Determination of soil organic matter Source: Category: Technical Articles Update Time: 2008-11-3 17:31:28 Read 152 times that of soil organic matter in the soil of various nutrients, in particular, it is an important source of nitrogen and phosphorus. It also contains humic acids and other substances that stimulate plant growth. Because it has colloidal properties, it can adsorb more cations, thus making the soil fertility and buffering. It can also loosen the soil and form structures that can improve the physical properties of the soil. It is also an essential carbon source and energy source for soil microorganisms. Therefore, in addition to the low-lying soil, in general, the amount of soil organic matter is an important indicator of the level of soil fertility.
Instruments, Equipment and Reagents
1. Instruments and equipment Hard test tubes, oil bath digestion devices (including oil baths and wire cages), adjustable temperature furnaces, stopwatches, automatic temperature control regulators.
2. Reagents
(1) 0.800 0molL-1 Potassium Dichromate Standard Solution: Dissolve 39.2245 g of potassium dichromate (K2Cr207, analytical grade) dried at 130°C in water and bring to volume to 1000 mL.
(2) 0.2molL-1FeS04 solution: Weigh 56.0g of ferrous sulfate (FeS047H20, chemically pure) in water, add 5mL of concentrated sulfuric acid, and dilute to 1L.
(3) Indicator 1 o-Phenanthroline indicator: Weigh out 1.485 g of o-phenanthroline (Analytical Pure) and 200.695 g of FeS047H and dissolve in 100 mL of water.
22-Carboxydiphenylamine (o-phenylanthranilic acid, also known as o-phenylaminobenzoic acid, C13H11O12N) Indicator: Weigh 0.25
g reagent in a small mortar and mortar, and then poured into a small 100mL beaker, add 0.1molL-1NaOH solution 12mL, and use a small amount of water to rinse the residual reagent in the mortar 100mL beaker, the beaker on a water bath to heat it Dissolve, dilute to 250 mL after cooling, place clear or filter, and use the supernatant.
Method and procedure
1. Sample Preparation Weigh 0.1 to 1g (accurate to 0.0001g) of 0.14mm (100 mesh) sieve air-dried soil samples, place 6-8 dry hard test tubes, and accurately add 0.8000 with a pipette. molL-1 potassium dichromate standard solution 5mL (if the soil contains chloride need to add (Ag2S040.1g), add concentrated H2S04 with a syringe
Shake 5mL thoroughly and cover the neck with a small funnel.
2. Measurement
1 Place 8 to 10 test tubes (1 to 2 blank tubes per cage) in a wire cage, place a paraffin oil bath at a temperature of 185 to 190°C, and control the electric furnace so that the temperature inside the oil bath Always maintain at 170～180°C, start timing when boiling liquid in the test tube, boil 5
Min, remove the test tube, and wipe the external oil of the test tube after it is cold.
2 After cooling, pour the contents of the test tube into a 250 mL conical flask. Wash the inside of the tube and the small funnel with water to make the total volume of the solution in the flask reach 60 to 70.
In mL, keep the sulfuric acid concentration in the mixture at 2～3molL-1, then add 12～15 drops of 2-carboxydiphenylamine indicator, and the solution is brownish red. With a standard 0.2molL-1 ferrous sulfate titration, during the titration constantly shaking the flask, until the color of the solution from the brown-purple to dark green (gray blue-green), which is the end of the titration. Such as with o-orpine porphyrin indicator, plus 2 to 3 drops of the indicator, the process of discoloration by the orange-blue-green-brick red is the end. Record Pml4 titration ml (y).
Simultaneously with the determination of each batch of samples, 2 to 3 blank tests are carried out, ie 0.5 g of powdered silica is taken in place of the soil sample, and the other steps are the same as those for the determination of the samples. Record the number of milliliters (Vo) of FeS04 titration, and take the average value.
3. Calculation
Where: c - concentration of potassium dichromate standard solution, molL-1
Vo - volume of FeS04 used for blank titration, mL;
3.0——Molar mass of 1/4 carbon atom, gmol-1
1.1—oxidation correction factor;
m - dry soil quality, g;
k - Coefficient of converting dry soil into dry soil.
Soil organic matter (gkg-1) = Soil organic carbon (kg-1) Х 1.742
Where: 1.724 is the average conversion coefficient of soil organic carbon exchanged into soil organic matter.
Note 1 contains more than 50gkg-1 machine, weighing soil 0.1g, containing 20 ~ 30gkg-1 machine, said soil sample 0.3g, less than 20gkg-1 were weighed more than 0.5g.
2 The presence of chloride in the soil can lead to high results. Because chloride can also be oxidized by potassium dichromate, the determination of organic matter in saline soil must be prevented by chloride interference. A small amount of chlorine can be added with a small amount of Ag2SO4 to precipitate the chloride. The addition of Ag2SO4 not only precipitates chloride but also promotes the decomposition of organic matter. The amount of Ag2SO4 can not be too much, about 0.1g, otherwise it will produce Ag2Cr2O7 precipitation, affecting the titration.
3 It is necessary to start the boiling on the surface of the solution in the tube before calculating the time. Master boiling standards as consistent as possible, and then continue to cook 5min, cooking time has a greater impact on the analysis results, it should be as accurate as possible.
4 The color of the boiled solution should generally be yellow or slightly greenish. If the color is green, the amount of potassium dichromate is insufficient. If the amount of ferrous sulfate consumed is less than 1/3 of the blank amount during titration, there is a possibility of incomplete oxidation, and redoing should be discarded.
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