Soil moisture refers to the moisture held in the soil pores, also known as soil moisture. Usually, the soil sample can be dried in an electric oven (temperature controlled at 105-110°C), and the amount of water released from the pores of the soil can be used as the soil moisture content. Soil moisture is not pure water, but a dilute solution. It also contains colloidal particles. The main source of soil moisture is atmospheric precipitation and irrigation water, in addition there is condensation of water vapor near the ground, rise of groundwater level, and moisture in soil minerals. How much the atmospheric precipitation seeps into the soil depends on the amount of precipitation, the intensity and nature of the precipitation. In general, the amount of precipitation is large, and there may be more water in the soil, but the soil moisture content is not necessarily high. Precipitation or quaternary precipitation with high intensity may easily cause ground loss. Therefore, the amount of water that penetrates into the soil is small. Continuous rainfall with low intensity is beneficial to the absorption and storage of soil moisture, and the soil moisture content is not necessarily low. According to its physical form, soil moisture can be divided into three kinds: solid, gaseous and liquid. Solid water exists only when it is frozen at a low temperature. Gaseous water often exists in the pores of the soil. Liquid water exists in the surface area of ​​the soil particles and intergranular pores. Under certain conditions, the three can be transformed into each other, of which the amount of liquid soil moisture is more. Soil moisture content can be expressed in the following ways: Percentage of soil water weight: The percentage of water actually contained in the soil as a percentage of the dry soil weight. which is W(%)=(W1-W2)/W2*100 In the formula, W (%) is the soil moisture content (percentage); W1 is the wet weight of the sample soil; W2 is the dry weight of the sample soil. Soil water volume percentage: refers to the percentage of soil water volume in unit soil volume. which is In the formula, W capacity (%) is soil volume water content (percentage); P is soil bulk density, ie dry soil weight per unit volume of undisturbed soil. The relationship between soil volume percentage and soil weight percentage is usually expressed by the following formula: W capacity (%) = W (%) × P Soil water layer thickness: refers to the total storage of soil moisture within a certain thickness of the soil layer, which is equivalent to a certain soil area, in a certain thickness of the soil layer there is a millimeter thick water layer. which is W thickness = H × W (%) × P × 10 Where W is the thickness of the soil water layer; H is the thickness of the soil layer; 10 is the unit conversion factor. Soil moisture determination methods at home and abroad are mainly the following: Titrimetric method, Karl Fischer method, weighing method, capacitance method, resistance method, γ-ray method, microwave method, neutron method, nuclear magnetic resonance method, time domain reflectometry method ( TDR), soil tension method, soil moisture sensor method, gypsum method and infrared remote sensing method. These soil moisture determination methods are not the same in the application of the position, the following highlights several methods commonly used in soil moisture determination methods. (1) Weighing method: also known as drying method, that is to take soil samples into the oven, dry 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 atom ions, and the fast neutron loses energy, thereby making it Slow 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) γ-ray method: Similar to the neutron instrument, 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) Time Domain Reflectometry: The Time Domain Reflectometry (TDR) method, which is based on the propagation of electromagnetic waves in a soil medium, the decay of conduction constants such as velocity depends on the nature of the soil, especially on the water content in the soil and Conductivity. The above five soil moisture determination methods have their own advantages and disadvantages. For example, the weighing method is simple and intuitive, and the neutron method can be repeatedly measured. The time domain reflection method is currently the most used method. Then, in the actual operation process, we must make some choices, consider in many aspects, and then determine which measurement method to use. Nowadays there are many products that can measure soil moisture. The following figure shows a soil moisture analyzer that can easily measure the soil moisture content. Figure 1 Soil Moisture Tester
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Soil moisture and its determination method