CN111398267A - Method for detecting content of available phosphorus in soil in sugarcane area - Google Patents

Abstract

The invention provides a method for detecting the content of available phosphorus in soil in a sugarcane area. The method comprises the following steps: firstly sampling soil to be detected in a sugarcane area, then leaching a soil sample by using a sodium bicarbonate lixiviant, adjusting a liquid to be detected of the soil sample to be yellowish by using a 2, 4-dinitrophenol indicator, a dilute sulfuric acid solution and a dilute sodium hydroxide solution, further adding a molybdenum-antimony anti-color-developing agent, determining a light absorption value of a color developing solution by using a 1 cm-diameter cuvette at a wavelength of 700nm to obtain the phosphorus content in the liquid to be detected of the soil sample, and finally calculating the content of available phosphorus in the soil of the sugarcane area according to the phosphorus content in the liquid to be detected of the soil sample. According to the technical scheme, the detection time is short, the detection limit, the accuracy and the precision are good, the accuracy of the detection result of the effective phosphorus content in the soil in the sugarcane area can be improved, and the labor amount and the labor intensity can be reduced. The technical scheme can be widely applied to detection of available phosphorus in neutral, slightly acidic and calcareous soil, and further provides scientific basis for application of phosphate fertilizer in sugarcane areas.

Description

Method for detecting content of available phosphorus in soil in sugarcane area

Technical Field

The invention relates to the technical field of element content detection, in particular to a method for detecting the content of available phosphorus in soil in a sugarcane area.

Background

The research results show that the nitrogen and the potassium can promote the growth of the stem diameter and the stem length of the sugarcane, the obvious effect on improving the yield of the sugarcane is achieved, and the phosphorus and the potassium can increase the sugar content of the sugarcane. It follows that phosphorus has a significant effect on the growth of sugar cane. In calcareous soil, the calcium ion is usually higher, and calcium phosphate precipitation is easily formed with phosphate radical ions, so that the phosphorus effectiveness is reduced, and the phosphorus in the soil in a sugarcane area is deficient. The sugarcane production and fertilization are mainly guided by soil nutrient analysis research and production experience, so that scientific basis can be provided for reasonable fertilization for improving the yield and quality of sugarcane by detecting the content of available phosphorus in the soil of sugarcane areas.

The soil available phosphorus is not referred to as phosphorus with a specific form, and the soil available phosphorus does not have a real ' quantity ' concept, and different quantities of available phosphorus can be measured by different methods for the same soil, so the soil available phosphorus detected by the soil available phosphorus only represents a relative index and has a certain statistical significance but cannot be judged as ' absolute content ' of the real ' available phosphorus in the soil. However, the index can reflect the phosphorus content of the soil to a certain extent, can be used for judging whether to apply a phosphate fertilizer or not, and can also be used as a recommended method for applying the phosphate fertilizer. The method for measuring the available phosphorus in the soil comprises a sodium bicarbonate leaching-molybdenum-antimony colorimetric resistance method (an Olsen method for short), a Bray 1 method, a resin method and the like, and the Olsen method has good reaction correlation with crops on phosphate fertilizers and wide application range, so the method is generally used for detecting the available phosphorus in the soil in various laboratories.

Disclosure of Invention

The invention provides a method for detecting the content of available phosphorus in soil in a sugarcane area, which is used for improving the accuracy of a detection result of the content of the available phosphorus in the soil in the sugarcane area.

The method for detecting the content of nitrogen, phosphorus and potassium elements in sugarcane comprises the following steps:

s1, sampling: sampling soil to be detected in a sugarcane area, and naturally drying the soil sample;

s2, crushing: grinding the naturally air-dried soil sample, and sieving by a 2mm sieve to obtain soil sample powder in the sugarcane area;

s3, weighing and leaching, namely weighing 2.5g of sugarcane area soil sample powder, putting the sugarcane area soil sample powder into a 200ml plastic bottle, injecting 50m of a sodium bicarbonate leaching agent with L, 25 +/-1 ℃ and 0.5 mol/L concentration into the plastic bottle, and then tightly covering a bottle cap;

s4, oscillation: placing the plastic bottle covered with the bottle cap in a full-temperature oscillation incubator at the temperature of 25 +/-1 ℃ and the oscillation frequency of 180 +/-20 r/min for oscillation for 25-30 min;

