🌿 Available phosphorus in soil — extraction with calcium lactate (Egner-Riehm method) and colorimetry of phosphomolybdenum blue
In short
Extraction of the phosphorus compounds from the soil with a calcium lactate solution acidified with hydrochloric acid (pH 3. The test is performed according to PN-R-04023:1996; Extractant: calcium lactate 0.04 N + HCl 0.02 N, pH 3.6. The procedure comprises 6 steps; it is used for: Assessment of the phosphorus status of arable soils and of grassland for the setting of the rates of phosphorus fertilisers (fertiliser advisory work, regional chemical and agricultural stations), Fertilisation plans and the correction of the rates by factors depending on the class of the status, Monitoring of over-fertilisation with phosphorus (the limit of 40 mg P₂O₅/100 g; 80 mg in carbonate soils) and protection of waters against eutrophication.
At a glance
- Standard: PN-R-04023:1996
- Category: Environment
- Procedure steps: 6
- Extractant: calcium lactate 0.04 N + HCl 0.02 N, pH 3.6
- Ratio of soil to solution: 5 g of dry soil : 250 cm³; 90 min of mixing (about 40 rev/min)
- Colour: phosphomolybdenum blue (molybdate + fotoreks, reducing agent SnCl₂); measurement at 660 nm after 30 min–6 h
Overview
The status of the standard: PN-R-04023:1996 has been withdrawn by PKN without replacement; the method remains in use in fertiliser advisory work.
The content of the forms of phosphorus available to plants in the soil is small — a mere 4 % or so of the total content, most often 0.2–8 mg/100 g; the available form is the anion H₂PO₄⁻. The basis of rational fertilisation is the assessment of the agrochemical state of the soil, the most important indicators of which are the reaction and the status of the available phosphorus, potassium and magnesium; such tests should be carried out once every 3–5 years (the National Chemical and Agricultural Station, IUNG-PIB, instruction No 251). The results are expressed in absolute values or on a 5-degree scale of the status (very low, low, medium, high, very high): a very low and a low content indicates the need for intensive fertilisation, a medium one — for fertilisation returning the uptake, a high and a very high one — for a limitation of the rates (the correction factors for phosphorus 1.5 / 1.25 / 0.5 / 0.25). Fertilisation with phosphorus is not recommended where the content exceeds 40 mg P₂O₅/100 g (80 mg/100 g in carbonate soils). For the determination of the available forms of phosphorus the Egner-Riehm method is in common use, standardised in PN-R-04023:1996 (withdrawn by PKN, with no replacing standard). The classes of the status according to the limit numbers: V — very low < 5.0; IV — low 5.1–10.0; III — medium 10.1–15.0; II — high 15.1–20.0; I — very high > 20.0 mg P₂O₅/100 g of soil.
Method principle
The phosphorus compounds are extracted from the soil with calcium lactate (CH₃·CHOH·COO)₂Ca acidified with hydrochloric acid: a solution 0.04 N with respect to the calcium lactate and 0.02 N with respect to the HCl, pH 3.6, well buffered both with respect to the hydrogen and to the calcium ions — the two factors determining the solubility of the phosphorus compounds in the soil. The presence of calcium causes only a slight dissolution of the humic compounds, thanks to which the filtrate is weakly coloured or colourless and suitable for colorimetry; the essential feature of the reagent is that it brings into solution such amounts of phosphorus as the plants are able to take up. The phosphorus in the filtrate is determined colorimetrically by measuring the intensity of the colour of phosphomolybdenum blue — the complex formed by the orthophosphate ions in an acid medium with molybdate in the presence of fotoreks and of tin(II) chloride (SnCl₂) as the reducing agent; the fotoreks fixes the colour, which without a fixative quickly fades in too acid a medium. The absorbance is measured at 660 nm at the earliest 30 min and at the latest 6 h after the development of the colour. The content is calculated as mg P₂O₅/100 g = a·100/s, where a — mg of P₂O₅ in 25 cm³ of the filtrate, s — the mass of soil corresponding to 25 cm³ of the filtrate (0.5 g with a weighed portion of 5 g and 250 cm³ of the extractant).
