kSIST FprEN 13650:2026
(Main)Soil improvers and growing media - Digestion by aqua regia for subsequent determination of elements
- Abstract
This document specifies a method for the digestion of soil improvers and growing media using an aqua regia digestion.
NOTE 1 Materials containing more than a mass fraction of about 28 % organic matter can require treatment with additional nitric acid. With high solute concentrations in extract solutions, spectral interferences and background enhancement can be expected.
NOTE 2 This method has been validated in an interlaboratory study with specific products that were present on the market during the study (as included in Annex B, being biowaste compost (dry heavy metals (HM) spiked, Rottegrad 3, milled, sieved at 0,5 mm, dried and spiked), green compost (dry HM spiked, Rottegrad 5, milled, sieved at 0,5 mm, dried and spiked), liquid digestate (HM spiked, sieved 0 mm to12 mm, spiked), expanded clay (dry HM spiked, milled, sieved at 0,5 mm, dried and spiked), mineral wool (fresh), soil improver (dry HM spiked, milled, sieved at 0,5 mm, dried and spiked) and growing media blend 1 (dry HM spiked, white peat, green compost, wood fibre, lime, inorganic fertilizer (NPK + micro elements), milled, sieved at 0,5 mm, dried and spiked)).
- Status
- Not Published
- Public Enquiry End Date
- 19-Dec-2025
- Technical Committee
- KAT - Soil quality
- Current Stage
- 5020 - Formal vote (FV) (Adopted Project)
- Start Date
- 13-Oct-2026
- Due Date
- 01-Dec-2026
- Mandate
- M/XXX - M/XXX
kSIST FprEN 13650:2026, published by SIST, is the standard for digesting soil improvers and growing media with aqua regia before elemental determination. It is written for laboratories that need a defined digestion step for subsequent chemical analysis of these materials. The document sets out two digestion routes, supporting sample preparation, blank testing, reporting, and validation guidance.
What does kSIST FprEN 13650:2026 specify?
kSIST FprEN 13650:2026 specifies a laboratory method for digesting soil improvers and growing media using aqua regia. The digestion is intended to prepare the sample for later measurement of elements, so the method matters wherever elemental content is reported from these materials.
The scope notes two practical limits. Materials with more than about 28 % organic matter may need extra nitric acid, and high solute concentrations can create spectral interferences and background enhancement during later analysis.
The document is organized into:
- Scope, references, terms and definitions, principle, reagents, apparatus, sample pre-treatment, sampling, procedure, test report, and validation.
- Annex A with adjustments for different digestion procedures and measurement apparatuses.
- Annex B with performance characteristics from the validation study.
| Annex | What it covers |
|---|---|
| Annex A | Adjustments for different final volumes and sample-weight calculations, including support for low dry matter liquid samples |
| Annex B | Validation performance characteristics for the method across different matrices |
What are the key requirements of kSIST FprEN 13650:2026?
kSIST FprEN 13650:2026 is built around controlled digestion, careful sample preparation, and a blank-corrected analytical workflow. In practice, the user must choose a digestion route, manage organic matter, and document the exact preparation used so the later element result can be interpreted correctly.
Digestion principle and two methods
Clause 4 gives two digestion procedures. Method A uses reflux heating for 120 min and then filtration or centrifugation, while Method B uses a heating block at 105 °C for 120 min and then filtration or centrifugation.
This matters because laboratories can match the method to their equipment, but they must keep the heating conditions and post-digestion handling consistent. The document also notes that the extracts are suitable for analysis with prEN 18259:2025 and prEN 18258:2025.
Sample pre-treatment and sampling
Clause 7 requires solid samples to be reduced below 0.5 mm according to prEN 13040-1:2025. Liquid samples are prepared according to the same part and may be digested as received or after drying at 70 °C.
Clause 8 states that sampling is not part of the method and points users to EN 12579:2024. In practice, this means the laboratory depends on a representative sample delivered in good condition, because the digestion method cannot correct for poor sampling.