s5, filtering: after the oscillation is finished, filtering the solution in the plastic bottle by using phosphorus-free qualitative mode until the filtrate is clear, wherein the obtained clear filtrate is the sample solution to be detected;

s6, color mixing, namely sucking 5.00m L sample liquid to be measured by a graduated pipette, injecting the sample liquid into a volumetric flask with the volume of 50m L, adding 10m L of purified water into the volumetric flask, dropwise adding a 2, 4-dinitrophenol indicator into the volumetric flask, and adjusting the liquid to be measured in the volumetric flask to be yellowish by using a dilute sulfuric acid solution and a dilute sodium hydroxide solution;

s7, adding color-developing agent, namely slowly adding 5.00m L molybdenum antimony color-developing resisting agent into the liquid to be detected, shaking the volumetric flask from slow to fast until bubbles disappear, and adding CO₂Fully discharging the gas, adding ultrapure water to a constant volume of 50m L scale mark, tightly covering a bottle cap, and shaking up to be tested;

s8, color development detection of available phosphorus: placing the liquid to be measured with constant volume at the temperature of 25 ℃ for standing for 30min, simultaneously preheating a spectrophotometer, using a 1cm optical diameter cuvette at the wavelength of 700nm, carrying out colorimetric measurement after zero point zero adjustment of a standard solution, keeping the smooth surface of the cuvette aligned with a light path all the time in the measurement process, holding the rough surface by hand, wiping off the liquid outside the cuvette, then placing the cuvette into a colorimetric tank, carrying out colorimetric measurement after zero point zero adjustment of a standard curve, and finding out the phosphorus content in the liquid to be measured from the standard curve;

and S9, calculating the content of available phosphorus in the soil according to the phosphorus content in the liquid to be detected.

As the above-mentioned detection method, it is preferable that in S3, the pH of the sodium bicarbonate lixiviant used is 8.5.

In the detection method as described above, preferably, in S9, the content of available phosphorus in the soil is calculated according to the following formula:

wherein rho is the concentration of phosphorus in the color-developing solution obtained from the standard curve and has a unit of mg/L, and rho₀The phosphorus concentration in the blank sample is determined from the standard curve in mg/L, V is the volume of the developing solution in m L, D is the ratio of the volume of the leaching agent in the sample to the volume of the sample taken, m is the mass of the sample in g, and 1000 is the coefficient for converting m L to L and g to kg.

The technical scheme provided by the invention has the advantages of short detection time consumption, good detection limit, accuracy and precision, and capability of improving the accuracy of the detection result of the effective phosphorus content in the soil in the sugarcane region and reducing the labor amount and labor intensity. The technical scheme can be widely applied to detection of available phosphorus in neutral, slightly acidic and calcareous soil, and further provides scientific basis for application of phosphate fertilizer in sugarcane areas.

Drawings

In order to more clearly illustrate the technical solution of the present invention, the drawings used in the description will be briefly introduced below. For a person skilled in the art, without inventive effort, further figures can be obtained from these figures.

FIG. 1 is a flow chart of the method for detecting the content of available phosphorus in soil in a sugarcane area provided by the invention.

Detailed Description

The technical solution implemented by the present invention will be clearly and completely described by embodiments with reference to the accompanying drawings. Terms used in the present invention generally have meanings commonly understood by those of ordinary skill in the art, unless otherwise specified. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

FIG. 1 is a flow chart of the method for detecting the content of available phosphorus in soil in a sugarcane area provided by the invention. As shown in figure 1, the method for detecting the content of available phosphorus in the soil in the sugarcane area comprises the following steps.

S1, sampling: sampling soil to be detected in a sugarcane area, and naturally air-drying the soil sample.

S2, crushing: and grinding the naturally air-dried soil sample, and sieving by using a 2mm sieve to obtain the soil sample powder in the sugarcane area.

S3, weighing and leaching, namely weighing 2.5g of sugarcane area soil sample powder, putting the powder into a 200ml plastic bottle, injecting 50m of L sodium bicarbonate leaching agent with the temperature of 25 +/-1 ℃ and the concentration of 0.5 mol/L into the plastic bottle, and then covering a bottle cap tightly.

Preferably, the pH of the sodium bicarbonate lixiviant used in this step is 8.5.

