Determination of plant moisture, dry matter and crude ash
Determination of plant moisture, dry matter and crude ash
2.1 Determination of plant moisture and dry matter
Plants are composed of water and dry matter. The water content is an indicator that reflects the physiological state and maturity of the plant. If the water content is too high, the plant will easily become leggy and lodging; if the water content is too low, the plant will easily wilt. Plants need the right amount of water to grow and thrive. When studying the effects of factors such as soil, fertilization, cultivation, and climate on plant growth and development and photosynthetic utilization efficiency, it is generally necessary to measure the water and dry matter accumulation status of the plant. The water content of fresh plants is generally 70 to 95%, with the leaves having higher water content, and young leaves being the highest. The stems have less water content, and the seeds have even less water content, generally 5 to 15%. The remaining part of fresh plants after removing water is matter, which includes organic matter and minerals. Among them, organic matter accounts for 90 to 95% of plant dry matter, and minerals account for 5 to 10%.
Moisture content measurement is also an important criterion for quality inspection of crop products and judging whether they are suitable for storage. In the analysis of plant components, the mass percentage of each component is calculated based on the whole dry sample. Because the moisture content of fresh samples varies greatly, the moisture content of air-dried samples will also change due to the influence of environmental humidity and temperature. Only by using completely dry samples for calculation (dry basis) can the values ??of each component content be relatively stable.
2.1.1 Methods for measuring moisture
There are many methods for measuring plant moisture, which should be appropriately selected according to the properties of plant sample components, requirements for analytical accuracy, and laboratory equipment conditions. . Commonly used methods include normal pressure and constant temperature drying, reduced pressure drying and distillation. Among them, the most commonly used normal pressure and constant temperature drying method has higher accuracy and is suitable for samples that do not contain easily pyrolyzed and volatile components. It is considered It is a standard method for determining moisture; however, it is not suitable for young plant tissues and samples containing sugar, dry oil or volatile oil. The reduced pressure drying method is suitable for samples containing easily pyrolyzed components; however, it is not suitable for samples containing volatile oils. The distillation method is suitable for samples containing volatile oils and dry oils, and is more suitable for samples containing more water, such as Fruits and vegetables etc. Others, such as infrared drying method, freeze drying method, microwave attenuation method, neutron method, Karl Fischer method, etc., require specific equipment and are difficult to promote and use.
Normal pressure and constant temperature drying method
Principle of the method: Plant samples are dried in an oven at 100-105°C, and the moisture is calculated from the drying weight loss of the sample (that is, the moisture weight) content. This method is suitable for plant samples that do not contain easily pyrolyzed and volatile components.
During the high-temperature drying process of plant samples, some components that are easily charred, decomposed and volatilized may be lost, causing positive errors in moisture measurement; it may also be due to incomplete removal of moisture (or during cooling or weighing). moisture absorption) or some grease, etc., may be oxidized and gain weight, resulting in negative error. However, under strictly controlled operating conditions, this method is still the standard method for measuring plant moisture.
Operation steps
1. To measure the moisture content of air-dried plant samples, take a clean aluminum box, open the lid, and place it in an oven at 100-105°C for 30 minutes. Take it out, cover it, and transfer it to a desiccator filled with silica gel to cool to room temperature (it takes about 30 minutes). Weigh immediately and quickly. Bake for another 30 minutes and weigh. If the difference between the two weighings is less than 1 mg, it can be regarded as reaching constant weight (m0).
Put about 3g of crushed (1mm) and mixed air-dried plant samples in an aluminum box that has reached constant weight. After accurately weighing (m1), place the lid under the box and move it. Place in an oven that has been preheated to about 115°C, close the door, adjust the temperature to 100-105°C, and bake for 4-5 hours. Take it out, cover it, move it to a desiccator, cool it to room temperature and then weigh it. Dry again in the same way for about 2 hours, and then weigh (m2) until the difference between the two weights is less than 2 mg.
2． Determination of the moisture content of fresh plant samples: Take a small beaker, put about 5g of clean pure sand and a small glass rod (you can omit it if the water is not much), move it into an oven at 100-105°C and bake it to constant weight (m0).
