weight (W1) is recorded.

2. Oven Drying

• The soil sample is placed in a thermostatic oven set at a constant temperature of 105°C to 110°C.
• The sample is dried for 24 hours until the weight becomes constant, ensuring all capillary and hygroscopic water is removed.

3. Calculation

• The dried sample is weighed again (W2). The weight of water is the difference between W1 and W2.
• The moisture content is calculated using the formula: % Moisture = [(W1 - W2) / W2] × 100.

[Soil Sample Before Drying]      [Soil Sample After Drying]
      +-----------+                    +-----------+
      |  Soil +   |                    |           |
      |  Water    |  ---(105°C)--->    |   Soil    |
      |           |                    |           |
      +-----------+                    +-----------+
      Weight = W1                      Weight = W2

Visual representation of the gravimetric moisture loss process.

Advantages / Applications

• Gravimetric Method: Serves as the standard "Gold Method" for the calibration of all other soil moisture sensors.
• Tensiometers: Highly useful for irrigation management in orchards and vegetable crops to measure soil water tension directly.
• TDR (Time Domain Reflectometry): Widely used in research and automated agriculture for rapid, non-destructive, and continuous monitoring of volumetric water content.

Summary

The determination of soil moisture is essential for optimizing irrigation, understanding plant-water relationships, and managing soil health. While direct methods like oven-drying provide the highest accuracy, indirect methods like electrical resistance, neutron probes, and TDR offer the efficiency required for real-time monitoring. Important terms to remember include field capacity, wilting point, gravimetric water content, and volumetric water content.