S4, oscillation: and (3) placing the plastic bottle covered with the bottle cap in a full-temperature oscillation incubator at the temperature of 25 +/-1 ℃ and the oscillation frequency of 180 +/-20 r/min for oscillation for 25-30 min.

S5, filtering: after the oscillation is finished, filtering the solution in the plastic bottle by using phosphorus-free qualitative method until the filtrate is clear, and obtaining clear filtrate which is the sample solution to be detected.

And S6, color mixing, namely sucking 5.00m L sample liquid to be measured by a graduated pipette, injecting the sample liquid into a volumetric flask with the volume of 50m L, adding 10m L of purified water into the volumetric flask, dropwise adding a 2, 4-dinitrophenol indicator into the volumetric flask, and adjusting the liquid to be measured in the volumetric flask to be yellowish by using a dilute sulfuric acid solution and a dilute sodium hydroxide solution.

S7, adding color-developing agent, namely slowly adding 5.00m L molybdenum antimony color-developing resisting agent into the liquid to be detected, shaking the volumetric flask from slow to fast until bubbles disappear, and adding CO₂And (3) fully exhausting the gas, adding ultrapure water to the constant volume of 50m L scale mark, tightly covering a bottle cap, and shaking up to be tested.

Due to the same ion effect of carbonate, the solubility of calcium carbonate is reduced, and the concentration of calcium in the solution is reduced, thereby being beneficial to extracting calcium phosphate salt and enabling HCO in the sodium bicarbonate alkali solution₃ ^-、CO₃ ^2-The plasma is beneficial to the replacement of the adsorbed phosphorus; the increase of pH in the subacid soil can hydrolyze aluminum phosphate and iron phosphate (Fe-P, Al-P) to partially extract, and the phosphorus content in the leaching solution can be measured by molybdenum-antimony anti-color development because the leaching solution has low concentrations of Al, Ca and Fe and does not generate re-precipitation of phosphorus.

Before the color developing agent is added, the solution to be detected reaches the specific acidity condition suitable for molybdenum blue color development by color mixing, and the molybdenum antimony anti-solution added with ascorbic acid is used as the color developing agent, so that the color development can be completed in a short time, and the color developing is stable, and the detection efficiency is improved.

S8, color development detection of available phosphorus: placing the liquid to be measured with a constant volume at the temperature of 25 ℃ for standing for 30min, preheating a spectrophotometer, using a 1cm optical diameter cuvette at the wavelength of 700nm, carrying out colorimetric determination after zero point zero setting of a standard solution, keeping the smooth surface of the cuvette aligned with a light path all the time in the determination process, holding a rough surface by hand, wiping off the liquid outside the cuvette, then placing the cuvette into a colorimetric tank, carrying out colorimetric determination after zero point zero setting of a standard curve, and finding out the phosphorus content in the liquid to be measured from the standard curve.

And S9, calculating the content of available phosphorus in the soil according to the phosphorus content in the liquid to be detected.

In this step, the content of available phosphorus in the soil can be calculated according to the following formula:

in the formula: ρ is the color development obtained from the standard curveThe concentration of phosphorus in the liquid is mg/L, rho₀The phosphorus concentration in the blank sample is determined from the standard curve in mg/L, V is the volume of the developing solution in m L, D is the ratio of the volume of the leaching agent in the sample to the volume of the sample taken, m is the mass of the sample in g, and 1000 is the coefficient for converting m L to L and g to kg.

The technical scheme provided by the invention shortens the detection time, increases the stability of the detection result and optimizes the limit of the environment on the detection. The lower detection limit is 1.90 mg/kg; compared with the traditional method, the method has the advantages of fast color development, good color development effect, stable generated color, small interference ions and color development at room temperature; the method has the advantages of good detection limit, accuracy and precision, meets the requirement of soil available phosphorus determination, can be widely applied to detection of available phosphorus in neutral, subacid and calcareous soil, and provides accurate experimental detection data for balanced fertilization in sugarcane areas.

The following provides an application test example of the method for detecting the content of available phosphorus in the soil in the sugarcane area.

