Some standard content:
1 Scope of application
National Standard of the People's Republic of China
Method for the determination of soil water content
UDC 631.423
GB 7172—87
This standard is used to determine the water content of various types of soil except gypsum soil and organic soil (soil containing more than 20% organic matter). 2 Principle of determination
The weight loss of soil sample when it is dried to constant weight at 105±2℃ is the mass ratio of water content in the soil sample. 3 Instruments and equipment
3.1 Soil drill;
3.2 Soil sieve: pore size 1mm,
Aluminum box: small diameter about 40mm, height about 20mm; large diameter about 55mm, height about 28mm; Analysis day: sensitivity 0.001g and 10.01g3.4
Small electric constant temperature oven:
3.6+ Desiccant: Contains color-changing silica gel or anhydrous calcium chloride. 4 Selection and preparation of samples
4.1 Samples: Select representative soil samples, crush them, pass them through a 1mm sieve, mix them evenly and set aside. 4.2 Fresh soil samples: Take representative fresh soil samples in the field with a soil drill, scrape off the upper floating soil in the soil drill, and crush about 20g of soil at the required depth in the middle of the soil drill. Put it into a large aluminum box with known mass, cover it tightly, put it into a wooden box or other container, bring it back to the room, wipe the surface of the aluminum box clean, weigh it immediately, and determine the moisture content as soon as possible. 5 Determination steps
5.1 Determination of moisture content of air-dried soil samples
Take a small aluminum box and bake it in a 105℃ constant temperature box for about 2h, transfer it to a desiccator and cool it to room temperature, weigh it, and determine the moisture content as soon as possible. 5 Determination stepswww.bzxz.net
5.1 Determination of moisture content of air-dried soil samples
Take a small aluminum box and bake it in a 105℃ constant temperature box for about 2h, transfer it to a desiccator and cool it to room temperature, weigh it, and determine the moisture content to 0.001g. Use a corner spoon to mix the air-dried soil sample, keep about 5g, evenly spread it in the aluminum box, cover it, and weigh it, and determine the moisture content to 0.001g. Open the lid of the aluminum box, put it under the box, and bake it in an oven preheated to 105±2℃ for 6h. Take it out, cover it, and weigh it, and determine the moisture content to 0.001g. 5.2 Determination of moisture content of fresh soil samples
Weigh the large aluminum box containing fresh soil samples on an analytical balance with an accuracy of 0.01g. Open the lid of the box, place it at the bottom of the box, and bake it in an oven preheated to 105±2℃ for 12 hours. Take it out, cover it, cool it to room temperature in the dryer t1 (about 30 minutes), and weigh it immediately. The moisture content of fresh samples should be determined in triplicate. Note: After baking for the specified time, weigh it once to reach "constant weight". Issued by the National Bureau of Standards on January 3, 1987
Implementation on August 1, 1987
Calculation of measurement results
6.1 Calculation formula
GB 7172--87
Water (analytical basis), %=
Water (decimal basis), %=
-mass of aluminum box after drying, g;
Formula 1. mo ---
-mass of aluminum box and sample before drying, g,
m2--mass of aluminum box and soil sample after drying, g. mt--m2
m2 -mo
6.2 The results of parallel determinations shall be expressed as arithmetic mean values, with one decimal place retained. (1)
6.3 The difference in the results of parallel determinations shall not exceed 0.2% for a soil sample with a moisture content of less than 5%, 0.3% for a moist soil sample with a moisture content of 5-25%, and 0.7% for a large-grained (particle size of about 10 mm) heavy moist soil sample with a moisture content of more than 15% (equivalent to a relative difference of no more than 5%).
Additional Notes:
This standard was proposed by the Ministry of Agriculture and Animal Husbandry of the People's Republic of China. This standard was drafted by the Department of Soil Chemistry of Beijing Agricultural University. The main drafters of this standard are Li Xikai and Yi Xiaolin. 90
Determination of soluble salt in soil by weight method The soluble salt in soil is the water-soluble salt contained in the soil extracted with a certain water-soil ratio and within a certain period of time. By analyzing the anion and cation composition of the soluble salt in the soil, and determining the type and content of the salt, the salinity and salt dynamics of the soil can be determined, because when the soluble salt in the soil reaches a certain amount, it will directly affect the germination and normal growth of crops. Of course, the impact of salt on crop growth is mainly determined by the content and composition of soluble salt in the soil, and the salt tolerance of different crops. In terms of salt composition: soda salt (sodium carbonate, sodium bicarbonate) is the most harmful to crops, followed by sodium chloride, and sodium sulfate is relatively light. When the soluble magnesium in the soil increases, it can also poison crops. Therefore, regular determination of the total amount of soluble salts in the soil and its salt composition can help us understand the degree of soil salinization and seasonal salt dynamics, and accordingly formulate measures to improve the utilization of saline-alkali soil. Usually, the amount of drying residue of water extract is used to represent the total amount of water-soluble substances in the soil. The drying residue includes not only the amount of mineral salts, but also soluble organic matter and a small amount of oxides such as silicon and aluminum. The total amount of salt is usually the sum of anions and cations in the salt, and the amount of drying residue is generally higher than the total amount of salt, because the amount of non-salt should be deducted due to rain. In addition, the measured total amount of soluble salt can also verify the analysis results of various anion and cation components in the system analysis. There are many methods for determining the total amount of soluble salt, including weight method, conductivity method, hydrometer method, and total anion and cation calculation method. Since the hydrometer method is relatively extensive and the total anion and cation calculation method is relatively time-consuming, only the general weight method is introduced here. 1 Preparation of test solution
1.1 Principle
Mix soil sample and water in a certain water-soil ratio. After a certain period of oscillation, extract the soluble salt in the soil into the solution. Then filter the water-soil mixture. The filtrate can be used as the test solution for soil soluble salt determination. 1.2 Instruments
Electric oscillator, vacuum pump (for air extraction), large-mouth plastic bottle (1000mL), bus filter tube and flat porcelain funnel, air extraction bottle (1 000 ml).
