🌿 Available potassium in mineral soils — the Egner-Riehm method (DL): extraction with calcium lactate acidified with hydrochloric acid, determination by flame photometry

Environment PN-R-04022

Overview

The Egner-Riehm method, in the DL version (calcium lactate + hydrochloric acid), is in Poland the official soil test for available phosphorus and potassium. The extraction consists in extracting the compounds of phosphorus and potassium from the soil by means of calcium lactate (CH₃·CHOH·COO)₂Ca acidified with hydrochloric acid. The solution is 0.04 mol·dm⁻³ with respect to the calcium lactate and 0.02 mol·dm⁻³ with respect to the hydrochloric acid; its pH is 3.6 (the IUNG instruction for fruit plants gives pH 3.55, and the survey of European tests in the Studies and Reports of IUNG-PIB — pH 3.7; before the work one has to keep to the value from one’s own procedure). The reagent is well buffered both with respect to hydrogen ions and to calcium ions — the two factors which most strongly influence the solubility of phosphorus compounds in the soil.

The mechanism is one of ion exchange: because of the high content of Ca²⁺ and H⁺ ions, the exchangeable cation K⁺ is displaced from the sorption complex. The presence of calcium causes at the same time only a slight dissolution of humic compounds, thanks to which a filtrate is obtained which is weakly coloured or altogether colourless, suitable for colorimetric determinations (the phosphorus from the same filtrate is determined colorimetrically as phosphomolybdenum blue). The essential property of this reagent is the ability to bring into solution such amounts of compounds of phosphorus and potassium as can be taken up by plants. The potassium is determined by the method of flame photometry, in which the intensity of the colour of the potassium spectrum is measured — and it is precisely this technique that the scope of the standard indicates.

The result is expressed most often as mg K₂O per 100 g of soil. For the assessment of the richness, limit numbers serve which for potassium — unlike for phosphorus — are different for each agronomic category of 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). For a correct assessment an analysis of the particle size distribution is therefore needed in parallel. The fertiliser advisory system in force in Poland is based on the balancing of P, K and Mg: the dose is to balance the removal with the yields, corrected according to the richness of the soil, and since the 1970s five classes of richness have been in force — very low, low, medium, high and very high.

The status of the standard. The PKN catalogue card states that PN-R-04022:1996 (published on 21.03.1996, 4 pages, KT 90 for Soil Tillage and Horticulture, ICS 13.080.10) was WITHDRAWN on 30.06.2017; its additional element PN-R-04022:1996/Az1:2002 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 still refers to it for the service of determining available potassium. Separately it is worth noting that for organic soils a different standard is used (PN-R-04024), and the one discussed concerns mineral soils only. IUNG-PIB has for years been working on replacing the Egner-Riehm and Schachtschabel tests by the universal Mehlich 3 method together with the conversion of the limit numbers by regression equations.

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 time of shaking, the manner of removing the interfering ions and the limit numbers we give after a laboratory textbook of the Białystok University of Technology and after publications of IUNG-PIB, and not after the standard; the divergences between the sources (the weighed portion and the volume with the ratio 1 : 50 preserved, the pH value of the extractant) we mark expressly. The limit of determination, the requirements of precision and the manner of calibrating the flame photometer the standard has not made available to us — they have to be taken from its text or from the procedure of the laboratory.

THE SCALE OF CITATIONS (read on 23.09.2026). Matching by the BEGINNING of the string: **23 entries at 22 laboratories**; by all forms — 24 at 22. A ratio of 1.0, i.e. one laboratory is exactly one line of the scope. The distribution of forms shows how consistently the Az1 amendment is written here: "PN-R-04022:1996+Az1:2002" in exactly this form 14 times (once more with a capital "AZ1" and once with the number of a laboratory instruction added after it), and beside it four records in three forms differing only in the spaces around the plus (" + Az1" — twice, " +Az1", "+ Az1") and one with a spaced-out PN element ("PN- R-04022"). The standard itself without the amendment occurs 3 times.

A CAUTION WHEN COUNTING THIS SERIES: the core "04022" also contains three records "PN-Z-04022-3:2001" — this is a DIFFERENT standard (protection of the purity of air), which enters the result when matching by the number alone. The figures given above do not include it, because they are counted from the beginning of the string with the "PN-R" element.