Applications
- Assessment of the phosphorus status of arable soils and of grassland for the setting of the rates of phosphorus fertilisers (fertiliser advisory work, regional chemical and agricultural stations)
- Fertilisation plans and the correction of the rates by factors depending on the class of the status
- Monitoring of over-fertilisation with phosphorus (the limit of 40 mg P₂O₅/100 g; 80 mg in carbonate soils) and protection of waters against eutrophication
- Soil science studies and teaching (calculation of the reserve of available P in the arable layer per 1 ha)
Key parameters
| Parameter | Value |
|---|---|
| Extractant | calcium lactate 0.04 N + HCl 0.02 N, pH 3.6 |
| Ratio of soil to solution | 5 g of dry soil : 250 cm³; 90 min of mixing (about 40 rev/min) |
| Colour | phosphomolybdenum blue (molybdate + fotoreks, reducing agent SnCl₂); measurement at 660 nm after 30 min–6 h |
| Result | mg P₂O₅/100 g of soil = a·100/s (s = 0.5 g for 25 cm³ of the filtrate) |
| Classes of the status | < 5.0 very low; 5.1–10.0 low; 10.1–15.0 medium; 15.1–20.0 high; > 20.0 very high |
| Correction of the P rates | very low ×1.5; low ×1.25; high ×0.5; very high ×0.25; > 40 mg/100 g — no fertilisation |
| Frequency of the tests | once every 3–5 years |
Standard
- Standard number
- PN-R-04023:1996
- Title (PL)
- Analiza chemiczno-rolnicza gleby — Oznaczanie zawartości przyswajalnego fosforu w glebach mineralnych
- Title (EN)
- Chemical and agricultural analysis of soil — Determination of available phosphorus content in mineral soils
Step-by-step procedure
-
Extraction
Weigh out 5 g of dry soil into a bottle of about 500 cm³, add 250 cm³ of the calcium lactate solution and mix it for 90 min on a rotary mixer (about 40 rev/min).
-
Filtration
Filter it through a phosphorus-free and potassium-free filter paper, discarding the first 20–30 cm³ of the filtrate.
-
Development of the colour
Take 25 cm³ of the filtrate into an Egner tube, add 2 cm³ of the mixture of molybdate with fotoreks and mix, add 1 cm³ of the solution of tin(II) chloride and mix again; leave the tubes in a dark place.
-
Calibration curve
Prepare the standard solutions in the same way, measure their absorbance at 660 nm and plot the calibration curve.
-
Measurement
Measure the absorbance of the sample at 660 nm at the earliest 30 min and at the latest 6 h after the development of the colour; read a — mg of P₂O₅ in 25 cm³ of the filtrate.
-
Calculations and assessment
Calculate mg P₂O₅/100 g = a·100/s (s = 0.5 g), assign the class of the status according to the limit numbers and apply the correction factor for the fertiliser rate; if necessary convert the reserve of P in the 20 cm arable layer (about 3 000 000 kg of soil/ha).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Rotary mixer (about 40 rev/min) | bottles of about 500 cm³ | — |
| Phosphorus-free and potassium-free filter papers | the first 20–30 cm³ of the filtrate discarded | — |
| Egner tubes, pipettes | 25 cm³ of the filtrate per determination | — |
| Spectrophotometer (660 nm) | calibration curve from standard solutions of phosphates | — |
| Laboratory balance | a weighed portion of 5 g of dry soil | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Extraction solution of calcium lactate acidified with HCl (0.04 N Ca lactate, 0.02 N HCl, pH 3.6) | — | extraction of the available phosphorus |
| Mixture of molybdate with fotoreks | — | formation of the phosphomolybdenum complex and fixing of the colour (2 cm³ per 25 cm³ of the filtrate) |
| Solution of tin(II) chloride SnCl₂ | — | reducing agent — development of the blue colour (1 cm³) |
| Standard solutions of phosphates | — | calibration curve at 660 nm |
Health and safety (OHS)
- The extraction solution and the reagents are acid — protection of the eyes and of the hands; treat the tin(II) chloride and the molybdates in accordance with the safety data sheets
- Collect the waste containing tin and molybdenum into a container for chemical waste
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-R-04023:1996 — “Chemical and agricultural analysis of soil — Determination of available phosphorus content in mineral soils”.
How does this method work?
The phosphorus compounds are extracted from the soil with calcium lactate (CH₃·CHOH·COO)₂Ca acidified with hydrochloric acid: a solution 0.04 N with respect to the calcium lactate and 0.
What is the measuring range and accuracy?
Extractant: calcium lactate 0.04 N + HCl 0.02 N, pH 3.6; Ratio of soil to solution: 5 g of dry soil : 250 cm³; 90 min of mixing (about 40 rev/min); Colour: phosphomolybdenum blue (molybdate + fotoreks, reducing agent SnCl₂); measurement at 660 nm after 30 min–6 h; Result: mg P₂O₅/100 g of soil = a·100/s (s = 0.5 g for 25 cm³ of the filtrate).
How long does the test take?
The procedure comprises 6 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Rotary mixer (about 40 rev/min), Phosphorus-free and potassium-free filter papers, Egner tubes, pipettes, Spectrophotometer (660 nm), Laboratory balance.
Where is this test used?
Assessment of the phosphorus status of arable soils and of grassland for the setting of the rates of phosphorus fertilisers (fertiliser advisory work, regional chemical and agricultural stations); Fertilisation plans and the correction of the rates by factors depending on the class of the status; Monitoring of over-fertilisation with phosphorus (the limit of 40 mg P₂O₅/100 g; 80 mg in carbonate soils) and protection of waters against eutrophication; Soil science studies and teaching (calculation of the reserve of available P in the arable layer per 1 ha).
What safety precautions apply?
The extraction solution and the reagents are acid — protection of the eyes and of the hands; treat the tin(II) chloride and the molybdates in accordance with the safety data sheets; Collect the waste containing tin and molybdenum into a container for chemical waste.
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-04023.