RU2585169C1 - Radiophysical method of determining composition of soil - Google Patents

Abstract

FIELD: soil science.

SUBSTANCE: invention relates to microwave method of determining content of physical clay and humus in soil, method involves measuring refraction index of soil with moisture content exceeding maximum content of associated water samples which is maintained in an airtight container for 1-2 days at room temperature, refraction index is measured at frequencies f₁ = 0.35 GHz and f₂ = 1.75 GHz, difference between refraction index Δn=n(f₁)-n(f₂) at frequencies f₁ and f₂ is measured simultaneously and absorption factor, difference of values of absorption Δκ=κ(f₁)-κ(f₂) and weight ratio of physical clay C in soil from relationship:

and weight fraction of humus in soil from relationship:

where C is content of alphitite in soil (weight fraction); Δn is refraction index difference; Δκ is difference of absorption values; H is content of humus in soil (weight fraction).

EFFECT: high accuracy of determining weight ratio of physical clay and humus in soil is technical result of invention.

1 cl, 2 dwg

Description

The invention relates to microwave measurement methods and can be used in agriculture, land reclamation, in the preparation of the land cadastre, for determining the particle size class of soils, etc.

The granulometric composition of soils and the content of humus in them are the most important characteristics that determine many properties and regimes of soils, including water and thermal, absorption capacity, transformation of substances and fertility (Shein E.V. Course in Soil Physics. - M.: Moscow State University, 2005.432 p.). The clay fraction plays the most significant role in the particle size distribution, since its content is an important indicator of hydrophysical (Nerpin S.V., Chudnovsky A.F. Soil Physics. - M .: Nauka, 1967. 584 pp.) And dielectric properties of the soil (Mironov VL, Fomin SV Temperature and Mineralogy Dependable Model for Microwave Dielectric Spectra of Moist Soils // PIERS Online, 2009. Vol. 5. No. 5. P. 411-415). The content of humus also affects the dielectric constant of the soil (Bobrov P.P., Kondratieva O.V., Mustakova M.M. Effect of the content of organic matter in soils on the dielectric constant in the frequency range 10 kHz - 8.5 GHz // Bulletin of the Siberian State Aerospace University named after Academician MF Reshetnev (Vestnik SibSAU). 2013. Special issue 5 (51). S. 95-97.).

Known direct (laser) and indirect methods for determining the particle size distribution of soils and soils. The most common was the traditional pipette sedimentation method in the modification of N.A. Kachinsky (Vadyunina A.F., Korchagina Z.A. Methods for determining the physical properties of soils and soils. - M.: Higher School, 1961. 345 p.). It is carried out in several stages: from separation of elementary soil particles, boiling to direct sampling of the suspension with a pipette at fixed intervals from a certain depth, which can take a total of up to 30 hours. This method is very time-consuming, and, in addition, during the analysis it is necessary to maintain a constant temperature and minimize possible vibrations.

Modern methods of conducting particle size analysis, based on the diffraction of x-rays by particles in suspension, can reduce the analysis time by several times, but require the use of expensive equipment.

A known remote method for determining the content of physical clay in soils (physical clay is understood as the fraction of particles less than 0.01 mm in size; see: A. Voronin, Fundamentals of Soil Physics. - M.: Moscow State University, 1986, 243 s), based on that the amount of unfrozen water in frozen uninhabited soils depends on the clay content and thermodynamic temperature (RF patent for the invention No. 2411505, IPC G01N 22/04 from 10.13.2009).

, где ε′ и ε″ - действительная и мнимая части комплексной диэлектрической проницаемости соответственно; n и κ - показатель преломления и показатель поглощения соответственно. В результате получают зависимость показателя преломления n=f(W) на частоте около 1 ГГц (Комаров С.А., Миронов В.Л. Микроволновое зондирование почв. - Новосибирск: Научно-издательский центр СО РАН. 2000. 289 с.), после чего по полученной зависимости находят максимальную объемную долю связанной влаги W_t и определяют содержание физической глины из соотношения между W_t и процентным содержанием глины. Достоинством метода является то, что определяемое таким образом содержание глины является входным параметром диэлектрической модели почв (Mironov V.L., Fomin S.V. Temperature and Mineralogy Dependable Model for Microwave Dielectric Spectra of Moist Soils // PIERS Online. 2009, Vol. 5. №5. P. 411-415).There is also known a radiophysical method for determining the fraction of physical clay in soil, based on measurements in laboratory conditions of the dependence on humidity W of a number of values of the complex refractive index of soil