1 materials, devices and reagents

1.1 materials

The soil samples are taken from 11 actual soil samples to be detected in the Yunnan sugarcane area in 2019 (the numbers are 2019001, 2019002, 2019003, 2019004, 2019005, 2019006, 2019007, 2019008, 2019009, 2019010 and 2019011), and a self-made reference soil sample (the number is 2014F391) and a certified standard soil sample (the number is GBW07414) with known effective phosphorus content (29 +/-3) mg/kg are obtained. All soil samples were air-dried, ground and treated through 2mm sieve mesh, and the results of the pH measurements of the soil used in the tests are shown in Table 1.

TABLE 1 soil pH table

1.2 apparatus

An electronic balance, a Mettler model P L2002/01, a spectrophotometer, Shanghai Hua model 722, a full-temperature oscillation incubator, Taicang Huamei biochemical instruments and factories, a QHZ-98A, instruments and equipment such as volumetric flasks, pipettes and the like commonly used in laboratories.

1.3 reagent solution

The reagent and water used in the method are prepared and taken strictly according to the requirements of soil available phosphorus detection standard, and comprise molybdenum-antimony stock solution, molybdenum-antimony color-developing resisting agent, ascorbic acid (levorotatory, optical rotation + 21-22 degrees), 2, 4-dinitrophenol indicator, sodium bicarbonate lixiviant (0.5 mol/L, pH8.5), phosphorus-free activated carbon powder, phosphorus standard stock solution (P is 100 mg/L), and phosphorus-free filter paper.

2Olsen method for determining available phosphorus in soil in sugarcane area

2.1 extraction of available phosphorus

Weighing 2.50g of air-dried soil sample into a dry 200m L plastic bottle, accurately adding 50m L (25 +/-1) DEG C sodium bicarbonate extractant with the concentration of 0.5 mol/L, tightly covering the bottle cap, uniformly mixing and fixing the plastic bottle to ensure that the sample liquid is fully and uniformly mixed and does not leak in the oscillation process, then oscillating in an oscillation incubator at the full temperature of 25 +/-1 ℃ and the oscillation frequency of (180 +/-20) r/min for 25-30 min, after oscillation, performing dry filtration by using non-phosphorus filter paper until the filtrate is clear, and simultaneously performing a blank experiment.

2.2 determination of available phosphorus in the filtrate

2.2.1 suction of the test solution

Accurately sucking 5.00m L filtrate into a 50m L volumetric flask by a clean graduated pipette, and if the soil contains high available phosphorus content, the sucking amount of the filtrate can be reduced, but the acidity of the solution during color development must be maintained.

The calculation result is calculated according to the dividing times of the taken filtrate.

2.2.2 color matching

Before adding the color developing agent, adding water of about 10m L m into a volumetric flask filled with 5.00m L of filtrate, adding 2-3 drops of 2, 4-dinitrophenol indicator, and then adjusting the pH value to be yellowish by using a dilute sulfuric acid solution and a dilute sodium hydroxide solution.

2.2.3 feeling of bubbles

Slowly adding 5.00m L Mo-Sb color-developing resisting agent into the filtrate, and shaking the volumetric flask from slow to fast until bubbles disappear and CO disappears₂Gas removal (CO formation to be prevented during shaking)₂Air bubbleOverflowing the bottle mouth to influence the measuring result), then fixing the volume, tightly covering the bottle cap, and shaking up to be measured.

2.2.4 color development assay of available phosphorus

Placing the liquid to be measured with constant volume at room temperature higher than 25 ℃ for standing for 30min, simultaneously preheating a spectrophotometer, using a 1cm optical diameter cuvette at the wavelength of 700nm, carrying out colorimetric determination after zero point zero adjustment of a standard solution, always keeping the smooth surface of the cuvette aligned with an optical path in the determination process, holding the rough surface by hand, wiping off the liquid outside the cuvette, then placing the cuvette into a colorimetric tank, carrying out colorimetric determination after zero point zero adjustment of a standard curve; and finding out the phosphorus content from the standard curve; blank experiments were also performed. In the determination process, the colorimetric ware is dyed by molybdenum blue, so that the color development value of the filtrate is larger, and therefore, if the colorimetric ware is polluted by the molybdenum blue, the colorimetric ware can be cleaned by dipping a cotton stick in alcohol, then repeatedly washed by pure water, and poured into the solution to be determined for determination.

