How To Measure Nitrogen In Soil
Determination of total nitrogen methods
Kjeldahl method
Weigh 1.0 g of soil sample and put it into the digestive tube, wet it with a small amount of water, add potassium permanganate for oxidation, reduce it with reduced iron powder and a small amount of sulfuric acid, add a piece of mixed catalyst and 10 ml of concentrated sulfuric acid, digest it at 400 ℃ for 1.5 ~ 2.0 h, add 20 ml of distilled water into the digestive tube after the digestive solution is cooled to room temperature, and automatically distil and titrate it with Kjeldahl nitrogen determinator.
The Kjeldahl instrument can automatically calculate the total nitrogen content of the sample according to the consumed volume of sulfuric acid standard solution.
Pros:
- Good precision
- low machine and low cost.
Automatic chemical analyzer
Weigh 0.3000 ~ 0.5000g of soil sample into a 100ml digestion tube, add 5mlh2so4, shake well, connect a condensation reflux device to the digestion tube, place it on the digestion furnace for low fire heating, control the temperature at about 280 ℃, raise the temperature to 370 ℃ after 5min, keep it for 5min, remove it, cool it slightly, add 10 drops of H2O2, continue heating and digestion for 12min (record the temperature at 370 ℃).
After cooling slightly, add H2O2 repeatedly and then boil. After that, the amount of H2O2 added each time is 8 drops, 6 drops, 4 drops and 2 drops, respectively. After digestion, cool and fix the volume into a 500ml volumetric flask and boil the sample blank simultaneously. Take the supernatant after standing overnight, or the solution filtered by dry phosphorus-free filter paper, and use smartchem140 automatic intermittent chemical analyzer to determine TN at the wavelength of 660nm. Set 3 repetitions for each sample and take its average value.
Pros:
- high accuracy, good stability
- high recovery rate
- simple operation.
Dumas combustion nitrogen determination method
Weigh 0.2g soil sample into nitrogen-free tin foil paper, extrude air and place it in the automatic sampler. The sample is entirely burned in the oxidation tube. After high-temperature copper powder reduction, porous ceramic adsorption and drying, the generated gas is detected by a TCD detector and calculated and quantified by a standard curve.
Reaction conditions:
the temperature of the combustion tube is 990 ℃, and the temperature of the reduction tube is 650 ℃. At the same time, the EDTA with a certain mass gradient is weighed and tested on the machine, and the standard curve is drawn.
Pros:
- Simple, fast and efficient, good precision and accuracy, environmental protection and no pollution.
- Batch samples testing
- The results of Dumas combustion nitrogen determination and Kjeldahl nitrogen determination are no significant difference.
Potassium dichromate method
There are ammonium nitrogen and nitrate nitrogen for available soil nitrogen. In the diffusion dish, the soil is hydrolyzed and reduced reaction under the strong alkaline and ferrous sulfate condition. So, making hydrolyzable nitrogen and nitrate nitrogen transform into ammonia and absorbed by boric acid solution.
Titrate the ammonia in the absorption solution with standard acid, and calculate the content of available ammonia according to the consumption of standard acid.
Reagent:
1) NaOH solution, C (NaOH) = 1.8 Moll: weigh 72.0 g of sodium hydroxide, dissolve with water, cool and dilute to 1L;
2) Boric acid solution, 20 g / L: weigh 20.00 g of boric acid, dissolve it with hot water (about 60 ℃), dilute it to 1L after cooling, and finally adjust the pH to 4.5 with dilute sodium hydroxide (measured with odometer);
3) Mixed indicator for nitrogen determination: weigh 0.066 g methyl red and 0.099 g bromocresol green into a mortar, respectively, add a small amount of 95% ethanol, grind until the indicator is completely dissolved, and then add 95% ethanol to 100 ml. Dilute hydrochloric acid or dilute sodium hydroxide should be used to adjust the pH value to 4.5 ( measure with an acidimeter);
4) Special glue: 40g powdered Arabic gum is dissolved in 50ml water heated to about 70 ℃, stirred to promote dissolution. After cooling, add 20ml glycerol and 20ml saturated potassium carbonate aqueous solution, mix well, cool it, and store it in a glass bottle for standby (put it in a dryer containing concentrated sulfuric acid to remove ammonia);
5) Sulfuric acid standard titration solution, C (1 / 2h2so4) = 0.0100 mol / L of hydrochloric acid standard titration solution, C (HCI) = 0.0100 mol / L;
6) Zinc ferrous sulfate powder: grind 50.0 g of ferrous sulfate, pass through a 0.25 mm sieve, mix with 10.0 g of zinc powder, and store in a brown bottle.
Steps
1)Sample
Weigh 2.00 g and 1.0 g of zinc ferrous sulfate powder of air dried samples through a 2 mm sieve, evenly spread them in the outer chamber of the diffusion dish, and gently rotate the diffusion dish horizontally to flatten the sample.
2) Sealing dish
Add 3ml boric acid solution into the inner chamber of the diffusion dish (outer chamber diameter 10cm, inner chamber diameter 4cm), and drop 1 titration nitrogen mixing indicator, then apply special glue on the edge of the outer chamber of the dish, cover the ground glass, and rotate it several times to completely bond the ground glass with the edge of the dish.
3) Alkali addition
Then slowly turn one side of the ground glass to expose a slit in the diffusion dish, quickly add l0 ml [C (NaOH) = 1.8 mollnaoh] solution into the outer chamber of the dish, and immediately cover it with ground glass.
4) Spread
Gently rotate the diffusion dish horizontally to fully mix the solution with the soil, fix it with a rubber band, put it into an oven at 40 ℃ and 1 ℃, and take it out after 24 h ± 0.5 H.
5) Titration
Use sulfuric acid standard titration or hydrochloric acid standard titration solution to titrate the absorption solution in the inner chamber from blue-green to reddish, and record the consumption of standard acid.
6) Contrast
Blank test at the same time as sample determination.
4. Result from the calculation
The mass fraction of nitrogen W calculates the content of available soil nitrogen, and the value is expressed in (mg/kg), which is calculated according to the following formula:
W=(V-V0)*c*14*1000/m
V – the volume of standard acid consumed by titrating soil sample, ml;
V0 – the volume of standard acid consumed in titration blank, ml;
C – concentration of the acid standard solution, mol / L;
M – the mass of air-dried sample, G;
14 – the molar mass of nitrogen, g / mol;
1000 g is converted into mass per kg.
