🌿 Available magnesium in soil — the Schachtschabel method (extraction with calcium chloride, spectrophotometric determination with titan yellow or AAS)
In short
The soil is flooded with a solution of calcium chloride of a concentration of 0. The test is performed according to PN-R-04020:1994 + Az1:2004; Form determined: available magnesium — the pool extracted with a CaCl₂ solution, treated as the mobile form, potentially accessible to plants. The procedure comprises 10 steps; it is used for: Tests of the richness of mineral soils carried out by the regional chemical-agricultural stations at the request of farmers, Establishing the doses of magnesium fertilisers in the fertiliser advisory system based on the balancing of P, K and Mg, Assessment of the need for liming and magnesium fertilising of acid soils — the main area of application indicated in the scope of the standard.
At a glance
- Standard: PN-R-04020:1994 + Az1:2004
- Category: Environment
- Procedure steps: 10
- Form determined: available magnesium — the pool extracted with a CaCl₂ solution, treated as the mobile form, potentially accessible to plants
- Extractant: calcium chloride 0.0125 mol·dm⁻³, pH 7.0 (the Schachtschabel test, 1960)
- Soil : solution ratio: 1 : 10 (m/v) — 5 g of soil per 50 cm³ of extractant (after the IUNG instruction)
Overview
The magnesium determined by this method is the so-called available form — not the whole pool of magnesium in the soil, but that part of it which a weak solution of a neutral salt extracts and which it has been agreed to regard as accessible to plants. The test was developed by Schachtschabel in 1960; the extractant is calcium chloride of pH 7.0, and the principle of the extraction is dissolution. In Polish fertiliser advisory work a solution of CaCl₂ of a concentration of 0.0125 mol·dm⁻³ is used. The method was adapted from German fertiliser advisory work in the post-war period and introduced into agricultural practice together with ready-made limit numbers; in the 1970s the range of the limit numbers was extended and a Polish Standard was drawn up for it.
The standard covers two methods of the final determination: the spectrophotometric method with titan yellow and the method of atomic absorption spectrometry (in the practice of the chemical-agricultural stations the determination is carried out by the AAS technique). Besides the determination itself, the standard lays down the purity of the reagents, the accuracy of the weighing, the preparation of the soil extract, the blank, the calculation of the results and the final result, and also the assessment of the content of magnesium in the soil. As the PKN catalogue card states, the determination of magnesium is to make possible a correct assessment of the quality of the soil in order to establish the fertiliser requirements, chiefly on acid soils requiring liming and magnesium fertilising.
The numerical result itself means nothing without reference to the classes of richness, and those for magnesium — unlike for phosphorus — depend on the agronomic category of the soil (very light, light, medium, heavy; the division according to the Polish Soil Science Society based on the share of the fraction below 0.02 mm). The same content of magnesium on a very light soil means a different class than on a heavy one, so for the assessment of the richness an analysis of the particle size distribution is needed in parallel. The classes of richness in Poland are on a five-step scale: very low, low, medium, high and very high.
The status of the standard — a thing to be checked before it is entered into a scope of accreditation. According to the PKN catalogue card the standard PN-R-04020:1994 (published on 3.10.1994, 4 pages, KT 90 for Soil Tillage and Horticulture, ICS 13.080.10) was WITHDRAWN on 25.09.2020; its additional element PN-R-04020:1994/Az1:2004 is withdrawn as well. The card does not indicate a replacing standard, and the test order form of a regional chemical-agricultural station describes it expressly as „norma wycof. bez zastąp." [standard withdrawn without replacement] — and nevertheless still refers to it for the service of determining available magnesium. In practice this means a method without a current standard, kept in circulation by the force of the fertiliser advisory system and the procedures of the stations.
What this description does not contain: we have not seen the full text of the standard — PKN makes it available only for a fee. The weighed portion, the volume of the extractant, the manner and the time of shaking and the limit numbers we give after publications of IUNG-PIB and an IUNG fertiliser instruction, and not after the standard; the differences between the sources we mark below expressly. The composition and the preparation of the titan yellow solution, the wavelengths of the measurement, the working conditions of the atomic absorption spectrometer, the limit of determination and the requirements concerning precision have to be taken from the text of the standard or from the testing procedure of the laboratory — we do not guess them.
Method principle
Air-dry soil, sieved through a sieve of 1 mm mesh, is flooded with a solution of calcium chloride of a concentration of 0.0125 mol·dm⁻³ in a soil : solution ratio of 1 : 10 (m/v) — according to the IUNG instruction 5 g of soil per 50 cm³ of solution. The studies of IUNG-PIB classify the Schachtschabel test as a method with the principle of „dissolution" (the extractant CaCl₂ of pH 7.0) — the forms of magnesium readily accessible to plants pass into the solution. The suspension is mixed and then filtered — one filter paper per sample. As to the time of the extraction the sources give two variants: the studies of IUNG-PIB comparing soil tests indicate 1440 min in the static method (without shaking), whereas the IUNG instruction concerning the fertilising of fruit plants, after Karczewska and Kabała, gives 120 minutes in a rotary shaker at 40 revolutions per minute; before the work it has to be established which variant applies in the given procedure. In the clear filtrate the magnesium is determined spectrophotometrically with titan yellow or by the method of atomic absorption spectrometry — both methods are in the standard, in agrochemical practice AAS is used. The result is converted into the content of magnesium in the soil (mg Mg per 100 g of soil) and compared with the limit numbers proper to the agronomic category of the soil tested.