2.3 phosphorus Standard Curve plotting

While measuring the soil sample, sucking 5.00m L phosphorus standard stock solution (P100 mg/L) into a 100m L0 volumetric flask, shaking up with water to a constant volume to obtain 5.00 mg/L1 phosphorus standard solution, sucking phosphorus standard solution (P5 mg/L2) 0.00m L3, 0.50m L4, 1.00m L5, 1.50m L, 2.00m L, 3.00m L, 4.00m L and 5.00m L into a 50m L volumetric flask, adding 5.00m L sodium bicarbonate lixiviant and 2-3 drops of 2, 4-dinitrophenol indicator, adjusting pH to slight yellow with dilute sulfuric acid solution and dilute sodium hydroxide solution, adding 5.00m L antimony color-resisting developer, slowly shaking to remove CO, and slowly adding₂Adding water to constant volume to obtain phosphorus standard series solution containing phosphorus 0.00 mg/L, 0.05 mg/L, 0.10 mg/L, 0.15 mg/L, 0.20 mg/L, 0.30 mg/L, 0.40 mg/L and 0.50 mg/L, measuring sample absorbance at 700nm with spectrophotometer (Shanghai Hua, model: 722), and calculating standard curve data as shown in Table 2.

TABLE 2 phosphorus determination standard curve table

2.4 measurement calculation

The content (mg/kg) of available phosphorus (P) in the soil was calculated as follows:

wherein rho is the concentration of phosphorus in the color-developing solution obtained from the standard curve and has a unit of mg/L, and rho₀The phosphorus concentration in the blank sample is determined from the standard curve in mg/L, V is the volume of the developing solution in m L, D is the ratio of the volume of the leaching agent in the sample to the volume of the sample taken, m is the mass of the sample in g, and 1000 is the coefficient for converting m L to L and g to kg.

Through to sugarcane district partial soil, the effective phosphorus detection of certified standard soil sample, see table 3 for details, indoor temperature is 25 ℃, weigh and examine 4 of examining the soil sample, the serial number divide into: 2019001, 2019002, 2019003 and 2019004, 2.50g of each of self-made reference soil samples (No. 2014F391) and certified standard soil samples (No. GBW07414) with known effective phosphorus content (29 +/-3) mg/kg, wherein each sample is subjected to parallel determination for 5 times, 1cm cuvette is used for color comparison, the soil-liquid ratio is 1:20, the oscillation time is 30min, the rotation speed (180 +/-20) r/min and the extraction temperature is 25 +/-1 ℃; the content of available phosphorus is detected by an Olsen method, and the average value, the standard deviation and the relative standard deviation of each soil sample are respectively calculated to detect the precision.

TABLE 3 Table of precision measurement results of available phosphorus in soil samples in sugarcane area

According to the results, the RSD of the 6 groups of soil samples is between 0.41 and 1.22 percent, and the standard requirements are met.

3 results and discussion

3.1 Effect of the cuvette on the spectrophotometric determination

The cuvettes have different optical path lengths, generally, optical path lengths of 0.5cm, 1cm, 2cm, 3cm, etc. are commonly used, and the optical path selected is determined according to the absorbance of the sample to be analyzed. When the concentration of the sample is small, a cuvette with large optical path length such as 2cm and 3cm is selected; when the concentration of the sample is higher, a cuvette with a shorter optical path length, such as 1cm, can be used. The measurement errors are different under different concentrations, the measurement errors can be reduced by selecting the optimum measurement concentration, and the measurement errors are relatively small when the light absorption value is between 0.1 and 0.7, so that the light absorption value of the measured solution is between 0.1 and 0.7 when the cuvette is selected. The determination and analysis of the effective phosphorus absorbance of 10 sugarcane region soil samples (2019002-containing 2019011) by comparing 1cm, 2cm, 3cm and 3 cuvettes with different optical path lengths are found (table 4), and when the 1cm cuvette is selected for colorimetric, the absorbance values of the measured sample liquid are all between 0.1 and 0.7; when 2cm and 3cm cuvettes are selected for colorimetry, the light absorption value of part of soil exceeds 0.7; in order to reduce the measurement error, a cuvette with the optical path length being smaller than that of the 1cm cuvette is selected when the effective phosphorus of the soil sample of the batch in the sugarcane area is detected. The cuvettes are directional and should be marked strictly according to the direction in the detection process; when the cuvette is cleaned, dilute nitric acid (1%) or absolute ethyl alcohol can be used for soaking for a moment, the molybdenum blue substances adsorbed on the wall of the cuvette are removed, and then the cuvette is repeatedly washed clean by pure water, dried and placed in a drying box for later use.