where ε ′ and ε ″ are the real and imaginary parts of the complex permittivity, respectively; n and κ are the refractive index and absorption coefficient, respectively. The result is a dependence of the refractive index n = f (W) at a frequency of about 1 GHz (Komarov S.A., Mironov V.L. Microwave sounding of soils. - Novosibirsk: Scientific and Publishing Center SB RAS. 2000. 289 p.), after which, according to the obtained dependence, the maximum volume fraction of bound moisture W _{t is found} and the content of physical clay is determined from the ratio between W _t and the percentage of clay. The advantage of the method is that the clay content thus determined is an input parameter to the dielectric model of soils (Mironov VL, Fomin SV Temperature and Mineralogy Dependable Model for Microwave Dielectric Spectra of Moist Soils // PIERS Online. 2009, Vol. 5. No. 5. P 411-415).

The most widespread in agrochemical and soil practice when determining humus in the soil was the method of I.V. Tyurina (Physico-chemical methods of soil research. - M.: Publishing House of Moscow University. 1980. S. 155-157). The method is based on the oxidation of soil humus with a solution of potassium dichromate in sulfuric acid followed by photocolorimetric determination of trivalent chromium equivalent to the humus content. The disadvantages of the method are the bulkiness and duration of preparation of samples for analysis, as well as the need to use consumables.

A known method for determining the content of phosphorus and humus in the soil (US Pat. USSR No. 1785572, IPC G01N 33/24 of 11/11/1990), according to which the content of thorium radionuclide in soils is determined; the concentration of thorium radionuclide, using the previously established correlation, determine the concentration of non-radioactive elements, such as phosphorus, and other parent components, such as humus. The disadvantage of this method of determining phosphorus and humus in the soil is the high error.

A known method of determining the content of humus in the soil using x-ray fluorescence analysis of arsenic and cobalt in soil samples (RF patent No. 2253865, IPC G01N 33/24 from 12.30.2003). The problem was solved by measuring their ratio and determining the humus content using a calibration graph. The disadvantage of this method is the complexity of the installation for x-ray fluorescence analysis. An x-ray source is required - a closed source of cadmium-109 radionuclide. The radiation detector requires cooling with liquid nitrogen.

The closest technical solution is the radiophysical method for determining the proportion of physical clay in the soil (RF patent for the invention No. 2467314, IPC G01N 22/04 of 06.22.2011), based on measurements in laboratory conditions of the refractive index at soil temperature t from 10 to 40 ° C and humidity exceeding the maximum content of bound water at two frequencies f ₁ = 0.35 GHz and f ₂ = 1.75 GHz. The mass fraction of physical clay C in the soil is found from the ratio:

where C is the content of physical clay in the soil (in mass fractions);

Δn = n (f ₁ ) -n (f ₂ ) is the difference in refractive index at two frequencies;

t - soil temperature (in ° C).

The method is based on the fact that when the frequency decreases below 1 GHz, the refractive index of moist soil containing a certain amount of clay increases. This increase is due to inter-surface polarization at the mineral particle-bound water interface and depends on the maximum content of bound water, since the refractive indices of film and capillary water in the frequency range below 1 GHz, as well as the refractive indices of the solid phase of the soil are practically independent of frequency (Mironov V.L., Bobrov P.P., Kondratieva O.V., Repin A.V. Measurement of the complex dielectric constant of various forms of soil moisture in the microwave range // Russian Scientific Conference Sensing of terrestrial cover synthetic aperture radar ", Ulan-Ude, 06.09-10.09.2010, electronic collection of reports, pp. 344-355). The more clay in the soil, the greater the maximum possible amount of bound water in it, the sharper the increase in the refractive index with decreasing frequency.

The disadvantage of this method is that it does not determine the content of humus in the soil, which covers the surface of soil particles with a thin layer. In this case, inter-surface polarization occurs already at the interface between organic matter and bound water. In this case, the increase in the refractive index with decreasing frequency is not so significant. Therefore, the method gives a significant error with a mass fraction of humus in the soil above 0.02.

The technical result of the proposed method is the expansion of its functionality by simultaneously determining the content of physical clay and humus in the soil.