Put about 5g of chopped and mixed fresh plant samples into a small beaker, mix with sand and then weigh (mL). Bake the cup and sample in an oven (air blast) at 50-60°C for about 3-4 hours. After the sample is crispy, crush it gently with a glass rod, and then bake it at 100-105°C (no air blast) for about 3 hours. 3 to 4 hours, cool and weigh, then bake in the same way for about 2 hours, then weigh again until constant weight (m2).
Result calculation
1. Plant sample moisture content () = (m1-m2)/(m1-m0)*100
2. Plant sample dry matter content () = (m2-m0)/(m1-m0)*100
Allowable deviation: The allowable error of two parallel measurement results is 0.2% (air-dried sample), and 0.5% (fresh sample).
Notes
Fresh plant tissue has a lot of water and should not be dried directly at 100°C, because the external tissue easily forms a dry shell at high temperatures, making it difficult to expel the moisture in the internal tissue. . Therefore, it is necessary to preliminarily dry at a lower temperature, and then increase the temperature to 100-150°C for drying;
In the analysis of plants and fertilizers, the calculation of moisture content is customarily based on the analysis of samples (air-dried or freshly wet samples).
Reduced Pressure Drying Note
Method Principle: Under reduced pressure, the boiling point of water is lowered, which allows the water in the plant sample to evaporate at a lower temperature. Calculate the moisture content of the sample from the difference in final weight. This method is suitable for samples containing easily pyrolyzed components, but not for samples containing volatile components.
Operating steps
Weigh the chopped and mixed plant samples equivalent to 1.000-2.000g of dry matter in an aluminum box (m0) that has been pre-baked to constant weight. The added box weight is m1. Stagger the lid of the box and place it in a vacuum drying oven that has been preheated to about 80°C. Then exhaust and reduce the pressure to the required low pressure, usually above 80kPa, while heating to 70±1°C. The sample must be dried for more than 5 hours before the water can be driven out. Carefully and slightly unscrew the piston of the air inlet duct to allow air to slowly enter the drying box (the air intake should not be too fast to avoid violent air flow and blowing away the dried samples). After the pressure in the box is balanced with the atmospheric pressure, open the door. , take out the aluminum box and close the lid, transfer it to a desiccator with silica gel, cool it, and weigh it. Repeat the above operation until constant weight (the difference between the two weighings is not greater than 4 mg) (m2).
Result calculation
Same as normal pressure and constant temperature drying method.
2.2 Determination of plant coarse ash (dry ashing method)
After the plant is burned, all the water in it is removed, the dry matter is carbonized and decomposed, and the final residue It is called "coarse ash". The content of crude ash in plant dry matter varies with plant species, varieties, different organs and parts, growth period, growth environment and other agricultural technical measures, but is generally 2 to 7%, with an average of about 5%.
Measuring the content of crude ash can help us understand the nutrient absorption and accumulation status of various plants in different growth stages and different organs, as well as the effects of factors such as soil, fertilization, climate, and cultivation management on changes in plant ash content. It is also one of the quality inspection projects for agricultural products and their processed products.
The ash obtained after burning plant samples at a certain temperature can not only be used to calculate the crude ash content, but also can be used to determine ash elements, such as phosphorus, potassium, sodium, calcium, magnesium and various other ash elements. Trace elements.
The method for measuring coarse ash currently adopts the simple, fast and economical dry ashing method.
Method Principle: Plant samples are carbonized at low temperature and burned at high temperature to remove all moisture and organic matter. The remaining non-combustible parts are oxides of ash elements, etc. After weighing, the crude ash content can be calculated. Because the ash obtained by burning inevitably contains a very small amount of unburned carbon particles and dust that is not easy to clean, and the composition of the ash after burning has changed (such as increase in carbonate, loss of chloride and nitrate, organic Phosphorus and sulfur are converted into phosphate and sulfate, and the weight changes), so the ash measured by the dry ashing method can only be called "coarse" ash.
The temperature during burning should be controlled at 525±25°C. It should not be too high or the burning should be too rapid.
