Some standard content:
1 Mass method
National Standard of the People's Republic of China
Determination of forest soil water content
Determination of forest soil water content This method is applicable to the determination of soil water content (soil mass humidity). 1.1 Key points of the method
UDC 634.0.114
GB 7833—87
The water content in the soil sample (natural wet soil) is dried to a constant mass in an oven at 105°C, and the percentage of the mass lost in the sample to the mass of the dried soil is calculated to obtain the mass percentage of soil water content (mass humidity). If the soil bulk density has been determined, multiply the mass percentage of soil water content by the soil bulk density to obtain the volume percentage of soil water content. 1.2 Main instruments
Oven, aluminum box, desiccator, balance (sensitivity 0.01g). 1.3 Determination steps
1.3.1 Weigh 20g of natural wet soil sample (accurate to 0.01g), put it in an aluminum box with known mass, cover the box, and weigh it, that is, the mass of the aluminum box plus wet soil.
1.3.2 Open the box cover, put it in an oven, and bake it at 105℃ to a constant mass (about 12h). Soil samples with a lot of organic matter (>8%) should not be baked for too long above 105℃. After taking it out, put it in a dryer to cool to room temperature (about 20~30min). 1.3.3 Take out the aluminum box from the dryer, cover the box cover, and weigh it, that is, the mass of the aluminum box plus dried soil. 1.4 Result calculation
Mass humidity (mass %) =.
Mass of dried soil, g
Where. m-
-Mass of wet soil, nominal.
Volume humidity (volume %) = mass humidity (mass %) × soil bulk density (g/cm3) / water density (g/cm3)…: (2) 1.5 Allowable deviation
The difference between the results of parallel determinations shall not exceed 0.3-0.7% (equivalent to a relative deviation of no more than 5%). Note: ① The error in the mass disc method for determining soil moisture content mainly lies in the representativeness of the sampling, the accuracy of the balance, and the loss of mass disc due to partial oxidation and decomposition of organic matter during the drying process at 105°C. At the same time, clay particles can still contain a considerable amount of adsorbed water that is not evaporated. Therefore, when determining the soil moisture disc by the mass method, the representativeness of the sampling must be paid attention to first, and the number of sampling repetitions must be increased to compensate for its lack of representativeness. ② Clay soils with a very high moisture disc can be baked for another 3-4 hours if necessary. The difference between the two weighings before and after is no more than 0.05g, which is a constant mass. www.bzxz.net
2 Neutron deceleration method
This method is suitable for the dynamic determination of soil moisture content (soil volumetric humidity) in the field. 2.1 Key points of the method
The neutron deceleration method is mainly used to monitor the moisture content of soil profiles in the field. Its advantages are simple operation (it can also record automatically): and it is not affected by factors such as soil type, composition, and structure for representative soil volumetric humidity at the same location and depth. Fast neutrons emitted from a certain radiation source are radiated into the soil. Once they collide with atomic nuclei, they may be scattered or absorbed, gradually lose energy and slow down. The smaller the mass of the colliding atomic nucleus, the greater the deceleration ratio, especially when it collides with hydrogen atomic nuclei. The National Bureau of Standards for Neutrons approved on June 4, 1987
Implementation on January 1, 1988
GB 7833-87
Alkali has the largest speed. In practice, it is found that the attenuation of fast neutrons in soil is proportional to the hydrogen content in the soil. Therefore, due to the difference in soil moisture, the change in the number of captured slow neutrons (i.e. thermal neutrons) can be reflected on the detector accordingly. If the relationship between the moisture content and the number of neutrons of various soils is calibrated in advance, the soil moisture content can be obtained based on the number of neutrons detected by the detector. 2.2 Main Instruments
Neutron Moisture Meter. It consists of two main parts: a. Detector (which contains a fast neutron radiation source and a slow neutron detector), b. Calibrator or rate meter, which monitors the slow neutron flux, which is proportional to the soil moisture content. The fast neutron radiation source can be radium 226-beryllium, radium 241-beryllium and other neutron sources. Recently, a trace amount of pure steel 252 is also used, and the risk of radiation damage is very small. If a fast neutron source with a long life is selected (such as the half-life of radium-beryllium is 1620 years), it can be used for many years without significant changes in radiation flux. Note that there are two types of detectors: insertion type and surface type. 2.3 Measurement steps
2.3.1 For the insertion detector, the probe must be placed in the casing in the soil at the measurement site, and inserted vertically into the soil to a certain depth for measurement. For the surface detector measurement, the surface must be flat, and the bottom of the detector is in close contact with the surface. The volumetric humidity of the soil from the surface to a depth of 20 to 30 cm can be measured.
2.3.2 Turn on the switch and read the count rate directly from the calibrator or rate meter. The count rate and the volumetric humidity of the soil are nearly linearly dependent.
2. 4 Result calculation
Convert the count rate read into the soil volumetric humidity by looking up the table or standard curve. Note: ① The effective volume for measuring water content depends on the concentration of hydrogen nuclei, that is, the humidity per unit volume of the soil, and the energy of the emitted fast neutrons. The commonly used bracelet-beryllium source actually measures a spherical volume, which is about 15 cm in diameter for moist soil, but may be as large as 50 cm or more for relatively dry soil. The relatively large volume monitored is conducive to the study of water balance. Large volumes are usually more representative of field soil than small samples. ② The content of manganese, iron, chlorine and other elements in soil (especially soil that has been immersed in seawater) is relatively high, and the absorption capacity of these elements is also strong. In addition, organic matter also contains a large amount of hydrogen. The mutual interference of these elements is also the main cause of measurement errors and needs to be corrected. The type and degree of wear of the casing also have some influence, so it should be replaced in time during use or re-calibrated as necessary during long-term use.
③ It is dangerous to use the instrument improperly. The danger comes from the radiation intensity of the neutron source, the exposure time and the distance between the neutron source and the operator. As long as you pay attention to the safe operating procedures, this instrument can be used safely. Additional notes:
This standard was proposed by the Ministry of Forestry of the People's Republic of China and is under the jurisdiction of the Forestry Research Institute of the Chinese Academy of Forestry. This standard was drafted by the Forest Soil Research Laboratory of the Forestry Research Institute of the Chinese Academy of Forestry.
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