The above paragraph briefly introduces the influence of soil moisture on plants. Insufficient or excessive soil moisture will affect the normal growth of plants. The main manifestation is that plants can easily die due to lack of water in the absence of soil moisture. When the soil moisture is insufficient, the plants will be relatively short and cannot grow normally; when the soil is rich in water, the plants can grow vigorously and at the same time produce high yield; when the soil moisture is too much, the roots of the plants will not be able to be produced. Death by normal breathing. Soil moisture measurements can be measured using a soil moisture monitoring system that enables long-term periodic monitoring of all types of soil and rapid measurement and recording in real time. At present , farmland soil moisture measurement methods emerge in an endless stream , such as drying method, tension meter method, neutron method or ray method, dielectric constant method or electromagnetic wave method, sensor method, electric resistance method or granular array method, capacitance method, photoelectric method, Various methods , such as thermal diffusion method , have their own scope of application and advantages and disadvantages. In this paper, the basic principles, advantages and disadvantages of several widely used farmland soil moisture measurement methods are summarized . On this basis, the development direction of future soil moisture measurement methods is proposed.

I: Several major methods for measuring soil moisture in farmland

At present , the widely used soil moisture measurement methods are : drying method, tension meter method, neutron method, time domain P frequency domain reflectometer. Details are as follows.

Drying method

Drying method , also called weighing method or soil drilling method. The general practice is: good soil samples placed in the drill previously weighed aluminum case with soil (soil volume measured if desired water content, use a ring knife samples) were weighed, and then placed in an oven together, at 105 to 110 °C (the temperature is too high , the organic matter is easy to carbonize and dissipate ). The temperature is baked to constant weight (the difference between the measurements of 3 h is not more than 3 mg). In the actual operation, it is generally dried for 12 to 14 h, and cooled in a desiccator for 20 min. Yes , the difference between the two weights is the soil moisture content.

The advantage of the drying method is that the hardware requirements are not high and the results are reliable for the sample itself. It is generally believed that the conventional drying method measured value is credible soil water, as a variety of other standard methods of calibration measurements of soil moisture. However, its shortcomings are also obvious . The drying method is time-consuming and laborious , and deep sampling is difficult . Sampling will destroy the soil , and continuous observation of soil moisture can not be achieved , which is greatly affected by soil spatial variability.

Tensiometer method

Tonometry method is very broad application of one kind of soil moisture measurement, which measures the soil matric potential, i.e., soil water suction. Before applying tensiometer soil moisture measurement, the relationship must be established between the water suction current and soil moisture, soil moisture characteristic curve commonly known. This relationship is influenced by soil texture and structure, to produce different pressure in a closed porous ceramic plate for the determination of soil pressure, the measured residual moisture content of the soil is different, soil water can be obtained corresponding to the characteristic curve.

Tension meter is a closed tube filled with water, one end of a porous ceramic head (pore diameter of about 110 ~ 115μm), can be inserted into the soil, and the other end connected to a vacuum table. Through the suction of the porous ceramic head , the water continues to flow until the soil water suction and the tension meter balance are reached , and the negative pressure value indicated by the pressure gauge is the soil water suction.

The advantage of the tension meter method is that it can accurately measure the matrix potential of wet soil , can be continuously observed at fixed points , is less affected by soil spatial variability , and has low equipment , suitable for monitoring of irrigation and water stress. The disadvantage is that the reading response is slow , it takes a long time to balance before reading , and the range is narrow . Only soil water suction of less than 8 kPa can be measured , and it is not suitable for extremely dry soil. In the long-term measurement process, in the case of high temperature and dry season , it is necessary to add water to the pipe , and the ceramic head is easily damaged , and needs to be regularly maintained or replaced , and the running cost is high. The soil water potential meter is developed by the measurement principle of the tension meter method and is suitable for any soil property monitoring.

Neutron method

Method principle neutron emitted from a neutron source of high energy neutrons into the soil, the soil after hydrogen atom (most of the water present in the molecule) collision energy attenuation, the attenuation of neutron energy may It detected by the detector, and establishing the number of neutrons as a function of soil moisture detected by the calibration can be converted to soil moisture content.

Neutron soil moisture content was measured just beforehand is embedded, the measurement does not destroy soil structure, high speed, accurate measurement results [1], continuous fixed-point observation, and no hysteresis, but the method can not achieve long-term neutron Large-scale dynamic monitoring [2] . Since the neutron method actually measures the water content of a sphere with a radius of about several to several tens of centimeters , the radius varies with the soil moisture content , so when the soil is in a dry or wet cycle , or for layered soil and topsoil , The measurement results of the neutron method are not reliable. For highly organic soils , hydrogen in organic matter also affects the determination of soil moisture by neutrons. In addition, before using neutron probe also requires field calibration, by soil texture and bulk density of impact, indoor and outdoor calibration curve quite different [3], while expensive neutron probe equipment, and the need for special protective equipment, one-time investment , especially human potential radiation hazard, and therefore not widely used.