Otherwise, part of the chloride of potassium and sodium will be volatilized and lost, and the phosphates and silicates of potassium and sodium are also easy to melt and the wrapped carbon particles will not be easily burned out; burning too fast will cause the particles to fly away. For samples containing a lot of acidic elements such as phosphorus, sulfur, and chlorine, in order to prevent these elements from escaping during burning, a certain amount of salts of the alkaline metal elements calcium and magnesium must be added to the sample to fix it, and then burn it. change. At this time, a blank measurement is performed to correct the amount of metal salt added.
Add a small amount of alcohol or pure olive oil to the sample to loosen the sample when burning, and obtain a nearly white coarse ash; you can also add a small amount of distilled water or concentrated HNO3 during the burning process, etc. , to accelerate the ashing of carbon particles.
Operating steps
Burn the numbered porcelain crucible in a high-temperature electric furnace at 600°C for 15 to 30 minutes, move it to the furnace door to cool slightly, and place it in a desiccator to cool to Room temperature, weigh, burn again, cool, and weigh until constant weight (m0).
In a crucible of known weight, weigh 2.000-3.000 g (m1) ground (1mm) dried plant sample, add 1-2mL ethanol solution (in order to promote uniform ashing of the sample), Keep the sample moist. Then place the crucible on the voltage-regulating electric furnace with the crucible cover tilted, adjust the temperature of the electric furnace to slowly heat and carbonize, and when the smoke is gone, move it to a high-temperature electric furnace, heat to 525°C, keep it for about 1 hour, and burn to ash. Until almost white. Move the crucible to the furnace door, wait until it is cooled to below 200°C, then move it to a desiccator to cool to room temperature, and weigh. Then burn again for 30 minutes, cool and weigh. Until the weight difference between the two before and after does not exceed 0.5mg, it is considered to be constant weight (m2).
Result calculation
Coarse ash content () = (m1-m2)/(m1-m0)*100
Notes
(1) For samples (seeds) with high phosphorus content, 3 mL of magnesium acetate ethanol solution (1.5) can be added to wet all samples first, and then carbonized and ashed. Temperatures as high as 800°C will not cause phosphorus loss. Samples containing higher sulfur and chlorine can be soaked with sodium carbonate or lime solution and then ashed. In order to prevent the loss of boron during ashing, the plant samples must be added with NaOH solution before ashing. The added amount must be blank corrected.
§3 Determination of plant macroelements
Nitrogen (N), phosphorus (N), and potassium (K) are the three major elements of plant nutrition. Plants have the largest demand for nitrogen. , are also most likely to be deficient, followed by phosphorus and potassium. Therefore, the measurement of plant nitrogen, phosphorus, and potassium content is the most common routine analysis item in plant nutrition research, such as diagnosing plant nitrogen nutrition levels and soil nitrogen supply conditions, and understanding plants. The amount of nitrogen absorbed from the soil, the effect of nitrogen fertilizer application, the relationship between nitrogen absorption by plants and other nutrients, and the development of plant nitrogen nutrition diagnostic indicators, all need to measure the nitrogen content of the whole plant or certain parts of the plant (sensitive parts and organs). Content.
The nitrogen (N) content of plants is about 0.3 to 5% of dry matter weight, the phosphorus (P) content is generally 0.05 to 0.5%, and the potassium (K) content is generally 1 to 5%. The contents of N, P, and K vary depending on plant types, organs, growth stages, and fertilization management levels. For example, the nitrogen content of soybean grains is 5.36%, and the stems are 1.75%; the nitrogen content of wheat grains is 2.2 to 2.5%, and the stems are only About 0.5%; rice grains contain 1.31% nitrogen, and stems contain 0.51%. Nitrogen content also often changes at different plant development stages. This also shows that when sampling and measuring different plants and formulating nitrogen, phosphorus, and potassium nutritional abundance and deficiency diagnostic indicators. When measuring, the plant growth period and tissue location must be noted. Only under the same circumstances will the measurement results have comparative significance and have reference value for guiding plant fertilization. Similarly, when applying various crop nutrition diagnostic indicators, they are only for interpretation and analysis. Results are for reference only.