Method principle

Air-dry soil sieved through a 1 mm sieve is flooded with an extraction solution of a concentration of 0.04 mol·dm⁻³ with respect to the calcium lactate and 0.02 mol·dm⁻³ with respect to the hydrochloric acid, of pH about 3.6, keeping a soil : solution ratio of 1 : 50 (m/v). The sources give a consistent ratio at different scales: 5 g of soil per 250 cm³ (the laboratory textbook of the Białystok University of Technology), 3 g per 150 cm³ (the survey of analytical conditions of IUNG-PIB), 2 g per 100 cm³ (the IUNG instruction for fruit plants). The Ca²⁺ and H⁺ ions from the reagent displace the exchangeable cation K⁺ from the sorption complex; at the same time acid hydrolysis releases the compounds of phosphorus, and for that reason both components are determined from the same extract. The mixing is carried out for 90 minutes in a rotary shaker (about 40 rev/min according to the textbook of the Białystok University of Technology, 30 rev/min according to the IUNG instruction), after which the suspension is filtered, the first 20–30 cm³ of the filtrate being discarded and phosphorus-free and potassium-free filter papers being used. From the filtrate 25 cm³ are taken, 2 cm³ of a 10-per-cent solution of oxalic acid are added in order to remove the calcium (the interfering ions) and it is left for 24 h for the precipitate of calcium oxalate to settle. Then the intensity of the colour of the potassium spectrum is measured in a flame spectrophotometer — first for the calibration curve containing the appropriate amounts of K₂O, then for the sample — and the content of potassium is read off from the curve. The result is converted into mg K₂O per 100 g of soil from the formula mg K₂O/100 g = a·100/s, where a is the milligrams of K₂O in 25 cm³ of the filtrate, and s — the mass of soil in grams corresponding to 25 cm³ of the filtrate (in the procedure described 0.5 g).

Applications

• Routine tests of the richness of mineral soils in the regional chemical-agricultural stations
• Establishing the doses of potassium fertilisers in the fertiliser advisory system based on the balance of the components (Naw-Sald and the tabular recommendations of IUNG)
• Fertilising of orchards, nurseries and plantations on the basis of a soil analysis — samples taken at least once every 3 years, from light soils more often
• Monitoring of the chemical fertility of soils in the country — the central database of results of the KSChR supplemented annually by about 250 thousand analyses of pH, P, K and Mg
• Comparative and calibration studies in the implementation of the universal Mehlich 3 method in place of the DL Egner and Schachtschabel tests
• Teaching and laboratory training in agricultural chemistry — the determination of phosphorus and potassium from one soil extract

Key parameters

Standard

Standard number

PN-R-04022:1996 + Az1:2002

Title (PL)

Analiza chemiczno-rolnicza gleby — Oznaczanie zawartości przyswajalnego potasu w glebach mineralnych

Step-by-step procedure

1. Taking and preparation of the sample

   Take the sample with an Egner stick or an auger, mix the individual samples from a uniform plot and set aside about 1 kg of soil for the analysis. Dry it in a shaded place to the air-dry state and sieve it through a sieve of 1 mm mesh.

2. The weighed portion

   Weigh out 5 g of dry soil into a bottle or a canister of a capacity of about 500 cm³ (at other scales keep the soil : solution ratio of 1 : 50).

3. Addition of the extractant

   Add 250 cm³ of the solution of calcium lactate with hydrochloric acid: a concentration of 0.04 mol·dm⁻³ with respect to the calcium lactate and 0.02 mol·dm⁻³ with respect to the HCl.

4. Extraction

   Mix for 90 min in a rotary shaker (about 40 rev/min).

5. Filtration

   Filter through a filter paper, discarding the first 20–30 cm³ of the filtrate; use phosphorus-free and potassium-free filter papers. From the same filtrate available phosphorus is also determined.

6. Removal of the interfering ions

   Take 25 cm³ of the filtrate into an Egner tube, add 2 cm³ of a 10-per-cent solution of oxalic acid in order to remove the calcium and leave the tubes for 24 h, until the precipitate of calcium oxalate has settled.

7. Calibration curve

   Measure on the flame spectrophotometer the intensity of the colour of the potassium spectrum for a series of standard solutions containing the appropriate amounts of K₂O.

8. Measurement of the sample

   Measure the intensity of the colour of the potassium spectrum of the sample tested on the same instrument and under the same conditions, and then read off the content of potassium from the calibration curve.

9. Calculation

   Calculate the content according to the formula mg K₂O/100 g = a·100/s, where a is the milligrams of K₂O in 25 cm³ of the filtrate, and s is the mass of soil (g) corresponding to 25 cm³ of the filtrate — in the procedure described 0.5 g.

10. Assessment of the richness

    Establish the agronomic category of the soil (it requires an analysis of the particle size distribution) and read off the class of richness from the limit numbers in force in mg K₂O/100 g: very light soil — very low <2.5, low 2.6–7.5, medium 7.6–12.5, high 12.6–17.5, very high >17.5; light — <5.0 / 5.1–10.0 / 10.1–15.0 / 15.1–20.0 / >20; medium — <7.5 / 7.6–12.5 / 12.6–20.0 / 20.1–25.0 / >25; heavy — <10.0 / 10.1–15.0 / 15.1–25.0 / 25.1–30.0 / >30.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The hydrochloric acid in the extraction solution is corrosive — prepare the solution under a fume hood, in gloves and protective goggles, pour the acid into the water, never the other way round.
• Oxalic acid and its 10-per-cent solution are toxic if swallowed and irritant — work in gloves, do not pipette by mouth, collect the residues into the chemical waste.
• A flame photometer works with a flammable gas and an open flame — an efficient extraction over the burner, checking of the tightness of the gas installation and trained operators are required; do not leave the instrument working unattended.
• The extraction bottles of a capacity of 500 cm³ in a rotary shaker have to be tightened and secured — a loss of tightness means acid solution spilt in motion.
• The drying and the sieving of soil produce mineral dust; treat samples from fields fertilised with natural fertilisers as material potentially contaminated microbiologically.

Frequently asked questions