TABLE 4 comparison data table for measuring soil light absorption value by cuvettes with different optical path lengths

3.2 Effect of soil-to-liquid ratio on leaching Effect

Through comparison of the results of leaching measurement of a soil sample (2014F391) under different soil-liquid ratios, the soil-liquid ratio in the Olsen method can directly influence the leaching effect of phosphorus (table 5), the higher the soil-liquid ratio is, the less obvious the leaching effect is, the soil-liquid ratio is lower than 1:20, and although the leaching effect is obvious, the less filtrate is; therefore, in the detection, the soil-liquid ratio is strictly controlled to be 1:20 according to the standard, so that the filtrate is sufficient and the leaching is complete. When the soil sample is weighed, the calibrated electronic balance is preheated for 30min, and after the lixiviant is added, the bottle cap is screwed down and shaken up and down for multiple times to ensure that the sample liquid is fully and uniformly mixed.

TABLE 5 influence of soil-liquid ratio on leaching effect

3.3 control of the oscillation time

Phosphorus dissolution and exchange were both related to action time, as found in a study of choice of shaking time (Table 6) on soil samples (2019001), homemade reference soil samples (code: 2014F391) and certified standard soil samples (code: GBW07414) of known effective phosphorus content (29. + -.3) mg/kg; the soil mass is 2.50g, under the same condition that the soil-liquid ratio is 1:20, when the oscillation time is less than 25min, the leaching of phosphorus is insufficient, and the phosphorus result is lower; the longer the oscillation time is, the higher the phosphorus detection result is, and the data shows that the leaching result is relatively close to the standard value of the phosphorus content of each soil sample when the oscillation time is 30min, so the oscillation time is most suitable to be controlled within 30 min.

Table 6 table of the results of extracting available phosphorus from soil at different oscillation times

3.4 selection of the oscillation frequency

According to the research (table 7) of different oscillation frequencies when the certified standard soil sample (number: GBW07414) with known content of available phosphorus (29 +/-3) mg/kg is used for extracting the available phosphorus, the different oscillation frequencies can influence the extraction result of the available phosphorus in the soil; if the oscillation frequency is slow, the measurement result will be low; the faster the oscillation frequency, the higher the leaching efficiency, but too fast frequency can cause sample liquid to spill out, can also influence its measuring result, so according to industry standard "soil detects the seventh part: the leaching frequency should be controlled to be (180 +/-20) r/min, which is most suitable for the provision in the determination of the effective phosphorus in the soil.

TABLE 7 influence of different oscillation frequencies on the determination of the available phosphorus content in soil (GBW07414)

3.5 control of the extraction temperature

When the soil available phosphorus is measured by using an Olsen method, the phosphorus measurement result is influenced by the leaching temperature and the temperature of the leaching solution, and the Olsen indicates that the soil available phosphorus detection result is increased by 0.43mg/kg within the extraction temperature range of 20-30 ℃ by taking 25 +/-1 ℃ as a standard, and the temperature is increased by 1 ℃, so that the accurate control of the leaching temperature and the temperature of the leaching solution is an important condition for ensuring the accuracy of the detection result.

According to the effective phosphorus content determination experiments carried out on a homemade reference soil sample (No. 2014F391) and a certified standard soil sample (No. GBW07414) with known effective phosphorus content (29 +/-3) mg/kg, under the same conditions, different leaching temperature pairs directly influence the determination result of the effective phosphorus (Table 8); the result of available phosphorus increases along with the gradual rise of the temperature, and the content of leached phosphorus is closer to the content of standard available phosphorus of the sample at the temperature of 25 ℃; therefore, the temperature of the oscillating box should be controlled at 25 +/-1 ℃ during leaching, so that the leaching temperature is the same as the temperature of the leaching solution, and the accuracy of the detection result is improved. In winter, when the room temperature is lower, the lixiviant can be heated properly by hot water bath, but can not exceed 35 ℃. When the temperature is higher in summer, the temperature is properly reduced by adopting a cold water bath method.