Reagents, blank test, and contamination control
Clause 5 requires analytical-grade acids and reagents, water with very low conductivity, and a test blank run through the same steps as the sample. This is important because the digestion is used for trace analysis, and even small contamination from reagents or glassware can affect the result.
The document also allows an antifoaming agent. That helps control foaming during the acid reaction, especially with organic or reactive samples.
Apparatus and digestion vessels
Clause 6 sets the apparatus requirements for both methods. Method A uses a suitable digestion vessel with reflux condenser and an absorption vessel, while Method B uses a temperature-resistant digestion tube and a heating block.
The vessel materials must be clean and inert enough not to release elements into the digest. In practice, this is what keeps the digestion set-up from becoming a source of false positives.
Handling of organic matter during digestion
Both methods add nitric acid to the aqua regia mixture, and both contain extra instructions if the sample has more organic matter than the base acid charge can oxidize. The user must either reduce the sample size or add additional nitric acid in controlled steps after the first reaction has subsided.
This matters because overloading the digestion can leave incomplete oxidation or generate dangerous reaction behavior. Clause 9 also warns that the work must be done by trained staff and in fume hood or closed force-ventilated equipment.
Reporting and validation
Clause 10 requires the test report to identify the digestion method, the sample, the analytical methods used, the dry-matter basis of the result, and any optional or non-standard details. That gives the downstream analyst or buyer enough information to judge comparability.
Clause 11 states that the method was validated in accordance with ISO 5725-2 and that repeatability and reproducibility data are given in Annex B. This matters because users can see that the method was checked across multiple matrices and laboratories.
What terms does kSIST FprEN 13650:2026 define?
kSIST FprEN 13650:2026 defines the main terms used to describe the digestion step and its chemistry.
- Aqua regia - A digestion solution made by mixing one volume of nitric acid with three volumes of hydrochloric acid.
- Digestion - The mineralization of organic matter and dissolution of the mineral part of a sample by reaction with a reagent mixture.
- Test portion - The weighed amount of sample actually taken into the digestion vessel for analysis.
- Blank test - A reagent-only run through the same steps as the sample to measure contributions from reagents and equipment.
- Dry matter (DM) - The sample portion remaining after moisture is accounted for, used to express results on a dry-matter basis.
- Limit of quantification (LOQ) - The lowest concentration that the analytical system can quantify for the element and matrix in question.
Who uses kSIST FprEN 13650:2026?
kSIST FprEN 13650:2026 is used by analytical laboratories that measure elements in soil improvers and growing media. It is also relevant to producers, quality managers, and control laboratories that need a reproducible digestion step before ICP-AES, mercury determination, or other element analysis.
The document is particularly relevant in fertilizer-related and soil-amendment work, where elemental limits and product characterization depend on a consistent digestion procedure. The validation work in Annex B also shows its relevance to laboratories comparing results across different matrices and product types.
What changed in kSIST FprEN 13650:2026 from the previous edition?
kSIST FprEN 13650:2026 supersedes EN 13650:2001. The European foreword states two significant technical changes:
- the heating block procedure has been added to the reflux digestion procedure;
- the scope has been expanded to include liming materials and preformed materials.
Which standards are used with kSIST FprEN 13650:2026?
kSIST FprEN 13650:2026 refers to several standards that support sampling, preparation, terminology, and validation.
- CEN/TS 17732:2022 - Provides the terminology used in the field of soil improvers and growing media.
- EN 12579:2024 - Covers sampling, which the document relies on because sampling is outside the digestion method itself.
- prEN 13039:2025 - Provides determination of organic content and ash, used when the sample’s organic load needs checking before digestion.
- prEN 13040-1:2025 - Covers sample preparation and moisture determination for chemical and physical tests.
- ISO 5725-2 - Provides the basis for repeatability and reproducibility validation.
- prEN 18259:2025 - Gives the follow-on determination of aqua regia soluble elements.
