Detailed introduction of common soil moisture measurement methods
Measuring soil moisture changes is a very important content in agricultural scientific research and production management, because only when we fully understand the soil moisture data can we have a targeted, scientific and reasonable management of soil moisture, and promote high-quality and high-yield crops. So how is soil moisture measured? Here are several commonly used soil moisture measurement methods.
(1) Weighing method: also known as drying method, that is, take the soil sample into the oven and dry it to constant weight. At this time, the free water in the soil moisture is completely lost in the form of steam, and the weight is then weighed to obtain the soil moisture content. The drying method also has some rapid determination methods such as infrared method, alcohol burning method and oven method.
(2) Neutron method: The neutron source is buried in the soil to be tested. The neutrons continuously emit fast neutrons. The fast neutrons enter the soil medium and collide with various atomic ions. The fast neutrons lose energy, thereby causing them Slow down. When fast neutrons collide with hydrogen atoms, they lose the most energy and are easier to slow down. The higher the moisture content in the soil, the more hydrogen atoms, and the greater the density of slow neutron clouds. The neutron meter determines the moisture content of the soil by measuring the functional relationship between the density of the slow neutron cloud and the water molecules.
(3) Gamma-ray method: Similar to the neutron instrument, the gamma-ray transmission method uses a radioactive source 137Cs to emit gamma rays, and the probe receives the energy of the gamma rays after penetrating the soil, which is converted to the soil moisture content.
(4) FDR (FrequencyDomainReflectometry): The probe of FDR is called a dielectric sensor. It is mainly composed of a pair of electrodes (metal rods or circular metal rings arranged in parallel) to form a capacitor. The soil in between acts as a dielectric, The oscillator forms a tuning circuit. FDR uses a 100MHz sinusoidal signal to reach the dielectric sensor through a specially designed transmission line. The impedance of the dielectric sensor depends on the dielectric constant of the soil substrate. FDR uses the sweep frequency to detect the resonance frequency (the amplitude is the largest at this time), and the frequency of resonance is different for different soil moisture content.
(5) Time domain reflection method: The propagation speed of high-frequency electromagnetic pulse in the soil along the transmission line depends on the dielectric properties of the soil. In a certain electromagnetic wave frequency range (50 M～10 GHz), the dielectric properties of minerals, air and water are constant, so the dielectric constant of the soil mainly depends on the volumetric water content of the soil (very weakly depends on the soil type , Compactness, bound water, etc.), so that the empirical equation of soil volumetric water content and soil dielectric constant can be established. TDR obtains the dielectric constant of the soil by measuring the propagation speed of high-frequency electromagnetic pulses in the soil, thereby calculating the soil moisture content.
(6) Ground penetrating radar method (GPR): When the high-frequency radar pulse reaches the interface of two layers of materials with significantly different dielectric properties, part of the signal is reflected, and the reflected signal is received by the receiving device and amplified. The magnitude of the reflected signal is determined by the difference in the dielectric constant of the two materials and the penetration depth of the radar wave. Soil water content is the main factor affecting the dielectric constant, and the penetration depth of radar pulses is significantly affected by the water content in the soil.
(7) Nuclear Magnetic Resonance Method (NMR): It uses the property that the initial amplitude value of the nuclear magnetic resonance signal generated by the excitation of different laser pulse moments is proportional to the free water content in the detection range to detect the moisture content.
(8) Separate tracer method (PT): pass the non-separated tracer and the separated tracer into the gas phase system, and the separated tracer is dissolved in water so that its migration in the gas phase is relatively lagging behind that of the non-separated tracer Tracer. And the hysteresis factor is a function of soil moisture content and Henry's constant.
At present, with the development of science and technology, in the field of industry, more professional instruments- soil moisture recorders are used to measure soil moisture. The instrument can quickly and accurately collect soil data through the cooperation of the sensor and the host, and display the records on the host. The soil moisture data can be obtained in a few simple steps, and the data can also be transmitted or exported wirelessly, which is very convenient. This method It has also become one of the soil moisture measurement methods widely used in the industry.
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