Applications
- Tests of the richness of mineral soils carried out by the regional chemical-agricultural stations at the request of farmers
- Establishing the doses of magnesium fertilisers in the fertiliser advisory system based on the balancing of P, K and Mg
- Assessment of the need for liming and magnesium fertilising of acid soils — the main area of application indicated in the scope of the standard
- Fertilising of fruit plants and plantations on the basis of a soil analysis (samples taken at least once every 3 years)
- Monitoring of the chemical fertility of soils on a national scale — the central database of results of the KSChR, about 250 thousand analyses of pH, P, K and Mg a year
- Scientific work on the calibration and the replacement of the existing soil tests by the universal Mehlich 3 method
Key parameters
| Parameter | Value |
|---|---|
| Form determined | available magnesium — the pool extracted with a CaCl₂ solution, treated as the mobile form, potentially accessible to plants |
| Extractant | calcium chloride 0.0125 mol·dm⁻³, pH 7.0 (the Schachtschabel test, 1960) |
| Soil : solution ratio | 1 : 10 (m/v) — 5 g of soil per 50 cm³ of extractant (after the IUNG instruction) |
| Time of extraction | the sources diverge: 1440 min by the static method (studies of IUNG-PIB) or 120 min in a rotary shaker at 40 rev/min (the IUNG instruction after Karczewska and Kabała) |
| Final determination | the spectrophotometric method with titan yellow or atomic absorption spectrometry (in agrochemical practice — AAS) |
| Form of the result and the limit numbers | mg Mg per 100 g of soil; five classes of richness (very low → very high), the thresholds different for each agronomic category of soil |
| Status of the standard | PN-R-04020:1994 + Az1:2004 withdrawn on 25.09.2020, without replacement; the chemical-agricultural stations still refer to it |
Standard
- Standard number
- PN-R-04020:1994 + Az1:2004
- Title (PL)
- Analiza chemiczno-rolnicza gleby — Oznaczanie zawartości przyswajalnego magnezu
Step-by-step procedure
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Taking the sample
Take the soil sample with an Egner stick or an auger (in their absence with a spade, cutting out slices of soil of comparable depth and width). Mix the individual samples from a uniform plot thoroughly and set aside about 1 kg of soil for the analysis. Do not take samples directly after the application of fertilisers or after heavy rainfall.
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Preparation of the sample
Dry the sample in a shaded place (air-dry mass), transfer it into a linen bag or a polyethylene bag, and before the analysis sieve it through a sieve of 1 mm mesh diameter.
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The weighed portion
Weigh out 5 g of air-dry soil; the accuracy of the weighing — as in the standard.
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Extraction
Add 50 cm³ of the solution of calcium chloride of a concentration of 0.0125 mol·dm⁻³ (soil : solution ratio 1 : 10 m/v) and carry out the extraction in the variant adopted in the procedure of the laboratory: statically for 1440 min or in a rotary shaker for 120 min at 40 rev/min.
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Filtration
Filter the suspension (one filter paper per sample) until a clear filtrate intended for the determination of magnesium is obtained.
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Blank
In parallel carry out a blank — without soil, through the whole course of the procedure; the standard provides for it as an element of the determination.
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Final determination
Determine the magnesium in the filtrate by one of the methods covered by the standard: spectrophotometrically with titan yellow or by the method of atomic absorption spectrometry, with calibration by standard solutions of a known concentration of magnesium.
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Calculation of the result
Convert the concentration of magnesium in the filtrate into the content in the soil and express it in mg Mg per 100 g of soil, taking into account the weighed portion, the volume of the extractant and the result of the blank.
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Assessment of the richness
Establish the agronomic category of the soil (an analysis of the particle size distribution is needed — the share of the fraction below 0.02 mm) and read off the class of richness from the limit numbers proper to that category: for very light soils very low <1.0, low 1.1–2.0, medium 2.1–4.0, high 4.1–6.0, very high >6.0 mg Mg/100 g; for light soils respectively <2.0 / 2.1–3.0 / 3.1–5.0 / 5.1–7.0 / >7.0; for medium soils <3.0 / 3.1–5.0 / 5.1–7.0 / 7.1–9.0 / >9.0; for heavy soils <4.0 / 4.1–6.0 / 6.1–10.0 / 10.1–14.0 / >14 (the source table gives for two neighbouring classes the same value 10.0 — when assessing a borderline result keep to the figures from the current edition of the IUNG recommendations).