- prEN 18258:2025 - Gives the follow-on determination of mercury in aqua regia extracts.
What does the kSIST FprEN 13650:2026 document contain?
kSIST FprEN 13650:2026 contains two complete digestion procedures, one based on reflux heating and one based on a heating block. It also contains instructions for reagents, apparatus, pre-treatment of solid and liquid samples, a safety warning, and a blank test procedure.
Annex A provides adjustment guidance for other final volumes and for calculating sample weight, including worked calculation logic for low dry matter samples. Annex B contains the validation data, including performance characteristics for seven matrices from an interlaboratory study, presented in tables for different elements.
The document therefore serves both as a laboratory operating method and as a basis for method comparison, reporting, and follow-on elemental analysis.
Relations
- Effective Date
- 28-Jan-2026
- Effective Date
- 28-Jan-2026
- Effective Date
- 28-Jan-2026
- Effective Date
- 28-Jan-2026
- Effective Date
- 28-Jan-2026
- Effective Date
- 28-Jan-2026
- Effective Date
- 28-Jan-2026
- Effective Date
- 28-Jan-2026
- Effective Date
- 28-Jan-2026
- Effective Date
- 28-Jan-2026
- Revises
SIST EN 13650:2002 - Soil improvers and growing media - Extraction of aqua regia soluble elements - Effective Date
- 01-Dec-2025
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Frequently Asked Questions
kSIST FprEN 13650:2026 is a draft published by the Slovenian Institute for Standardization (SIST). Its full title is "Soil improvers and growing media - Digestion by aqua regia for subsequent determination of elements". This standard covers: This document specifies a method for the digestion of soil improvers and growing media using an aqua regia digestion. NOTE 1 Materials containing more than a mass fraction of about 28 % organic matter can require treatment with additional nitric acid. With high solute concentrations in extract solutions, spectral interferences and background enhancement can be expected. NOTE 2 This method has been validated in an interlaboratory study with specific products that were present on the market during the study (as included in Annex B, being biowaste compost (dry heavy metals (HM) spiked, Rottegrad 3, milled, sieved at 0,5 mm, dried and spiked), green compost (dry HM spiked, Rottegrad 5, milled, sieved at 0,5 mm, dried and spiked), liquid digestate (HM spiked, sieved 0 mm to12 mm, spiked), expanded clay (dry HM spiked, milled, sieved at 0,5 mm, dried and spiked), mineral wool (fresh), soil improver (dry HM spiked, milled, sieved at 0,5 mm, dried and spiked) and growing media blend 1 (dry HM spiked, white peat, green compost, wood fibre, lime, inorganic fertilizer (NPK + micro elements), milled, sieved at 0,5 mm, dried and spiked)).
This document specifies a method for the digestion of soil improvers and growing media using an aqua regia digestion. NOTE 1 Materials containing more than a mass fraction of about 28 % organic matter can require treatment with additional nitric acid. With high solute concentrations in extract solutions, spectral interferences and background enhancement can be expected. NOTE 2 This method has been validated in an interlaboratory study with specific products that were present on the market during the study (as included in Annex B, being biowaste compost (dry heavy metals (HM) spiked, Rottegrad 3, milled, sieved at 0,5 mm, dried and spiked), green compost (dry HM spiked, Rottegrad 5, milled, sieved at 0,5 mm, dried and spiked), liquid digestate (HM spiked, sieved 0 mm to12 mm, spiked), expanded clay (dry HM spiked, milled, sieved at 0,5 mm, dried and spiked), mineral wool (fresh), soil improver (dry HM spiked, milled, sieved at 0,5 mm, dried and spiked) and growing media blend 1 (dry HM spiked, white peat, green compost, wood fibre, lime, inorganic fertilizer (NPK + micro elements), milled, sieved at 0,5 mm, dried and spiked)).
kSIST FprEN 13650:2026 is classified under the following ICS (International Classification for Standards) categories: 65.080 - Fertilizers. The ICS classification helps identify the subject area and facilitates finding related standards.
