🌿 Determination of total nitrogen in soil (ammonium, nitrate, nitrite and organic nitrogen) by the modified Kjeldahl method — 0,2–1 g of air-dried soil with salicylic acid in sulfuric acid and sodium thiosulfate, digestion with a catalyst mixture of potassium sulfate, copper sulfate and titanium dioxide (instead of selenium) at up to 400 °C, distillation of ammonia into boric acid and titration with sulfuric acid; result in mg/g of dry matter, PN-ISO 11261

Environment PN-ISO 11261

Overview

WHAT THE STANDARD COVERS. Scope from the PKN catalogue card of PN-ISO 11261:2002 (Polish version), in full (our translation): “A method is given for the determination of total nitrogen (ammonium-N, nitrate-N, nitrite-N and organic nitrogen) in soil. Nitrogen in N-N linkages, N-O linkages and some heterocyclics (especially pyridine) is only partially determined. The method is suitable for all soils”. We read the text of the standard in the sample of ISO 11261:1995 — the French version (first edition, 1995-03-01) in full: foreword, Clauses 1–10 and Annex A (bibliography); the formula of Clause 8 and Table 1 were read from the page images.

ACCORDING TO THE TEXT OF ISO 11261:1995 (complete). The standard was prepared by ISO/TC 190 “Soil quality”, subcommittee SC 3 “Chemical methods and soil characteristics”; Annex A is informative. Scope (1): determination of the total nitrogen content of soil (in ammonium, nitrate, nitrite and organic form); nitrogen in N-N and N-O linkages and in some heterocycles (e.g. pyridine) is only partially determined; the standard is applicable to all types of soil. References (2): ISO 3696 (water for laboratory use), ISO 5725 (precision), ISO 11464 (pretreatment of samples), ISO 11465 (dry matter and water content, gravimetric method). Principle (3): Kjeldahl-type digestion, but with a titanium dioxide (TiO2) catalyst instead of selenium (note 1 — titanium dioxide is less harmful to the environment than selenium). Reagents (4): of recognized analytical grade, water of grade 2 according to ISO 3696; salicylic acid in sulfuric acid — 25 g of salicylic acid in 1 l of concentrated sulfuric acid (ρ = 1,84 g/cm³); catalyst mixture — 200 g of potassium sulfate, 6 g of copper sulfate pentahydrate and 6 g of titanium dioxide with anatase crystal structure, ground and mixed until homogeneous; sodium thiosulfate pentahydrate powdered so as to pass a 0,25 mm sieve; sodium hydroxide c(NaOH) = 10 mol/l; boric acid ρ(H3BO3) = 20 g/l; indicator — 0,1 g of bromocresol green and 0,02 g of methyl red in 100 ml of ethanol; sulfuric acid c(H+) = 0,01 mol/l. Apparatus (5): 50 ml digestion flasks or tubes, digestion block (note 2 — glass test tubes in holes of an aluminium block may be used), distillation apparatus preferably of the Parnas-Wagner type, burette graduated in 0,01 ml or less. Sample pretreatment (6): according to ISO 11464 (note 3 — nitrogen losses may occur in samples with a high content of ammonium or nitrate nitrogen, so excessive drying, 105 °C, should be avoided). Procedure (7): a test portion of air-dried soil from 0,2 g (expected nitrogen content about 0,5 %) to 1 g (about 0,1 %) is placed in the flask, 4 ml of salicylic acid in sulfuric acid is added and mixed until the soil is well wetted; the mixture is left for at least a few hours (or overnight); 0,5 g of sodium thiosulfate is added through a dry funnel reaching into the bulb of the flask and the mixture is heated carefully until foaming stops; after cooling, 1,1 g of catalyst mixture is added and heated until clear — boiling gently for at most 5 h so that the sulfuric acid condenses at about 1/3 of the neck height and the temperature of the solution does not exceed 400 °C (note 4 — in most cases 2 h of boiling is sufficient); after digestion the flask is cooled, about 20 ml of water is added slowly with stirring, the insoluble residue is suspended and transferred to the distillation apparatus, rinsing three times with water; 5 ml of boric acid is placed in a 100 ml conical flask under the condenser so that the end of the condenser is immersed; 20 ml of sodium hydroxide is run slowly through the funnel of the apparatus, about 40 ml of distillate is collected (the amount depends on the apparatus), the end of the condenser is rinsed, a few drops of indicator are added and the distillate is titrated with sulfuric acid to a violet colour (note 5 — potentiometric titration is also possible, end point pH = 5,0; note 6 — with steam distillation a rate of up to about 25 ml/min is applicable, distillation is stopped after about 100 ml of distillate has been collected); a blank test is carried out under the same conditions without soil. Result (8): wN = (V1 − V0) · c(H+) · MN / m · (100 + wH2O) / 100, in milligrams per gram, where V1 and V0 are the volumes of sulfuric acid in ml used for the sample and the blank, c(H+) — hydrogen ion concentration of the acid in mol/l (for 0,01 mol/l sulfuric acid c(H+) = 0,02 mol/l), MN = 14 g/mol, m — mass of air-dried soil in g, wH2O — water content in percent by mass on oven-drying according to ISO 11465; the result is rounded to two significant figures. Precision (9): according to ISO 5725, from a study in 1992 in 14 laboratories on 4 soils (Table 1, after elimination of outliers): at a content of 0,98 mg/g r = 0,17 and R = 0,76 mg/g; at 3,11 mg/g r = 0,33 and R = 1,62 mg/g; at 6,70 mg/g r = 0,54 and R = 3,00 mg/g; at 10,88 mg/g r = 0,74 and R = 2,50 mg/g; the difference between two determinations should not exceed 15 % of the total nitrogen content below 2 mg/g and 10 % above 2 mg/g. Test report (10): reference to the standard, complete identification of the sample, results, and details of operations not included in the standard or regarded as optional and circumstances that may have affected the result.

