How To Measure Organic Matter In Soil
Soil organic carbon refers to a special, complex, stable macromolecular organic compound produced by organic residues through microbial action. It is an essential part of the soil and closely related to soil fertility, so measuring the organic carbon in soil is necessary.
Physical analysis method
The TOC physical analysis method is to burn the organic carbon in the soil sample at a high temperature of 1100 ° to release carbon dioxide and then collect the TOC value converted into the sample with a highly sensitive detector.
The TOC value multiplied by a coefficient of 1.724 is the content of organic matter in the soil.
Pros:
high precision, stability and good repeatability,
Cons:
it is the total carbon content in the soil, which is easily affected by inorganic carbon such as carbonate and highly concentrated carbon in the sample.
Chemical analysis method
Under heating conditions, the organic carbon in the soil sample is oxidized by excess potassium dichromate sulfuric acid solution, and the hexavalent chromium (Cr6 +) in potassium dichromate is reduced to trivalent chromium (Cr3 +).
Its content is directly proportional to the sample’s organic carbon content. We can measure the absorbance at the wavelength of 585 nm and calculate the organic carbon content according to the content of trivalent chromium (Cr3 +). Dry the soil sample to constant weight at 105 ± 5 ° C, then calculate the dry substance content based on the mass difference of the soil sample before and after drying.
Preparation of samples
Place the soil sample in a clean white enamel tray and spread it into a thin layer of 2 ~ 3 cm thick. First, remove the residues such as plants, insects and stones, crush the soil with a wooden hammer, and dry it naturally. When drying, turn it several times a day.
Thoroughly mix the dried soil and use the quartering method to take two copies, one retained and one passed through the 2 mm soil sieve to determine dry matter content. Take out 10 ~ 20 g of the sample passing the 2 mm sieve for further fine grinding, pass the 60 mesh (0.25 mm) soil sieve, and put it into a brown stoppered glass bottle to measure the organic carbon.
Determination of dry substance content
Sample processing
Take an appropriate amount of fresh soil samples and lay them on a clean enamel plate or glass plate to avoid direct sunlight at the ambient temperature of less than 40 ° C. dry them naturally, remove stones, branches and other impurities, and pass them through a 2mm sample sieve. After crushing the soil block > 2mm, pass the 2mm sample sieve, mix well and wait for measurement.
Sample determination
The container with lid and lid dried at 105 ± 5 ° C for 1h, cooled slightly, then covered the lid, and then placed in the dryer for cooling for at least 45min. Measure the mass of the container with lid as M0, accurate to 0.01g. Put 10 ~ 15g air-dried soil sample to the weighed container with a sample spoon, cover the container cover, and determine the total mass as M1, accurate to 0.01g.
Remove the container cover, put the container together with the air-dried soil sample into the oven, dry it to constant weight at 105 ± 5 ° C, and dry the container cover simultaneously. Cover the container and put it in a dryer for cooling for at least 45 minutes. After taking it out, measure the total mass of the M2 container with a cover and the dried soil immediately, accurate to 0.01g.
Steps
Drawing of the calibration curve
1) Weigh 10.00g of glucose and dissolve it in an appropriate amount of water. After dissolution put it in a 1000 ml volumetric flask, fix the volume with water and shake well.
2) Measure 0.00, 0.50, 1.00, 2.00, 4.00 and 6.00 ml glucose standard solution respectively into a 100 ml digestion glass tube with a plug, and the corresponding organic carbon mass is 0.00, 2.00, 4.00, 8.00, 16.0 and 24.0mg respectively.
3) Add 0.1g mercuric sulfate and 5.00 ml potassium dichromate solution respectively, and shake well. Add 7.5 ml of sulfuric acid slowly and shake gently.
4) Turn on the thermostatic heater and set the temperature to 135 ℃. When the temperature rises to nearly 100 ℃, open the plug of the above plug digestion glass tube and put it into the heating hole of the thermostatic heater. When the instrument temperature displays 135 ℃, start timing and heating for 30 minutes. Please take out the plug digestion glass tube and cool it to room temperature in a water bath. Slowly add about 50ml of water to each digestion glass tube with a plug and continue to cool to room temperature. Then fix the volume with water to a 100 ml scale, add a stopper and shake well.
5) At the wavelength of 585 nm, measure the absorbance with a 10 mm cuvette and water as a reference. The calibration curve is drawn with the zero concentration corrected absorbance as the Y and the corresponding organic carbon mass (mg) as the X.
6) Do not add the sample to the digestion glass tube with a plug for the blank test, and the measurement is done according to the above steps.
Sample determination
Weigh about 0.5g of the sample accurately and carefully add it into a 100ml digestion glass tube with a stopper to avoid sticking to the wall. Add reagents according to the standard curve drawing steps for digestion, cooling and constant volume.
Let the test solution stand for 1h after constant volume, take about 80ml of supernatant into the centrifuge tube, centrifuge for 10min at 2000 R / min, and then let it stand until it is clear; Or directly stand in the digestion glass tube with plug until it is clear. Finally, take the supernatant at the wavelength of 585 nm and measure the absorbance with water as the reference.