TABLE 8 influence of different leaching temperatures on the determination of the effective phosphorus content in soil

3.6 Effect of tinting on assay results

The soil available phosphorus is determined by the Olsen method with NaHCO at pH8.5₃The solution is used for extracting available phosphorus in soil and phosphorus in calcareous soil, and the available phosphorus and the phosphorus in the calcareous soil are mostly in the forms of monocalcium phosphate and dicalcium phosphate, and the pH value of the solution is 8.5 NaHCO₃Extraction of soilWhen the phosphorus is available, the solubility of calcium carbonate is reduced due to the same ion effect of carbonate, and the concentration of calcium in the solution is reduced, so that the extraction of calcium phosphate salt is facilitated, and HCO in the sodium bicarbonate alkali solution can be reduced₃ ^-、CO₃ ^2-The plasma is beneficial to the replacement of the adsorbed phosphorus; the increase of pH in the subacid soil can hydrolyze aluminum phosphate and iron phosphate (Fe-P, Al-P) to partially extract, and the phosphorus content in the leaching solution can be measured by molybdenum-antimony anti-color development because the leaching solution has low concentrations of Al, Ca and Fe and does not generate re-precipitation of phosphorus.

The pH detection of direct filtrate and filtrate after color mixing after leaching 4 soil samples to be detected (serial numbers: 2019001, 2019002, 2019003 and 2019004), home-made reference soil samples (serial number: 2014F391) and certified standard soil samples (serial number: GBW07414) with known effective phosphorus content (29 +/-3) mg/kg (serial number: 2014F391) is found (Table 9), the pH of the filtrate is alkaline due to strict control of the pH of the leaching liquor during leaching, proper acidity conditions are required for the generation of phosphomolybdic heteropoly acid, and the color developing effect is influenced by over-acid or under-shortage, so in the molybdenum blue colorimetric determination of phosphorus, the control of acidity is extremely important, and in order to maintain the acidity of the liquid to be detected during color development, the pH of the leaching liquor needs to be adjusted to be slightly yellow before adding a color developing agent, and the molybdenum blue color development is carried out under the specific acidity conditions, so as to ensure the accuracy of experimental results.

TABLE 9 comparison of pH of direct and toned filtrates

3.7 selection of absorption wavelength

Spectrophotometry is a method for carrying out quantitative and qualitative analysis on a substance to be detected by measuring the absorbance of the light of the substance in a certain wavelength range or at a specific wavelength according to the beer's law; different test standards have different requirements on the absorption wavelength of the liquid to be tested, before the test, the absorption spectrum of the liquid to be tested is measured (table 10) by a certified standard soil sample (number: GBW07414) with known effective phosphorus content (29 +/-3) mg/kg, the absorbance at 700nm is the maximum, and the position with the maximum absorption wavelength of 700nm is selected as the measurement wavelength of the experiment, so that higher sensitivity and better linear relation are obtained.

TABLE 10 Absorbance Table for Standard soil sample (GBW07414) measured at different wavelengths

4 conclusion

Experiments show that when the sodium bicarbonate lixiviation agent is used for lixiviation in the sodium bicarbonate lixiviation-molybdenum antimony colorimetric resistance method, the soil-liquid ratio is strictly controlled to be 1:20, the oscillation time is 30min, the frequency (180 +/-20) r/min, the lixiviation temperature is 25 +/-1 ℃, and the light absorption value and other key factors are measured by a 1cm cuvette; the influence of color mixing on the pH of the filtrate to be detected is mainly researched, the pH of the filtrate is alkaline due to the limitation of the leaching environment, so that the liquid to be detected reaches a specific acidity condition suitable for molybdenum blue color development by color mixing before the color developing agent is added, and the molybdenum antimony anti-solution added with ascorbic acid is used as the color developing agent, so that the color development can be completed in a short time and is stable, and the experimental efficiency is improved; the ultraviolet and visible wavelength full scanning is carried out, the wavelength of the maximum absorption peak is found to be 700nm as the wavelength of the experiment, the RSD of 6 groups of soil samples is measured to be between 0.41% and 1.22% (Table 3), and the standard requirements are met. Compared with NY/T1121.7-2014, the method shortens the experimental time, increases the stability of the experimental result, and optimizes the limit of the environment on the experiment.