The specified technical result is achieved by the fact that in the radiophysical method for determining the composition of the soil, based on measuring the refractive index of the soil with a moisture content exceeding the maximum content of bound water, the samples of which are kept in an airtight container for 1-2 days at room temperature, and the refractive index is measured on frequencies f ₁ = 0.35 GHz and f ₂ = 1.75 GHz, find the difference in refractive indices Δn = n (f ₁ ) -n (f ₂ ), new is that at frequencies f ₁ and f ₂ simultaneously with the measurement refractive index and measure and the absorption index, find the difference in absorption indicators Δκ = κ (f ₁ ) -κ (f ₂ ) and determine the mass fraction of physical clay C in the soil from the ratio:

and the mass fraction of humus in the soil from the ratio:

where C is the content of physical clay in the soil (in mass fractions);

Δn is the difference in refractive indices;

Δκ is the difference in absorption indicators;

H is the humus content in the soil (in mass fractions).

The inventive method is implemented as follows. Soil samples are moistened to a moisture content greater than W _t and kept in an airtight container for 1-2 days at room temperature. Measure the refractive indices and absorption at frequencies f ₁ = 0.35 GHz and f ₂ = 1.75 GHz. Find the difference in the refractive indices Δn = n (f ₁ ) -n (f ₂ ) and the absorption Δκ = κ (f ₁ ) -κ (f ₂ ); by the formula (1) determine the mass fraction of clay, and by the formula (2) - the mass fraction of humus in the soil.

The verification results of the proposed method are shown in FIG. 1 and FIG. 2.

As experimental data, we took part of the spectra of the complex refractive index of soils given in the work (Bobrov P.P., Kondratyeva O.V., Mustakova M.M. Effect of the content of organic matter in soils on the dielectric constant in the frequency range 10 kHz - 8, 5 GHz / Bulletin of the Siberian State Aerospace Dent named after Academician MF Reshetnev "(Bulletin of SibSAU). 2013. Special issue 5 (51). P. 95-97). Additional measurements were also made of the refractive index and soil absorption index at room temperature. The content of physical clay in soils was measured by the sedimentation method and ranged (in mass fractions) in different samples from 0 to 0.72, the humus content was measured by the Tyurin method and ranged (in mass fractions) from 0 to 0.071. For each soil, several measurements were carried out at different humidity in the range of soil moisture content from 7 to 36%.

Calculation of differences in refractive indices Δn = n (f ₁ ) -n (f ₂ ), where f ₁ = 0.35 GHz and f ₂ = 1.75 GHz and absorption indices Δκ = k (f ₁ ) -κ (f ₂ ) showed that the differences Δn and Δκ are weakly dependent on soil moisture and are statistically related to the content of clay and humus. The square of the correlation coefficient is 0.92-0.96.

The regression equations, allowing to determine the mass fraction of clay C and humus H in the soil by the difference in refractive indices and absorption Δn and Δκ at room temperature, have the form:

Verification of the proposed method was carried out as follows. The regression equations (1) and (2) were compiled from measurements of the refractive index and absorption index of seven soil samples. Using the obtained regression equations, the content of physical clay and humus was found in three soil samples that were not included in the sample from which the regression equations were compiled.

The verification results of the proposed method are shown in FIG. 1 and FIG. 2. Here, Series 1 is the values of the content of physical clay and humus by which the regression equations were compiled; Series 2 is the values of the content of physical clay and humus for samples not included in the sample for compiling the regression equations. The fact that the points in Row 2 fit well on the regression line indicates good reproducibility of the results.

Thus, the claimed method provides the simultaneous measurement of the content of physical clay and humus during one experiment. In addition, the clay and humus contents determined in this way can serve as input parameters for dielectric models used in determining soil moisture using remote microwave methods.

Claims (1)

A radiophysical method for determining the composition of the soil, based on measuring the refractive index of the soil with a moisture content exceeding the maximum content of bound water, the samples of which are kept in an airtight container for 1-2 days at room temperature, and the refractive index is measured at frequencies f ₁ = 0.35 GHz and f ₂ = 1.75 GHz, find the difference in refractive indices Δn = n (f ₁ ) -n (f ₂ ), characterized in that at frequencies f ₁ and f ₂ simultaneously with the measurement of the refractive index, the absorption index is also measured, the difference is found show absorption factors Δκ = κ (f ₁ ) -κ (f ₂ ) and determine the mass fraction of physical clay C in the soil from the ratio:
C = -0.9655 Δn ² + 2.1760 Δn-0.0485 Δκ ² -0.7105 Δκ-0.0065
and the mass fraction of humus in the soil from the ratio:
H = -0.3706 · Δn ² + 0.5046 · Δn-0.1958 · Δκ ² -0.0798 · Δκ-0.0136,
where C is the content of physical clay in the soil (in mass fractions);
Δn is the difference in refractive indices;
Δκ is the difference in absorption indicators;
H is the humus content in the soil (in mass fractions).

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