Can nitrogen, phosphorus and potassium in soil be measured by ordinary spectrophotometer?

Only phosphorus can be

Determination of soil alkaline nitrogen

I. Aims and requirements

Master the diffusion method for the determination of soil alkaline nitrogen.

II. Contents and principles

Soil hydrolyzable nitrogen or alkaline nitrogen includes inorganic nitrogen (ammonium nitrogen, nitrate nitrogen) and easily hydrolyzable organic nitrogen (amino acids, acyl ammonium and easily hydrolyzable proteins). When treating soil with alkali, the easily hydrolyzed organic nitrogen and ammonium nitrogen are converted to ammonia, while nitrate nitrogen is first converted to ammonium by ferrous sulfate. The ammonia is absorbed with boric acid and then titrated with standard acid to calculate the hydrolyzable nitrogen content.

Three, the main instruments and reagents preparation

Main instruments: electronic balance, diffusion dish, wool glass, thermostat, semi-micro buret (5 ml)

Reagents:

(1) 1.07molL-1Na0H: weighing 42.8 grams of NaOH dissolved in water, cooled to dilute to 1 liter.

(2) 2% H3BO3 indicator solution: weigh 20 grams of H3BO3 with 900 ml of water, slightly heated to dissolve, cooled, add 20 ml of mixed indicator (0.099 grams of bromocresol green and 0.066 grams of methyl red dissolved in 100 ml of ethanol). Then the solution was adjusted with 0.1 m0lL-1NaOH to a reddish purple color (pH about 5) finally diluted with water to 1000 mL, mixed well and stored in a bottle.

(3) 0.005molL-1H2SO4 standard solution: take concentrated H2S04 1.42 ml, add 5000 ml of distilled water, and then calibrated with standard alkali or borax (Na2B407?10H2O).

(4) alkaline glycerol: add 40 grams of gum arabic and 50 ml of water in a beaker, warmed to 70 - 80 ℃ stirring to promote solubility, cooling for about 1 hour, add 20 ml of glycerol and 30 ml of saturated K2CO3 aqueous solution, stirring cool, centrifugation to remove the foam and insoluble matter, the clear liquid stored in a glass bottle. Standby. (5) ferrous sulfate powder: FeSO4.7H2O (three) finely ground, packed into a glass bottle, stored in a cool place.

Four, the operation method and experimental steps

1, weighing through a 1 mm sieve of air-dried soil samples 2 grams (accurate to 0.01 grams) and ferrous sulfate powder 0.2 grams of uniformly spread in the diffusion of the dish in the outer chamber, gently rotate horizontally the diffusion of the dish, so that the sample is spread flat.

2, in the inner chamber of the diffusion dish, add 2 ml of 2% boric acid solution containing indicator, and then in the outer chamber of the edge of the dish coated with alkaline glycerin, cover the glass, and rotate it, so that the glass and the edge of the diffusion dish is completely bonded, and then slowly turn away from one side of the glass, so that the diffusion dish reveals a slit, and then quickly add 10 ml of 1.07 molL-1NaOH liquid in the diffusion dish in the outer chamber. Immediately rotate the cover tightly, gently rotate the diffusion dish horizontally on the experimental bench, so that the solution was fully mixed with the soil, and fixed with a rubber band; then carefully put into the 40 ℃ thermostat.

3, 24 hours later removed, with a microburette to 0.005molL-1 H2SO4 standard solution titration diffusion dish chamber boric acid solution absorbed ammonia, the end point of the purple-red color.

Take another diffusion dish, do a blank test, without soil, other steps with the same soil.

V. Homework

Calculate the content of alkaline nitrogen in soil according to the following formula

C × (V-V0) × 14

Hydrolyzed nitrogen in soil (mgkg-1) = ------------------- --- ×1000

W

C-- Concentration of H2S04 standard solution

V- - Volume of H2S04 standard solution used in sample determination

V0-- Volume of H2S04 standard solution used in blank determination

14-- Molar mass of nitrogen

< p>1000--Conversion coefficient

W-- Weight of soil (g)

VI. Precautions

The type and concentration of alkali, the ratio of soil and liquid, the temperature and time of hydrolysis in the process of determination all have a significant influence on the value. The factors such as alkali type and concentration, soil-liquid ratio, temperature and time of hydrolysis during the measurement process have certain influence on the value of the measured value. In order to obtain reliable and comparable results, the measurement must be carried out in strict accordance with the specified conditions.

VII. Reference index

Soil hydrolyzable nitrogen (mgkg-1) class

<25 very low

25-- 30 low

50--100 medium

100-- 150 High

Determination of quick-acting phosphorus in the soil

Knowing the supply of quick-acting phosphorus in the soil has a direct significance in guiding the application of fertilizer. There are many methods for the determination of quick-acting phosphorus in soil, and the results are not the same due to the different extractants. In general, calcareous and neutral soils are extracted with sodium bicarbonate, and acidic soils are extracted with acidic ammonium fluoride.

(A) sodium bicarbonate method

1, the principle of the method

Neutral, calcareous soil in the fast-acting phosphorus, mostly in the state of monocalcium phosphate and dicalcium phosphate exists, with 0.5M sodium bicarbonate can be extracted into solution, and then the solution to be measured with molybdenum antimony antimixed chromogenic agent at room temperature for reduction to make the yellow antimony phosphorus molybdenum polypeptide reduction to phosphorus molybdenum colorimetry.