kSIST FprEN 13650:2026 has the following relationships with other standards: It is inter standard links to SIST EN ISO 13161:2021, SIST EN ISO 22125-2:2020, SIST EN ISO 23548:2025, SIST EN ISO 11665-6:2020, SIST EN ISO 13165-3:2025, SIST EN ISO 18589-5:2021, SIST EN ISO 19581:2025, SIST EN ISO 11665-5:2020, SIST EN ISO 18589-1:2021, SIST EN ISO 13163:2022, SIST EN 13650:2002. Understanding these relationships helps ensure you are using the most current and applicable version of the standard.
kSIST FprEN 13650:2026 is associated with the following European legislation: EU Directives/Regulations: 2019/1009; Standardization Mandates: M/564, M/564 AMD 1, M/564 AMD 2, M/XXX. When a standard is cited in the Official Journal of the European Union, products manufactured in conformity with it benefit from a presumption of conformity with the essential requirements of the corresponding EU directive or regulation.
kSIST FprEN 13650:2026 is available in PDF format for immediate download after purchase. The document can be added to your cart and obtained through the secure checkout process. Digital delivery ensures instant access to the complete standard document.
Standards Content (Sample)
SLOVENSKI STANDARD
oSIST prEN 13650:2025
01-december-2025
Izboljševalci tal in rastni substrati - Razklop z zlatotopko za določanje elementov
Soil improvers and growing media - Digestion by aqua regia for subsequent
determination of elements
Bodenverbesserungsmittel und Kultursubstrate - Aufschluss mit Königswasser zur
anschließenden Bestimmung der Elemente
Amendements du sol et supports de culture - Digestion à l’eau régale pour le dosage
ultérieur des éléments
Ta slovenski standard je istoveten z: prEN 13650
ICS:
65.080 Gnojila Fertilizers
oSIST prEN 13650:2025 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
oSIST prEN 13650:2025
oSIST prEN 13650:2025
DRAFT
EUROPEAN STANDARD
prEN 13650
NORME EUROPÉENNE
EUROPÄISCHE NORM
October 2025
ICS 65.080 Will supersede EN 13650:2001
English Version
Soil improvers and growing media - Digestion by aqua
regia for subsequent determination of elements
Amendements du sol et supports de culture - Digestion Bodenverbesserungsmittel und Kultursubstrate -
à l'eau régale pour le dosage ultérieur des éléments Aufschluss mit Königswasser zur anschließenden
Bestimmung der Elemente
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee
CEN/TC 223.
If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations
which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.
Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 13650:2025 E
worldwide for CEN national Members.
oSIST prEN 13650:2025
prEN 13650:2025 (E)
Contents Page
European foreword . 3
Introduction . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Principle . 5
5 Reagents . 6
6 Apparatus . 6
6.1 General. 6
6.2 Method A — Thermal heating under reflux conditions . 6
6.3 Method B — Thermal heating with a heating block with containers . 6
7 Sample pre-treatment . 7
8 Sampling . 7
9 Procedure . 7
9.1 Safety . 7
9.2 Blank test . 7
9.3 Method A — Thermal heating under reflux conditions . 8
9.4 Method B — Thermal heating with a heating block with containers . 9
10 Test report . 9
11 Validation of the method . 10
11.1 Validation in accordance with ISO 5725-2. 10
11.2 Performance characteristics. 10
Annex A (informative) Adjustment to different digestion procedures and measurement
apparatuses . 11
A.1 Adjustment of the procedure to different final volumes . 11
A.2 Calculation of the sample weight . 11
A.2.1 General. 11
A.2.2 Calculation of the weight of the sample (dry matter) . 12
A.2.3 Calculation of the weight of the sample for analysis . 12
Annex B (informative) Performance characteristics of the method . 13
Bibliography . 32
oSIST prEN 13650:2025
prEN 13650:2025 (E)
European foreword
This document (prEN 13650:2025) has been prepared by Technical Committee CEN/TC 223 “Soil
improvers and growing media”, the secretariat of which is held by NEN.