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Report
Give the content of available magnesium, the agronomic category of the soil and the class of richness, and also the standard referred to together with the information about its withdrawal — the laboratory should indicate on what procedure it based the determination.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Laboratory sieve of 1 mm mesh | preparation of the air-dry sample for the extraction | — |
| Analytical balance | a weighed portion of 5 g of soil; the accuracy of the weighing is laid down by the standard | — |
| Bottles or containers for the extraction with a rotary shaker | the variant with shaking — 40 rev/min; in the static variant a shaker is not needed | — |
| Filtration set with filter papers | one filter paper per sample | — |
| UV-Vis spectrophotometer | the variant of the determination with titan yellow | — |
| Atomic absorption spectrometer (AAS) | the variant used in the chemical-agricultural stations | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Calcium chloride — extraction solution 0.0125 mol·dm⁻³ | — | the extractant of the Schachtschabel test, pH 7.0; it is its concentration that decides the comparability of the result with the limit numbers — do not replace it with a solution from another method |
| Titan yellow — the colour reagent for magnesium | — | the spectrophotometric variant indicated in the scope of the standard; the composition, the preparation of the solution and the wavelength of the measurement are given by the standard |
| Standard solutions of magnesium | — | for plotting the calibration curve; preparation in accordance with the technique of the final determination |
| Water and reagents of the purity laid down in the standard | — | the standard lays down the purity of the reagents — the blank is an element of the procedure |
Health and safety (OHS)
- A solution of calcium chloride is a reagent of little chemical aggressiveness, but dusts and solutions of salts act as irritants on the eyes — work in goggles and gloves, do not pipette by mouth.
- The variant with titan yellow is work with an organic dye and its solutions — prepare them under a fume hood, avoid contact with the skin and the inhalation of the dust of the reagent in the solid form.
- An atomic absorption spectrometer with a flame means flammable gases under pressure and an open flame — an efficient extraction over the burner, checking of the tightness of the gas installation and training of the operators are required.
- During the drying and the sieving of soil mineral dust arises — work with ventilation, if necessary in a dust mask.
- Soil samples may come from areas fertilised with sludge or with natural fertilisers — treat them as material potentially contaminated microbiologically, wash the hands after the work.
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-R-04020:1994 + Az1:2004 — “Analiza chemiczno-rolnicza gleby — Oznaczanie zawartości przyswajalnego magnezu”.
How does this method work?
Air-dry soil, sieved through a sieve of 1 mm mesh, is flooded with a solution of calcium chloride of a concentration of 0.0125 mol·dm⁻³ in a soil : solution ratio of 1 : 10 (m/v) — according to the IUNG instruction 5 g of soil per 50 cm³ of solution.
What is the measuring range and accuracy?
Form determined: available magnesium — the pool extracted with a CaCl₂ solution, treated as the mobile form, potentially accessible to plants; Extractant: calcium chloride 0.0125 mol·dm⁻³, pH 7.0 (the Schachtschabel test, 1960); Soil : solution ratio: 1 : 10 (m/v) — 5 g of soil per 50 cm³ of extractant (after the IUNG instruction); Time of extraction: the sources diverge: 1440 min by the static method (studies of IUNG-PIB) or 120 min in a rotary shaker at 40 rev/min (the IUNG instruction after Karczewska and Kabała).
How long does the test take?
The procedure comprises 10 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Laboratory sieve of 1 mm mesh, Analytical balance, Bottles or containers for the extraction with a rotary shaker, Filtration set with filter papers, UV-Vis spectrophotometer, Atomic absorption spectrometer (AAS).
Where is this test used?
Tests of the richness of mineral soils carried out by the regional chemical-agricultural stations at the request of farmers; Establishing the doses of magnesium fertilisers in the fertiliser advisory system based on the balancing of P, K and Mg; Assessment of the need for liming and magnesium fertilising of acid soils — the main area of application indicated in the scope of the standard; Fertilising of fruit plants and plantations on the basis of a soil analysis (samples taken at least once every 3 years); Monitoring of the chemical fertility of soils on a national scale — the central database of results of the KSChR, about 250 thousand analyses of pH, P, K and Mg a year; Scientific work on the calibration and the replacement of the existing soil tests by the universal Mehlich 3 method.
What safety precautions apply?
A solution of calcium chloride is a reagent of little chemical aggressiveness, but dusts and solutions of salts act as irritants on the eyes — work in goggles and gloves, do not pipette by mouth.; The variant with titan yellow is work with an organic dye and its solutions — prepare them under a fume hood, avoid contact with the skin and the inhalation of the dust of the reagent in the solid form.; An atomic absorption spectrometer with a flame means flammable gases under pressure and an open flame — an efficient extraction over the burner, checking of the tightness of the gas installation and training of the operators are required.; During the drying and the sieving of soil mineral dust arises — work with ventilation, if necessary in a dust mask.; Soil samples may come from areas fertilised with sludge or with natural fertilisers — treat them as material potentially contaminated microbiologically, wash the hands after the work.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-R-04020.