According to all the steps of sample analysis, repeating n (n is more than or equal to 7) blank tests, converting each measurement result into the concentration in the sample, calculating the standard deviation of n parallel measurements, and calculating the detection limit according to the following formula, wherein the MD L is t (n-1, 0.99) × S, wherein n is the parallel measurement frequency of the sample, t is t distribution (single side) when the degree of freedom is n-1 and the confidence is 0.99, and S is the standard deviation of the n parallel measurements, when n is 7, t is 3.143, 4 times of detection limit is taken as the measurement lower limit of the method, the detection limit is 0.47mg/kg, and the detection lower limit is 1.90mg/kg, compared with the traditional Olsen method, the method has the advantages of fast color development, good color development effect, stable generated color, small interfered ions, color development at room temperature, good detection limit, good accuracy and precision, meets the requirement of soil effective phosphorus measurement, can be widely applied to soil, the theoretical and the effective detection of soil fertilization is more accurate and the basis is provided.

Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims (3)

1. A method for detecting the content of available phosphorus in soil in a sugarcane region is characterized by comprising the following steps:

s1, sampling: sampling soil to be detected in a sugarcane area, and naturally drying the soil sample;

s2, crushing: grinding the naturally air-dried soil sample, and sieving by a 2mm sieve to obtain soil sample powder in the sugarcane area;

s3, weighing and leaching, namely weighing 2.5g of sugarcane area soil sample powder, putting the sugarcane area soil sample powder into a 200ml plastic bottle, injecting 50m of a sodium bicarbonate leaching agent with L, 25 +/-1 ℃ and 0.5 mol/L concentration into the plastic bottle, and then tightly covering a bottle cap;

s4, oscillation: placing the plastic bottle covered with the bottle cap in a full-temperature oscillation incubator at the temperature of 25 +/-1 ℃ and the oscillation frequency of 180 +/-20 r/min for oscillation for 25-30 min;

s5, filtering: after the oscillation is finished, filtering the solution in the plastic bottle by using phosphorus-free qualitative mode until the filtrate is clear, wherein the obtained clear filtrate is the sample solution to be detected;

s6, color mixing, namely sucking 5.00m L sample liquid to be measured by a graduated pipette, injecting the sample liquid into a volumetric flask with the volume of 50m L, adding 10m L of purified water into the volumetric flask, dropwise adding a 2, 4-dinitrophenol indicator into the volumetric flask, and adjusting the liquid to be measured in the volumetric flask to be yellowish by using a dilute sulfuric acid solution and a dilute sodium hydroxide solution;

s7, adding color-developing agent, namely slowly adding 5.00m L molybdenum antimony color-developing resisting agent into the liquid to be detected, shaking the volumetric flask from slow to fast until bubbles disappear, and adding CO₂Fully discharging the gas, adding ultrapure water to a constant volume of 50m L scale mark, tightly covering a bottle cap, and shaking up to be tested;

s8, color development detection of available phosphorus: placing the liquid to be measured with constant volume at the temperature of 25 ℃ for standing for 30min, simultaneously preheating a spectrophotometer, using a 1cm optical diameter cuvette at the wavelength of 700nm, carrying out colorimetric measurement after zero point zero adjustment of a standard solution, keeping the smooth surface of the cuvette aligned with a light path all the time in the measurement process, holding the rough surface by hand, wiping off the liquid outside the cuvette, then placing the cuvette into a colorimetric tank, carrying out colorimetric measurement after zero point zero adjustment of a standard curve, and finding out the phosphorus content in the liquid to be measured from the standard curve;

and S9, calculating the content of available phosphorus in the soil according to the phosphorus content in the liquid to be detected.

2. The assay of claim 1, wherein in S3, the sodium bicarbonate lixiviant used has a PH of 8.5.

3. The method according to claim 1 or 2, wherein in S9, the content of available phosphorus in the soil is calculated as follows:

wherein rho is the concentration of phosphorus in the color-developing solution obtained from the standard curve and has a unit of mg/L, and rho₀The phosphorus concentration in the blank sample is determined from the standard curve in mg/L, V is the volume of the developing solution in m L, D is the ratio of the volume of the leaching agent in the sample to the volume of the sample taken, m is the mass of the sample in g, and 1000 is the coefficient for converting m L to L and g to kg.

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