This document is currently submitted to CEN Enquiry.
This document will supersede EN 13650:2001.
prEN 13650:2025 includes the following significant technical changes with respect to EN 13650:2001:
— the heating block procedure has been added to the reflux digestion procedure;
— the scope has been expanded including liming materials and preformed materials.
This document has been prepared under a standardization request addressed to CEN by the European
Commission. The Standing Committee of the EFTA States subsequently approves these requests for its
Member States.
oSIST prEN 13650:2025
prEN 13650:2025 (E)
Introduction
Aqua regia is applied to digest different matrices for the subsequent determination of many elements.
For example, a multi-matrix standard for aqua regia digestion of soil, treated biowaste, sludge and waste
was prepared by CEN/TC 444 “Environmental characterization of solid matrices”. CEN/TC 260
“Fertilizers and liming materials” published a standard with a similar procedure for organic and organo-
mineral fertilizers.
It is known that the digestion of soil improvers or growing media with aqua regia will not necessarily
lead to complete element recoveries and that the extract from a test sample may not reflect the total
concentrations of the target analytes. However, for most applications the result obtained based upon
digestion methods specified in this document are considered to be fit for the intended purpose.
oSIST prEN 13650:2025
prEN 13650:2025 (E)
1 Scope
This document specifies a method for the digestion of soil improvers and growing media using an aqua
regia digestion.
Materials containing more than about 28 % (m/m) organic matter will require treatment with additional
nitric acid. With high solute concentrations in extract solutions, spectral interferences and background
enhancement can be expected.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
CEN/TS 17732:2022, Soil improvers and growing media — Terminology
EN 12579:2024, Soil improvers and growing media — Sampling
prEN 13039:2025, Soil improvers and growing media — Determination of organic content and ash
prEN 13040-1:2025, Soil improvers and growing media — Sample preparation — Part 1: Sample
preparation for chemical and physical tests, determination of dry matter content, moisture content and
laboratory bulk density
3 Terms and definitions
For the purposes of this document, the terms and definitions given in CEN/TS 17732:2022 and the
following apply.
3.1
aqua regia
digestion solution obtained by mixing 1 volume of nitric acid (mass fraction of 65 % to 70 %) and
3 volumes of hydrochloric acid (mass fraction of 35 % to 37 %)
3.2
digestion
mineralization of the organic matter of a sample and dissolution of its mineral part, more or less
completely, when reacting with a reagent mixture
4 Principle
A test portion is digested with aqua regia according to one of the following heating procedures:
— Method A: reflux for 120 min ± 10 min, followed by filtration/centrifugation;
— Method B: heating block at 105 °C ± 5 °C for 120 min + 10 min, followed by filtration/centrifugation.
oSIST prEN 13650:2025
prEN 13650:2025 (E)
5 Reagents
5.1 General
Use only acids and reagents of recognized analytical grade to avoid high blank values for subsequent
analytical measurements. Use a test blank solution throughout the procedure applying all steps with the
same amount of acids, but without a sample.
5.2 Water, having an electrical conductivity not higher than 0,2 mS/cm at 25 °C, free of the elements
to be determined.
5.3 Hydrochloric acid, c(HCl) = 12 mol/l; ρ ≈ 1,18 g/ml; 37 % mass/volume.
5.4 Nitric acid, c(HNO ) = 15 mol/l; ρ ≈ 1,42 g/ml; not less than 65 % mass/volume.
5.5 Nitric acid, c(HNO ) = 0,5 mol/l. Dilute 35 ml nitric acid (5.4) to 1 l with water (5.2).
5.6 Antifoaming agent, e.g. n-dodecane (C H ) or n-octanol (C H O) are suitable.
12 26 8 18
6 Apparatus
6.1 General
Usual laboratory apparatus. All glassware and plastics ware shall be adequately cleaned and stored to
avoid any contamination.