1.3 Operation steps
1.3.1 Weigh 100g (accurate to 0.1g) of soil sample that has passed through No. 18 sieve (1mm mesh), put it into a 1000mL large-mouth plastic bottle, and add 500mL of carbon dioxide distilled water. 1.3.2 Plug the plastic bottle with a rubber stopper and oscillate it on the oscillator for 8 minutes. 1.3.3 After shaking, vacuum filter immediately. If the soil sample is not too sticky or the alkalinity is not high, you can use a flat porcelain funnel to filter until it is filtered. For samples with heavy soil and high alkalinity, vacuum filter with a bus filter tube. Store the clear liquid in a 500mL triangular flask and plug it with a rubber stopper for later use. If the solution of potassium and sodium ions is not to be determined for the time being, it should be divided into small plastic bottles of about 50mL for storage. 1.4 Description
1.4.1 Water-soil ratio: The water-soil ratio directly affects the extraction of soluble salts in the soil. Therefore, the extracted water-soil ratio should not be changed casually, otherwise the analysis results cannot be compared. Usually, a water-soil ratio of 5:1 is used for extraction. In order to study the dynamics of salt more in line with the actual situation, some people use field wet soil to squeeze the extracted solution on a soil press, and then analyze it in the same way. 1.4.2 Soil soluble salt extraction (oscillation) time: Experiments have shown that after 2 minutes of water-soil reaction, all soluble fluorides, carbonates and sulfates in the soil can be dissolved in water. If the reaction time is prolonged, medium-soluble salts and sparingly soluble salts (calcium sulfate and calcium carbonate) will enter the solution. Therefore, it is recommended to use the method of oscillating for 3 nm and then filtering immediately. The longer the oscillation and placement time, the greater the error in the analysis results of soluble salts.
1.4.3 Filter as quickly as possible. For soils with heavy texture or high alkalinity, use a Pasteur filter tube for vacuum filtration. Sometimes clay will block the pores of the Pasteur filter tube, causing the filtration speed to slow down. At this time, the Pasteur filter tube can be removed and a balloon can be used to inflate and pressurize the tube so that the clay adsorbed on the tube wall falls off in a shell shape. Then continue to vacuum filtration to speed up the filtration speed. 91
1.4.4 In the air The carbon dioxide partial pressure and the carbon dioxide dissolved in distilled water will affect the solubility of calcium carbonate, magnesium carbonate and calcium sulfate, and correspondingly affect the amount of salt in the water extract. Therefore, distilled water without carbon dioxide must be used to extract the sample. 1.4.5 If the total amount of salt is determined by the conductivity method, after oscillating for 3 minutes, let it settle for half an hour before determination. 1.4.6 The test solution cannot be left for too long (generally not more than 1 day), otherwise it will affect the determination of calcium, magnesium, carbonate and bicarbonate. 2 Determination method
2.1 Principle
Absorb a certain amount of the test solution, evaporate it to dryness, and the weight obtained is the total amount of dried residue (this value is generally close to or slightly higher than the total amount of salt). After the total amount of dried residue is removed of organic matter with hydrogen peroxide, its weight is weighed again to obtain the total amount of soluble salt. 2.2 Instruments
Porcelain evaporator III (100 mL), analytical balance, electric oven, water bath. 2.3 Operation steps
2.3.1 Take 50~100mL of the clear liquid to be tested and put it into a porcelain evaporation blood of known weight, and put the blood on a water bath to evaporate (a sand bath can also be used).
2.3.2 Wipe the outside of the porcelain evaporation blood with filter paper, put it in a 100~105℃ oven to dry for 4 hours, then move it to a dryer to cool, and weigh it with an analytical balance (generally cool for 30 minutes).
2.3.3 The weighed sample is placed in the oven for another 2 hours and then weighed again until constant weight (i.e. the difference between the two weights is less than 0.0003g), and the drying residue is obtained.
2.3.4 Continue to heat the above drying residue in a water bath with 15% hydrogen peroxide solution to remove organic matter, and then evaporate it to dryness according to the above method, and weigh it to constant weight to obtain the total amount of soluble salts.
2.4 Calculation of results
The results are calculated according to formula (1):
Drying residue (% Weighing blood and residue) = Weighing blood weight × 100W
Wherein: W = the volume of the test liquid absorbed, which is equivalent to the weight of the soil sample (e.g. 50mL is absorbed, which is equivalent to 10g of sample). 2.5 Explanation
+ (1)
2.5.1 The amount of the test liquid absorbed should be determined according to the amount of salt. If the salt content is >0.5%, absorb 25mL; if the salt content is <0.5%, absorb 50mL or 100mL. Keep the salt content between 0.02 and 0.2g. Too much salt will absorb water and it is not suitable to weigh to constant weight. Too little salt will result in large error. ·
2.5.2 The temperature during evaporation should not be too high. Too high, otherwise, the solution will be lost due to boiling, especially when it is close to evaporation, more attention should be paid, and this phenomenon can be avoided by evaporating on a water bath.
2.5.3 Since salt easily absorbs water in the air, it should be cooled and weighed under the same time and conditions. 2.5.4 When adding hydrogen peroxide to remove organic matter, it is sufficient to moisten the residue. This can avoid excessive foaming during the decomposition of hydrogen peroxide, which will cause the salt to splash. Therefore, it is necessary to repeatedly process in small amounts until the residue turns completely white. However, when there is iron in the solution and yellow iron oxide appears, it should not be mistaken for the color of organic matter. 92
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