STATUS (catalogue card as of 03.10.2026). The card of PN-ISO 11261:2002 (Polish version, published 10-01-2002, 9 pages, price group E) has no “Withdrawn” header: Health, Environment and Medicine Sector, KT 191 Soil Chemistry, ICS 13.080.10, “Introduces: ISO 11261:1995 [IDT]”. The PKN search engine (query “PN-ISO 11261”, 03.10.2026) shows only this edition. International document: ISO 11261:1995 has the status “Published” in the iTeh catalogue, stage 90.20 — periodical review started 15-07-2026 (read 03.10.2026).

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database, data up to 17.09.2026, read 03.10.2026). The number 11261 appears in 7 records at 7 laboratories: AB 176, AB 333, AB 646, AB 696, AB 894, AB 945, AB 1436. In the current scopes on the PCA website (read 03.10.2026, issues from 19.08.2024 to 29.06.2026) six laboratories give “PN-ISO 11261:2002”, and AB 333 — “PN ISO 11261:2002” (without hyphen). The “Laboratories” tab (“PN-ISO 11261”) shows 6 laboratories — all except AB 333, whose entry without the hyphen does not start with this key; checked with the tab query on the production server labcoda.pl on 03.10.2026.

WHAT THE LABORATORIES TEST (items in PCA scopes). “Total nitrogen content” (AB 176, AB 333, AB 646, AB 696), “Kjeldahl nitrogen content” (AB 894, AB 945) and “Total Kjeldahl nitrogen content” (AB 1436), in all cases “Titrimetric method”. Object: soil (AB 333, AB 696, AB 894, AB 945, AB 1436), soil, ground, sewage sludge and bottom sediment (AB 176), ground, bottom sediments, sewage sludge and waste with eight codes (AB 646). Ranges: (50–80000) mg/kg (AB 176), (0,18–25) g/kg (AB 333), (0,02–18,5) % (AB 646), (0,1–5) % (AB 696), (0,7–80) g/kg (AB 894), (0,01–4,00) % (AB 945), (1,0–25) g/kg (AB 1436). Next to it are dry matter according to PN-ISO 11465 (AB 176, AB 333), total nitrogen after dry combustion according to PN-ISO 13878 and carbon according to PN-ISO 10694 (AB 696; AB 945 — carbon).

