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Soil · Air-dried soil, sieved to < 2 mm

Available Nitrogen, Phosphorus and Potassium in Soil

Estimates the plant-available pool of the three main nutrients. Nitrogen that is easily mineralised is released by alkaline permanganate and distilled as ammonia. Phosphorus is extracted with sodium bicarbonate (Olsen) or acid fluoride (Bray) and measured as molybdenum blue. Exchangeable potassium is displaced by ammonium acetate and measured by flame photometry.

  • Environmental & water
  • Distillation–titration (N), molybdenum-blue colorimetry (P), flame photometry (K)
  • Kjeldahl
  • UV-Vis
  • Flame photometer
  • Analytical balance
  • Burette & glassware

Reference: Typical soil-testing methods: Subbiah and Asija (1956) for available N; Olsen et al. (1954) for neutral and alkaline soils or Bray and Kurtz (1945) for acid soils for available P; neutral normal ammonium acetate extraction for available K. Follow the manual adopted by your soil-testing programme.

PRINCIPLE

Why this method works

These are not total contents; they are 'available' fractions defined by the extractant. (1) Nitrogen: alkaline potassium permanganate oxidises and hydrolyses the easily decomposable organic matter, releasing ammonia, which is distilled into boric acid and titrated with standard acid. (2) Phosphorus: 0.5 M NaHCO₃ at pH 8.5 (Olsen) extracts phosphate from calcium, aluminium and iron phosphates in neutral to alkaline soils; Bray-1 (dilute HCl with NH₄F) suits acid soils. The phosphate reacts with ammonium molybdate and is reduced, usually by ascorbic acid, to a blue complex whose absorbance is proportional to concentration. (3) Potassium: ammonium ions in neutral 1 N ammonium acetate exchange with K⁺ on soil colloids; potassium in the extract emits light at about 766–768 nm in a flame, and the flame photometer reads this against standards.

Instruments used

MATERIALS

Reagents & standards

ReagentGrade / specification
N: Potassium permanganate0.32% KMnO₄ solution
N: Sodium hydroxide2.5% NaOH solution
N: Boric acid with mixed indicator2% H₃BO₃ with bromocresol green–methyl red indicator
N: Standard acid0.02 N H₂SO₄ (standardised)
P: Olsen extractant0.5 M NaHCO₃ adjusted to pH 8.5; activated charcoal (P-free) if the extract is coloured
P: Bray-1 extractant (acid soils)0.03 N NH₄F in 0.025 N HCl
P: Colour reagentAmmonium molybdate–antimony potassium tartrate with ascorbic acid (Murphy and Riley type)
P: StandardKH₂PO₄ stock (dried), diluted to working standards of 0–1.0 mg P/L in the final volume
K: ExtractantNeutral 1 N ammonium acetate, pH 7.0
K: StandardKCl stock (dried), working standards for example 0–40 mg K/L prepared in ammonium acetate
SAMPLE PREPARATION

Preparing the sample

  1. 01

    Prepare soil

    Air-dry, crush and sieve to < 2 mm.

  2. 02

    N distillation

    20 g soil + KMnO₄ + NaOH; distil NH₃ into boric acid.

  3. 03

    N titration

    Titrate distillate with 0.02 N H₂SO₄.

  4. 04

    P extraction

    2.5 g soil + 50 mL Olsen reagent; shake 30 min; filter.

  5. 05

    P colour

    Aliquot + colour reagent; read at about 880 nm.

  6. 06

    K extraction & read

    5 g soil + 25 mL NH₄OAc; shake; filter; read on flame photometer.

Procedure

  1. 1Available N: weigh 20 g soil into the distillation flask. Add 20 mL water, 100 mL of 0.32% KMnO₄ and 100 mL of 2.5% NaOH (volumes as in your manual). Distil immediately and collect about 100 mL of distillate in 20 mL of 2% boric acid containing mixed indicator. Titrate with 0.02 N H₂SO₄ from green back to the original pink. Run a reagent blank.
  2. 2Available P (Olsen): weigh 2.5 g soil into a 100 mL bottle, add a pinch of P-free charcoal if needed and 50 mL of 0.5 M NaHCO₃ (pH 8.5). Shake for 30 minutes and filter through a P-free filter paper.
  3. 3Pipette 5 mL of filtrate into a 25 mL volumetric flask. Acidify carefully to release CO₂ (as described in your manual), add the ascorbic-acid molybdate reagent, make to volume and allow the colour to develop (typically 10–30 minutes).
  4. 4Read absorbance at about 880–882 nm against a reagent blank. Prepare standards in the same matrix and plot a calibration curve.
  5. 5For acid soils, use Bray-1 extractant instead (typically 1 g soil + 7 mL extractant, shaken briefly) and read the same way.
  6. 6Available K: weigh 5 g soil, add 25 mL of neutral 1 N ammonium acetate, shake for about 5 minutes and filter. Aspirate the filtrate into the flame photometer, calibrated with K standards prepared in ammonium acetate.
  7. 7Include a reagent blank, a reference soil and one duplicate per ten samples for each nutrient.

Standard preparation

  1. 1P: dissolve dried KH₂PO₄ to give a 50 mg P/L stock; dilute to a 2 mg P/L intermediate; transfer 0, 1, 2, 3, 4, 5 mL into 25 mL flasks with 5 mL extractant to give 0–0.4 mg P/L in the final volume (extend if needed).
  2. 2K: dissolve dried KCl to give a 1000 mg K/L stock; prepare 0, 5, 10, 20, 30, 40 mg K/L in ammonium acetate. Set zero with the blank and full scale with the top standard.
  3. 3N: standardise the 0.02 N H₂SO₄ against a primary standard (for example sodium carbonate or borax) and check recovery with an ammonium sulphate standard.