Time domain reflectometer (TDR)

TDR (T ime Domain Reflectometry) method is one kind of the dielectric constant, the basic principle is the speed of high-frequency electromagnetic pulses propagating along the transmission line in the soil depends on the soil dielectric constant Ka, Ka and governed mainly by the water content of soil (20 At °C , the Ka of natural water, air and soil particles are 80 , 1 , 3, 5, respectively . The dielectric constant Ka of the soil is calculated according to the propagation frequency of electromagnetic waves in the medium, and the soil moisture content is calculated by the empirical relationship between the soil dielectric constant and the soil volumetric water content (θ v) .

Ka frequency of electromagnetic waves when the 1 MHz ~ 1GHz, the reciprocating electromagnetic wave propagation velocity (V) in the form of the following relationship electrode (length L) are: Ka = (cP V) 2 = (ct / 2L) 2

(1) where : c is the speed of light , c = 3 × 108 mP s ; t is the transit time of electromagnetic waves , s . The reflection of electromagnetic waves at various points is obvious , and t can be accurately measured , so that Ka can be calculated by the formula (1) . T opp et al. used TDR to measure the propagation time of electromagnetic waves , and obtained the empirical formula of the propagation time in most soils and soil volumetric water content (θv) [4].

:θv = - 513 ×10- 2+ 2192 ×10- 2Ka - 515 ×10- 4K2a + 413×10- 6Ka3 when θv ≤016

(2) However, this empirical relationship applies only when Ka → 1 or Ka → 80136 (20 ° C) , and is mainly applicable to sandy soil.

TDR is a mainstream method for determining the moisture content of the soil newly developed, has many advantages, such as nuclear radiation, extremely fast and can be fixed in situ continuous measurement, and the measurement values are accurate. In conventional soils , the instrument has a measurement error of less than 5% . Generally do not need calibration, wide measurement range (0 to 100% water content), easy to operate, can be used indoor and outdoor, long TDR probe embedded in the soil, when needed and then connected to the TDR measurements. In addition , TDR is little affected by soil salinity and can measure surface soil moisture content ( neutron method is not acceptable ) . However , the measured value of TDR is affected by temperature, bulk density, and soil quality [5 - 6 ]. In soils with high conductivity ( such as saline-alkali soil ), the measurement accuracy is also reduced , and the organic matter content is high, the bulk density is particularly high or special. Low and heavy soils need to be recalibrated before they can be used. At present , the use of TDR in China mainly depends on imports , and the price is high , and its application is also limited.

Frequency domain reflectometer (FDR)

The principle of measuring soil moisture content by FDR (Frequency Domain Reflectometry) is similar to that of TDR [7]. Using the principle of electromagnetic pulse , the change of apparent permittivity K a of soil is tested according to the frequency of electromagnetic wave propagation in soil . These changes are transformed into A millivolt signal proportional to the volumetric water content of the soil.

FDR probe by one pair of electrodes (e.g., metal rods arranged in parallel) consisting of a capacitor, soil therebetween function as a dielectric, with the capacitor consisting of an oscillator tuning circuit, the oscillator frequency F and the capacitance C soil nonlinear inverse relationship:

F = 12Ï€ L1C +1Cb015

(3) where : L is the inductance of the oscillator ; Cb is the capacitance associated with the instrument. Since the soil capacitance C increases as the soil moisture content increases , the correlation between the oscillator frequency F and the soil moisture content is established.

Compared with the TDR, FDR have in the selection and design of the geometry electrode operating frequency greater freedom, the probe may also be connected to the FDR most conventional data acquisition, in order to achieve automatic and continuous monitoring. But FDR at low frequencies (≤ 20 MHz) work, more vulnerable than TDR soil salinity, density and impact of clay. In addition , FDR lacks control and some details compared to pure TDR waveform analysis.

FDR has the advantages of simple and safe, fast and accurate, fixed-point continuous, automatic, wide-range, and less calibration [8]. It not only has high measurement accuracy , but also has low price . It can measure single-point or multi-point vertical depth. Although there are still some problems in the FDR method , such as the FDR reading is affected by the soil pores and moisture near the electrode (as is the case with TDR ), for the FDR using the probe , the probe, the probe and the soil are in good contact with the reliability of the measurement results. Impact, etc. , but the FDR method is indeed far superior to other existing assays.

two: Conclusion

In many methods of measuring soil moisture content, the drying method is simple and intuitive, reliable results, other measurement methods may be used as a calibration, but the drying method sampling difficulties, damage the soil, sampling hole in left field, a crop will be cut off Some roots affect the soil water movement and cannot be continuously observed at a fixed point ; the neutron method can be periodically and repeatedly measured at different depths in situ without destroying the soil , but the vertical resolution of the instrument is poor , the surface measurement is difficult , and the person There is a radiation hazard. The author believes that the overall performance is higher

It is also a more advanced time domain reflectometry (TDR) method and an economical frequency domain reflectometry (FDR) method , all of which have the advantages of mature technology, high precision, continuous positioning, and portability. Since TDR is fast, accurate, continuous and non-radiation measured in situ, etc., and more used in portable moisture determination. FDR because of simple, safe, fast and accurate, point-continuous, automated, wide range, less calibration, etc., the probe can be used with traditional data logger is connected, showing a good momentum of development in automatic and continuous monitoring of soil moisture. In short , the monitoring of intelligent soil moisture in farmland is still the development trend of soil moisture monitoring in the future.