Depending upon the concentration of the element of interest, particular care should be exercised with
respect to the effective cleaning of the vessels.
6.2 Method A — Thermal heating under reflux conditions
6.2.1 Digestion vessel, temperature-resistant and capable of containing the mixture of sample and
digest solution, for example a quartz vessel. The digestion vessel shall have a volume of at least 5 times
of the volume of the aqua regia used. The inner wall of the vessel shall be inert and shall not release
substances to the digest in excess of the purity requirements of the subsequent analysis.
Silica or borosilicate glass vessels may be used instead of quartz vessels.
It may be necessary to periodically clean the digestion vessels with a suitable surfactant to remove
persistent deposits.
6.2.2 Reflux condenser, adaptable to the digestion vessel (6.2.1).
6.2.3 Absorption vessel, volatile species trap, in an open digestion system capable of trapping one or
more volatile measurement species, adaptable to the reflux condenser (6.2.2).
6.2.4 Heating device, for example a heating mantle, thermostatic controlled, or an aluminium block
thermostat.
6.3 Method B — Thermal heating with a heating block with containers
6.3.1 Digestion tube, 50 ml plastic tube, temperature-resistant and capable of containing the mixture
of sample and digest solution, with a screw cap from e.g. polypropylene.
The part of the tube not being heated and the screw cap function as a condenser but are not really a reflux
system. The material of the tube and screw cap need to be tested to be sure that release of elements of
oSIST prEN 13650:2025
prEN 13650:2025 (E)
interest does not take place. Other materials and vessels with other volumes than mentioned above are
allowed to be used if suitability has been proven.
6.3.2 Temperature controlled heating block, heating block able to heat the tube(s) to a temperature
of 105 °C ± 5 °C.
6.4 Sample containers, plastics and glass containers are both suitable.
6.5 Filter paper, usually with a pore size of 0,45 µm and resistant to the diluted aqua regia final
digestion solution.
6.6 Volumetric flasks, usually of nominal capacity of 50 ml or 100 ml.
6.7 Analytical balance, with an accuracy of 1 mg or better.
6.8 Boiling aids, anti-bumping granules or glass beads, diameter 2 mm to 3 mm, acid washed.
7 Sample pre-treatment
Prepare the solid sample by reducing the particle size below 0,5 mm in accordance with Clause 10 of
prEN 13040-1:2025. The residual moisture content in the analysed sample is determined in accordance
with prEN 13040-1:2025, Clause 11, using 10 g of sample.
Prepare the liquid samples in accordance with Clause 9 of prEN 13040-1:2025. Liquid samples may be
digested as such or after drying at 70 °C. The (residual) moisture content in the analysed sample is
determined in accordance with prEN 13040-1:2025, Clause 11, using 10 g of sample.
8 Sampling
Sampling is not part of the method specified in this document. Follow EN 12579:2024 dealing with soil
improvers and growing media. It is important that the laboratory receives a sample that is representative
of the product under consideration. The sample should not have been damaged or changed during
transport or storage.
9 Procedure
9.1 Safety
WARNING — Persons using this document should be familiar with usual laboratory practice. Some of the
reagents used in this document are highly corrosive and very toxic. Safety precautions are absolutely
necessary.
All procedures should be performed in a fume hood or in closed force-ventilated equipment. By the use
of strong oxidising reagents, the formation of explosive organic intermediates is possible, especially when
dealing with samples with a high organic content. Avoid contact with the chemicals and the gaseous
reaction products.
IMPORTANT — It is absolutely essential that tests conducted according to this document be carried out
by suitably trained staff.
9.2 Blank test
Carry out a reagent blank test digestion in parallel with the determination, using the same procedure and
the same quantities of all the reagents as in the determination, but omitting the test portion. The
laboratory shall define acceptable limits.
oSIST prEN 13650:2025
prEN 13650:2025 (E)
NOTE The measurement of a blank is introduced to determine the contribution of the extracting solution,
glassware, digestion tube and filter paper used to the measured value.