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. Name of the characteristic: three laboratories write “Kjeldahl nitrogen”, while the standard determines total nitrogen — ammonium, nitrate, nitrite and organic (1), and the procedure provides for treatment with salicylic acid in sulfuric acid and sodium thiosulfate before digestion (7) — a result without this step is not a result according to this standard. Object: the standard applies to soils (1) — the items with sewage sludge, bottom sediment and waste (AB 176, AB 646) go beyond its scope. Range: the standard gives no range of application; test portions of 0,2–1 g correspond to an expected content of about 0,5–0,1 % (7), and precision was determined for 0,98–10,88 mg/g (9) — the upper limits of all seven items (from 25 g/kg to 18,5 %) lie above the highest content of the precision study (10,88 mg/g, i.e. 10,88 g/kg). Sample: excessive drying (105 °C) risks loss of nitrogen in samples rich in ammonium or nitrate nitrogen (note 3) — the test portion is taken from air-dried soil and converted to dry matter with the water content according to ISO 11465 (8). Digestion: the temperature of the solution must not exceed 400 °C, boiling time — at most 5 h (7). Titrant concentration: the formula contains the hydrogen ion concentration c(H+), not the concentration of sulfuric acid — for 0,01 mol/l sulfuric acid c(H+) = 0,02 mol/l is substituted (8); the standard specifies reagent 4.7 directly as c(H+) = 0,01 mol/l. Confusing these values changes the result twofold. Result: the content is given in mg/g with two significant figures (8), and the difference between two determinations should not exceed 15 % (below 2 mg/g) or 10 % (above 2 mg/g) (9).

WHAT WE DO NOT GIVE. The standard (read in full) gives no limit of quantification, range of application or requirements for soils — so we do not give them either. Nor do we give safety warnings beyond the text of the standard.

Method principle

The soil is treated with salicylic acid in sulfuric acid and sodium thiosulfate and then digested in sulfuric acid with a titanium dioxide catalyst; after alkalization, ammonia is distilled into boric acid and titrated with sulfuric acid, and total nitrogen in mg/g of dry matter is calculated from the volume difference relative to the blank.

Applications

• Soils of all types — total nitrogen (ammonium, nitrate, nitrite and organic) (scope of the standard); 7 laboratories in PCA scopes
• Ground, sewage sludge, bottom sediments and waste — in the PCA scopes of two laboratories, outside the scope of the standard
• Soil testing together with dry matter (PN-ISO 11465), nitrogen after dry combustion (PN-ISO 13878) and carbon (PN-ISO 10694)

Key parameters

Standard

Standard number

PN-ISO 11261:2002 (wersja polska; ISO 11261:1995 IDT)

Title (PL)

Jakość gleby — Oznaczanie azotu ogólnego — Zmodyfikowana metoda Kjeldahla

Title (EN)

Soil quality — Determination of total nitrogen — Modified Kjeldahl method

Step-by-step procedure

1. Sample

   Prepare the soil according to ISO 11464, without excessive drying (105 °C); weigh 0,2–1 g of air-dried soil. PN-ISO 11261:2002 current (PKN catalogue card as of 03.10.2026).

2. Pretreatment

   Add 4 ml of salicylic acid in sulfuric acid, mix, leave for at least a few hours; add 0,5 g of sodium thiosulfate and heat carefully until foaming stops.

3. Digestion

   After cooling, add 1,1 g of catalyst mixture and boil gently until clear, at most 5 h, solution temperature ≤ 400 °C (2 h is usually sufficient).

4. Distillation

   Cool, slowly add about 20 ml of water, transfer to the apparatus; after adding 20 ml of NaOH 10 mol/l distil about 40 ml of distillate into 5 ml of boric acid.

5. Titration

   Add the indicator and titrate with sulfuric acid c(H+) = 0,01 mol/l to violet; carry out a blank without soil.

6. Result

   wN = (V1 − V0) · c(H+) · 14 / m · (100 + wH2O) / 100 in mg/g, two significant figures; check the difference of two determinations (15 % / 10 %).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The standard replaces selenium with titanium dioxide as catalyst, because it is less harmful to the environment (note 1)
• Sodium thiosulfate is added through a dry funnel, the mixture is heated carefully until foaming stops, and water after digestion is added slowly with stirring to the cooled flask (7)

Frequently asked questions