(1) Drying method: Also called weight determination method, that is to take the soil sample into the oven and dry it to constant weight. At this time, the free water in the soil moisture is lost in the form of steam and the weight is weighed to obtain the soil moisture content. Drying methods include infrared, alcohol burning, and oven methods.
(2) Neutron method: the neutron source is buried in the soil to be measured, the neutron source continuously emits fast neutrons, the fast neutron enters the soil medium and collides with various atomic ions, and the fast neutron loses energy, so that It slows down. When fast neutrons collide with hydrogen atoms, the energy loss is greatest, and it is easier to slow down. The higher the moisture content in the soil, the more hydrogen atoms there are, and the greater the density of slow neutron clouds. The neutron meter determines the water content in the soil by measuring the relationship between the density of the slow neutron cloud and the water molecules.
(3) Gamma ray transmission method: Similar to the neutron meter, the γ-ray transmission method uses 137Cs as a radioactive source to emit γ-rays, and the probe receives the energy of γ-rays transmitted through the soil, and is converted with the soil moisture content.
(4) Soil Moisture Sensor Method: Currently used sensors are various, including ceramic moisture sensor, electrolyte moisture sensor, polymer sensor, piezoresistive moisture sensor, photosensitive moisture sensor, microwave moisture sensor, and capacitive moisture sensor.
(5) Frequency domain reflectometry: The FDR (Frequency Domain Reflectometry) method measures the volumetric content of soil moisture by measuring the change in electrolyte constants. These changes are converted into millivolt signals proportional to soil moisture. For example, TZS series moisture measurement produced by Hangzhou Top Instrument Co., Ltd. utilizes this technology. Its advantage is high measurement accuracy and low price. It can measure single-point or multi-point vertical sections.
(6) Time Domain Reflectometry: TDR (Time Domain Reflectometry) method, which is based on electromagnetic wave propagation in the soil medium, the attenuation of its conduction constants such as velocity depends on the nature of the soil, especially depends on the water content in the soil And conductivity.
Comparison of the above methods:
In many methods of measuring soil moisture, the drying method is simple and intuitive, but the sampling will interfere with the continuity of soil moisture in the field. The sampling holes left in the field will cut off some roots of the crop and affect the movement of soil moisture.
The neutron method can be periodically repeated at different depths in situ without destroying the soil, but the vertical resolution of the instrument is poor, surface measurement is difficult, and radiation hazards are healthy.
The γ-ray transmission method has many advantages over the neutron method, and it has higher vertical resolution than the neutron meter, but γ-rays also harm human health.
The sensor method is now more commonly used as an economical frequency domain reflectometer method (FDR). It has the advantages of mature technology, high precision and portability. Domestic products have reached the standard.
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