2, the operation procedure

Weigh through 1 mm sieve air-dried soil 2.5 g (accurate to 0.01 g) in 250 ml triangular flask, add 50 ml of 0.5 M NaHCO3 liquid, and then a dime-spoon of phosphorus-free activated charcoal, corked tightly, at 20 - 25 ℃, oscillation for 30 minutes. Remove from the vial and filter through a dry funnel and phosphorus-free filter paper into a triangular vial, and do a reagent blank test at the same time. Aspirate 10 ml of filtrate in a 50 ml measuring flask, with molybdenum antimony reagent 5 ml of color development, and fixed with distilled water, shaking, at room temperature higher than 15 ° C for 30 minutes under the conditions of colorimetry with red filters or 660 nm wavelength of light, with the transmittance of the blank solution for 100 (i.e., the optical density of 0), read out the optical density of the determination of the solution, the color development of the solution in the standard curve to find out the concentration of phosphorus ( mgkg-1).

Standard curve preparation:

Pick up the standard solution containing phosphorus (P) 5mgkg-1 0, 1, 2, 3, 4, 5, 6 ml, were added to the 50 ml volumetric flask, add 0.5M NaHCO3 liquid 10 ml, add water to about 30 ml, and then add molybdenum and antimony anticolorant 5 ml, shaking, and then the capacity of 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 mgkg-1 phosphorus standard series of solutions, and the solution to be measured at the same time than the color, read the absorption value, in the grid coordinate paper to the absorption value for the vertical coordinate, the number of phosphorus mgkg-1 for the horizontal coordinate, plotted as a standard curve.

1. Calculation of results

Phosphorus mgkg-1 number of color developing solution × volume of color developing solution × multiplicity of sampling

Quick-acting phosphorus mgkg-1 in soil = ----- ---------- ---------- ---------

Dry soil weight (g) )

Phosphorus mgkg-1 of color developing solution: Phosphorus mgkg-1 of color developing solution obtained from the working curve

Volume of color developing solution: 50 ml

Total volume of leaching solution (50 ml)

Splitting times = -----. ----------------- -

Number of milliliters of leachate extracted

4. Preparation of main apparatus and reagents

Apparatus: Reciprocating oscillator, spectrophotometer or photoelectric colorimeter

Reagents:

(1) 0.5M NaHCO3 leaching agent (pH=8.5)

Weighing 42.0g of NaHCO3 dissolved in 800 ml of water, diluted to 990 ml, adjusted pH to 8.5 with 4M NaOH solution, then diluted to 1 liter, stored in bottle, if more than one month, pH should be recalibrated before use.

(2)Phosphorus-free activated carbon powder

Soak the activated carbon powder with 1:1 HCl overnight, then pump and filter with a flat funnel, wash with water until there is no HCl, and then add 0.5M NaHCO3 liquid overnight, pump and filter on a flat funnel, wash with water to clean the NaHCO3, and then finally check it until it is free of phosphorus, and then dry it for spare.

(3) molybdenum antimony anti-reagent

Weighing antimony potassium tartrate (KSbOC4H4O6) 0.5 grams, dissolved in 100 ml of water to make a 5% solution.

Another 20 grams of ammonium molybdate dissolved in 450 milliliters of water Xu added 208.3 milliliters of concentrated sulfuric acid, add and stir, and then 0.5% of the potassium antimony tartrate solution 100 milliliters added to the ammonium molybdate liquid, and finally added to 1 liter, shaken well, stored in a brown bottle, this is the molybdenum and antimony mixed solution.

Before use (the same day) weigh 1.5 grams of levo-ascorbic acid solution in 100 milliliters of molybdenum-antimony mixture, mix well. This is molybdenum antimony reagent. (The expiration date is 24 hours, if stored in the refrigerator, the expiration date is longer.)

(4) phosphorus standard solution

Weigh 0.439 grams of KH2PO4 (105 ℃ baking 2 hours) dissolved in 200 ml of water, add 5 ml of concentrated H2SO4, transferred to a 1-liter flask, water, water, this is the 100 mgkg-1 phosphorus standard solution, which can be stored for a long time. Take this solution diluted 20 times that is 5 mgkg-1 phosphorus standard solution, this solution should not be stored for a long time.

Determination of soil quick potash

(I) Objectives and requirements Understand the principle of determination of soil quick potash; grasp the main methods and steps for the determination of soil quick potash. (ii) Determination of soil potassium fastness by flame photometer method. (iii) Instruments and supplies Flame photometer. Flame photometer, analytical balance, oscillator, 100mL triangular flask, 1000ml, 100ml, 50mL volumetric flasks, 20ml, 40ml pipettes, 10ml pipettes, earwash balls, etc. (D) Principle and method Potassium is one of the nutrients needed for plant growth, and the congestive elements in the soil can be divided into four states; potassium-containing minerals in the soil (this is insoluble potassium); non-substitutive God (for slow-acting potassium); substitutive potassium; water-soluble potassium (fast-acting potassium). The potassium that plants can utilize is the potassium that exists in water-soluble and substitutive states, of which the main one is substitutive potassium. Therefore, the measurement of soil potassium substitution can truly reflect the supply of quick-acting potassium in the soil.

(E) operation steps (1) weighing soil a leaching weighing through the lmm sieve air-dried soil samples 5g (accurate to O.01g, into the 100ml triangular flask, add 50mL 1mol / L neutral ammonium acetate solution plugged tightly the rubber stopper. (2) Oscillation and filtration: fix the triangular flask on the oscillator, oscillate for about 15 minutes; filter immediately; the filtrate is contained in a 50mL volumetric flask, and the volume is fixed to the scale. (3) Determination The filtrate in the above volumetric flask was determined by flame photometer together with potassium standard series solution. Plot the standard curve on the grid paper, and then check the corresponding ppm of the solution to be measured

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