9.3 Method A — Thermal heating under reflux conditions
This procedure may be used for all samples of soil improvers and growing media if a sufficiently sensitive
method for determination is used. For some liquid samples with a low dry matter content, a higher intake
volume or a pre-concentration step may be necessary (see Annex A). Pre-concentration may be carried
out by heating an amount of liquid sample to approximately 70 °C to evaporate excessive water. The pre-
concentration factor shall be taken into account in the final calculations.
Weigh approximately 1 g to 3 g, to the nearest 0,001 g of the test sample, and transfer to the digestion
vessel (6.2.1). In case of dry samples moisten the test portion with about 0,5 ml to 1,0 ml water (5.2). Add
while mixing, 21,0 ml ± 0,1 ml of hydrochloric acid (5.3) followed by 7,0 ml ± 0,1 ml of nitric acid (5.4),
drop by drop if necessary to reduce foaming. Connect the reflux condenser (6.2.2) to the digestion vessel
(6.2.1). Fill the absorption vessel (6.2.3) with approximately 15 ml nitric acid (5.5). Connect the
absorption vessel to the reflux condenser and let stand at room temperature until any effervescence
almost ceases to allow for slow oxidation of the organic matter in the sample.
The time of standing at room temperature can have an influence on the digestion rate of aqua regia. For
consistency, it is recommended to start heating as soon as possible after the first strong reaction has
ceased.
NOTE It is common practice to add hydrochloric acid directly to the reaction vessel, followed by the nitric acid,
and to assume that the aqua regia forms in the reaction vessel. However, problems have been encountered with
materials rich in carbonates, where a substantial portion of the hydrochloric acid has been consumed before the
nitric acid can be added. Under such circumstances, the correct formation of aqua regia is doubtful.
The amount of aqua regia is sufficient only for oxidation of about 0,85 g of organic matter (corresponding
to about 0,5 g of organic carbon). If there is any doubt about the amount of organic matter present, carry
out a determination according to prEN 13039:2025. If there is more than 0,85 g of organic matter in the
test sample reduce the quantity of sample analysed or proceed as follows.
Allow the first reaction with aqua regia to subside. Then add an extra 1 ml of nitric acid (5.4) only to every
0,17 g of organic matter above 0,85 g. Do not add more than 10 ml of nitric acid at any time and allow any
reaction to subside before proceeding further.
Add a few roughened glass beads (6.8) and connect the digestion vessel (6.2.1) to the heating device
(6.2.4). Raise the temperature of the reaction mixture slowly until reflux conditions are reached and
maintain for 2 h ensuring that the condensation zone is lower than 1/3 of the height of the condenser,
then allow to cool. Add the content of the absorption vessel to the digestion vessel via the reflux
condenser, rinsing both the absorption vessel and condenser with further 10 ml of diluted nitric acid
(5.5).
Transfer quantitatively the solution content of the digestion vessel into a suitable sized volumetric flask
and add water (5.2) to the volume mark.
Alternatively, another procedure may be applied, such that the adjustment to volume with the solid
residue still present shall be carried out immediately after digestion.
If the measurement solution contains particles due to precipitation which may clog nebulizers or
interfere with an injection of the sample into the instrument, the sample may be centrifuged, allowed to
settle, or filtered (6.5).
The extracts are suitable for analysis using prEN 18259:2025 and prEN 18258:2025.
oSIST prEN 13650:2025
prEN 13650:2025 (E)
9.4 Method B — Thermal heating with a heating block with containers
This procedure may be used for all samples of soil improvers and growing media if a sufficiently sensitive
method for determination is used. For some liquid samples with a low dry matter content, a higher intake
volume or a pre-concentration step may be necessary (see Annex A). Pre-concentration may be carried
out by heating an amount of liquid sample to approximately 70 °C to evaporate excessive water. The pre-
concentration factor shall be taken into account in the final calculations.