Calculation
Soil dry substance content
wdm=（m2-m0）/(m1-m0)*100
WDM — dry matter content in soil sample,%;
M0 — the mass of container with cover, G;
M1 — total mass of container with a cover and air-dried soil sample, G;
M2 — total container mass with a cover and dried soil, G.
Soil organic carbon content
m3=m*wdm/100
ω= (A -A0)/m3/1000*100
M — sample quantity, G;
M3 – the mass of dry matter in the sample, G;
ω—— Content of organic carbon in soil samples (calculated by dry weight, mass fraction),%;
A — absorbance of sample digestion solution;
A0 — absorbance of the blank test;
How to measure the components of soil organic carbon (SOC)
1) Soluble organic carbon (DOC)
Weigh 5g of dried soil into the centrifuge tube, add distilled water according to the water-soil ratio of 20:1, shake at 180rpm · min-1 for 30min at 25 ℃, centrifuge for 10min, and pass 0.45 μ M filter membrane, collect the filtrate and measure it with TOC instrument.
We can measure the soluble organic carbon by TOC instrument after shaking centrifugation with deionized water with a water soil ratio of 5:1.
Take 10g of air-dried soil and add deionized water (the water-soil ratio is 2.5:1, V / W), then shake for 30min and centrifuge at 4000 rpm · min-1 for 10min, and use glass fiber filter membrane (0.45 μ m) Filtration and TOC measurement.
2) Easily oxidized organic carbon (EOC)
Put 1.5g of air-dried soil sample into 100ml centrifuge tube, add 333M mol · L-1 potassium permanganate KMnO4 25ml, oscillate at 25 ℃ for 1h, centrifuge for 5min (rotating speed 2000rpm · min-1), take the supernatant and measure it with TOC instrument. (or 1:250 dilution with deionized water by colourimetry)
3) Total soil carbon / total organic carbon (TC / TOC) and total inorganic carbon (TIC)
Samples need to be ground, sieved and dried before analysis.
Step:
Place the soil sample on the air-drying plate and spread it into a thin layer of 2 ~ 3cm thick. First, remove the residues such as plants, insects and stones, crush the soil block with an iron hammer or porcelain grinding rod, turn it several times a day and dry it naturally. Thoroughly mix the air-dried soil and use the quartering method to take two parts, one for retention and one for grinding, until all pass the 2mm (10 mesh) soil screen. Take 10 ~ 20g sieved soil samples, grind them until they all pass the 0.097mm (160 mesh) soil sieve, put them into a brown glass bottle with a stopper and wait for measurement.
We can measure the TOC content in soil by subtraction (indirect method) or direct method.
Subtraction method (indirect method): weigh a certain quality of soil samples treated in the above steps to test TC and TIC respectively, and subtract the two items to get the value of TOC.
Direct method:
- Weigh a certain quality of the soil sample treated in the above steps into the ceramic boat (weighing about 100mg).
- Add 10% hydrochloric acid drop by drop.
- Observe the foaming condition.
- Drop until there are no bubbles.
After adding acid, put the ceramic boat containing soil samples in a 70 ℃ oven to dry, then take it out. It can be processed overnight to ensure that the samples are thoroughly dried and remove TIC from them. The TOC content was directly determined by the combustion method.
4) Light group organic carbon (LFOC)
Weigh 20g of the air-dried soil sample, place it in a 250ml centrifuge tube, add 50ml of sodium tungstate (1.6g · cm-3) solution, oscillate at low speed for 30min, and the vibration speed is 200rpm · min-1, to separate the light group and stable group in the soil sample.
Then, take 10ml of sodium tungstate solution with the same density to wash the soil particles attached to the inner wall of the centrifugal tube and let it stand overnight. The next day, the centrifuge tube and its contents were centrifuged at 5000 rpm for 30 minutes, and the suspended solids floating in the solution on the upper layer of the centrifuge tube were collected in 1 μ M polycarbonate film so that we could get the light fraction (LF) of soil.
This component is free soil organic substance; dry it in an 80 ° C oven to constant weight, weigh it respectively, grind the soil sample in a disc mill and measure it with a TOC instrument.
5) Particulate organic carbon (POC)
Weigh about 12g of air-dried soil, add 0.5% sodium hexametaphosphate, oscillate on a reciprocating shaker for 18h, and pass 250 and 53 um the sieve respectively, according to the separated soil, evaporate all samples left on the sieve, weigh and measure with TOC instrument.
6) Microbial biomass carbon (MBC)
Weigh 6 pieces of about 11g of air-dried soil, spray water to the field with a water holding capacity of 70%, and after incubation at 25 ℃ for 15 days, put 3 pieces of soil into a vacuum dryer. Put a beaker containing 20ml chloroform at the bottom of the dryer. Vacuum the dryer until the chloroform solution boils, fumigate for 15min, and then incubate for 24h.
Open the dryer, remove the beaker containing chloroform, and remove the residual chloroform in the soil by repeated vacuum pumping. Extract K2SO4 with a water-soil ratio of 4:1 and 0.5m (0.5mol / L), shake for 30min, filter, absorb 5ml of filtrate and dilute it 10 times. Meanwhile, the non-sterilized soil is extracted using the above method by TOC meter.
MBC=EC/KEC
EC = OC fumigation – OC not fumigated
KEC = conversion coefficient (0.45 for TOC instrument)