Notes for analysts

  • Use Olsen for neutral, calcareous and alkaline soils and Bray-1 for acid soils; results from the two are not interchangeable.
  • The 2.24 × 10⁶ kg/ha factor assumes a 15 cm plough layer with a bulk density of about 1.5 g/cm³. Report mg/kg as the primary result.
  • Phosphorus is easily contaminated by detergents. Wash glassware with phosphate-free detergent and acid rinse.
  • Distillation uses hot caustic solutions. Check the apparatus seals and keep the receiver tube under the boric acid surface to avoid ammonia loss.
TEST CONDITIONS

Method parameters (typical)

N: soil mass / extractant20 g; 0.32% KMnO₄ + 2.5% NaOH; distillation into 2% boric acid
N: titrant0.02 N H₂SO₄ (1 mL = 0.28 mg N)
P (Olsen): ratio / time1 : 20 (2.5 g + 50 mL), 30 minutes shaking
P: measurementMolybdenum blue (ascorbic acid), about 880 nm, 1 cm cell
K: ratio / time1 : 5 (5 g + 25 mL), about 5 minutes shaking
K: measurementFlame photometer, K filter (about 766–768 nm)
Reporting basismg/kg; often also kg/ha using 2.24 × 10⁶ kg soil per hectare (0–15 cm layer)
BEFORE YOU REPORT

System suitability & sequence

ParameterLimit
P calibration curveCorrelation coefficient typically ≥ 0.995
K calibrationStandards read back within the tolerance in your SOP; re-check a mid standard every 10 samples
N recoveryAmmonium sulphate standard typically recovered at 95–105%
Reference soilEach nutrient within control-chart limits
BlanksReagent blanks low and consistent

Typical sequence

  1. 1Reagent blanks
  2. 2Calibration standards (P, K)
  3. 3Reference soil
  4. 4Samples with one duplicate per ten
  5. 5Mid-range check standard every 10 samples (P, K)
CALCULATION

Calculation with a worked example

Formula
N (kg/ha) = [(T − B) × Nₐ × 14 × 1000 ÷ W] × 2.24; P (mg/kg) = C × Vf × (Ve ÷ Va) ÷ W; K (mg/kg) = C × Ve ÷ W

Where

T, B
Acid titre for sample and blank (mL)
Nₐ
Normality of H₂SO₄ (0.02 N)
14
mg of N per milliequivalent
W
Soil mass (g)
C
Concentration read from the calibration curve (mg/L)
Vf
Final colour volume for P (mL)
Ve, Va
Extract volume (mL) and aliquot taken (mL)
2.24
Factor from mg/kg to kg/ha (2.24 × 10⁶ kg soil/ha)

Given

N: W, T, B
20 g; 8.2 mL; 0.2 mL of 0.02 N H₂SO₄
P: W, Ve, Va, Vf
2.5 g; 50 mL; 5 mL aliquot; 25 mL final
P: C from curve
0.32 mg P/L
K: W, Ve
5 g; 25 mL
K: C from flame photometer
12.0 mg K/L

Working

  1. 1

    N: (8.2 − 0.2) × 0.02 × 14 = 8.0 × 0.28 = 2.24 mg N in 20 g soil

  2. 2
    N: 2.24 × 1000 ÷ 20 = 112 mg/kg; × 2.24 = 250.9 kg N/ha
  3. 3

    P: 0.32 mg/L × 25 mL = 8 µg P in the 5 mL aliquot

  4. 4
    P: in the 50 mL extract = 8 × (50 ÷ 5) = 80 µg; per 2.5 g soil = 80 ÷ 2.5 = 32 mg P/kg
  5. 5

    P: 32 × 2.24 = 71.7 kg P/ha; as P₂O₅ = 71.7 × 2.29 = 164 kg P₂O₅/ha

  6. 6
    K: 12.0 mg/L × 25 mL = 300 µg K in 5 g soil; 300 ÷ 5 = 60 mg K/kg
  7. 7

    K: 60 × 2.24 = 134.4 kg K/ha; as K₂O = 134.4 × 1.205 = 162 kg K₂O/ha

Available N ≈ 251 kg/ha; available P = 32 mg/kg (≈ 164 kg P₂O₅/ha); available K = 60 mg/kg (≈ 162 kg K₂O/ha)

ACCEPTANCE CRITERIA

What result passes

  • Results are interpreted against the soil-test rating chart used by your state or national soil-testing programme, then converted into fertiliser recommendations.
  • For available N by alkaline permanganate, a commonly quoted Indian rating is below about 280 kg/ha low, 280–560 kg/ha medium and above 560 kg/ha high. Check your programme's current chart for P and K classes.
METHOD VALIDATION

Validation parameters (ICH Q2(R2))

ParameterTypical requirement
LinearityP and K calibration curves over the working range
AccuracyReference soils and proficiency-testing samples; N recovery from ammonium sulphate
PrecisionDuplicates and reference-soil control charts for each nutrient
LOQFrom replicate reagent blanks
SelectivityCharcoal or blank correction for coloured Olsen extracts; matrix-matched K standards
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