Weigh an amount of not more than 2 g of the test portion (typically 0,5 g to 1 g of dry sample), to the
nearest 0,001 g, and transfer to the digestion vessel (6.3.1). In case of dry samples moisten the test
portion with a few drops of water (5.2). Add 12,0 ml + 0,1 ml hydrochloric acid (5.3) followed by
4,0 ml + 0,1 ml nitric acid (5.4). Let stand at room temperature until any effervescence almost ceases to
allow for slow oxidation of the organic matter in the sample.
The amount of aqua regia is sufficient only for oxidation of about 0,50 g of organic matter (corresponding
to about 0,3 g of organic carbon). If there is any doubt about the amount of organic matter present, carry
·
out a determination according to prEN 13039:2025 If there is more than 0,50 g of organic matter in the
test sample reduce the quantity of sample analysed or proceed as follows.
Allow the first reaction with aqua regia to subside. Then add an extra 1 ml of nitric acid (5.4) only to every
0,17 g of organic matter above 0,50 g. Do not add more than 10 ml of nitric acid at any time and allow any
reaction to subside before proceeding further.
NOTE For some elements, e.g. chromium, the additional volume of HNO is essential in order to have a sufficient
recovery upon digestion.
The time of standing at room temperature can have an influence on the digestion rate of aqua regia. For
consistency, it is recommended to start heating as soon as possible after the first strong reaction has
ceased.
Loosely screw on the tube cap (not very tight!) and place the digestion vessel on the heating block (6.3.2)
and slowly increase the temperature to the boiling point. Keep the temperature on the boiling point
during 120 min + 10 min.
Let the vessel cool down to room temperature and fill up with water (5.2) to the volume mark.
If a non-graduated digestion tube is used, transfer quantitatively the solution into a suitable sized
volumetric flask and add water (5.2) to the volume mark.
Alternatively, another procedure may be applied, such that the adjustment to volume with the solid
residue still present shall be carried out immediately after digestion.
If the measurement solution contains particles due to precipitation which may clog nebulizers or
interfere with an injection of the sample into the instrument, the sample may be centrifuged or filtered
(6.5).
The extracts are suitable for analysis using prEN 18259:2025 and prEN 18258:2025.
10 Test report
The test report may be prepared separately or in conjunction with the test report of the subsequent
analytical method.
The test report shall include the following information:
a) the digestion method used, together with a reference to this document (including its year of
publication);
b) a complete identification of the sample;
oSIST prEN 13650:2025
prEN 13650:2025 (E)
c) all the analytical methods used;
d) the results of the determination expressed as mass/mass on dry matter basis;
e) any details not specified in this document, or which are optional, as well as any other factor which
may have affected the results.
11 Validation of the method
11.1 Validation in accordance with ISO 5725-2
This standardized reference method was validated in accordance with ISO 5725-2 [2]. All relevant data
were reported in an interlaboratory study report, which has been published [6].
11.2 Performance characteristics
The performance characteristics (reproducibility and repeatability) of the method were determined in
an interlaboratory study. All data are given in Annex B. The method has been validated for seven different
matrices, as listed in Tables B.1 to B.19.
oSIST prEN 13650:2025
prEN 13650:2025 (E)
Annex A
(informative)
Adjustment to different digestion procedures and measurement
apparatuses
A.1 Adjustment of the procedure to different final volumes
Volumetric flasks or graduated plastic tubes for 150 ml, 100 ml or 50 ml may be used for the final volume
adjustment. Table A.1 gives recalculated parameters for the different final volumes.
Table A.1 — Recalculated parameters for different final volumes
V V V m V
F HNO3 HCl DM max
ml ml ml g ml
150 7 21 1,5 to 3 6
100 4,7 14 1 to 2 4
50 2,3 7 0,5 to 1 2
VF – final volume of a volumetric flask or a graduated plastic tube
m – weight of the sample recalculated
...