Water & Nutrient Management:
Soil Moisture-Based Irrigation Systems

Neutron Probe

Working Principle

Fast neutrons are emitted from a decaying radioactive source (241Am/9Be) and when they collide with particles having the same mass as a neutron (i.e., protons, H+), they slow down dramatically, building a "cloud" of "thermalized" (slowed-down) neutrons. Since water is the main source of hydrogen in most soils, the density of slowed-down neutrons formed around the probe is nearly proportional to the volume fraction of water present in the soil.

Description

The probe configuration is in the form of a long and narrow cylinder, containing a source and detector. Measurements are made by introducing the probe into an access tube (previously installed into the soil). It is possible to determine soil moisture at different depths by hanging the probe in the tube at different depths. The soil moisture is obtained from the device based on a linear calibration between the count rate of slowed-down neutrons at the field (read from the probe), and the soil moisture content obtained from nearby field samples.

Advantages

• Robust and accurate (±0.005 ft3ft-3)
• Inexpensive per location (i.e., a large number of measurements can be made at different points with the same instrument)
• One probe allows for measuring at different soil depths
• Large soil sensing volume (sphere of influence with 4-16 in. radius, depending on moisture content)
• Not affected by salinity or air gaps
• Stable soil-specific calibration

Drawbacks

• Safety hazard, since it implies working with radiation. Even at 16 in. depth, radiation losses through soil surface have been detected
• Requires certified personnel
• Requires soil-specific calibration
• Heavy, cumbersome instrument
• Takes relative long time for each reading
• Readings close to the soil surface are difficult and not accurate
• Manual readings; cannot be automated due to hazard
• Expensive to buy
• The sphere of influence may vary according to the following reasons:

1. It increases as the soil dries, because the hydrogen concentration reduces, so that the probability of collision is smaller and thereby fast neutrons can travel further from the source.
2. It is smaller in fine texture soils, because they can hold more water, thus the probability of collision is higher.
3. If there are layers with large differences in water content due to changes in soil physical properties, the sphere of influence can have a distorted shape.

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This page was last updated on June 15, 2010.