Unit 3: Chemical Analysis of Primary Nutrients in Organic Manures

I. Orientation: Basis of Nutrient Analysis

Chemical analysis of organic manures measures the concentration of plant nutrients in a complex material containing organic matter, moisture, ash, and mineral salts. The sample is first made representative, then dried, ground, digested, and analysed by a method suited to each nutrient. Results are commonly reported on an oven-dry basis as percent nutrient or as oxide equivalents such as P₂O₅ and K₂O.

• Representative sampling: Collect portions from several locations in the manure heap, mix thoroughly, and reduce the composite sample before laboratory analysis.
• Moisture correction: Nutrient concentration on a dry basis is calculated from the fresh-sample value using:
  
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    Nutrient (%) on dry basis =
    Nutrient (%) as received × 100 / Dry matter (%)

• Wet digestion: Concentrated acids convert organic nutrients into soluble mineral forms. Nitric acid, sulphuric acid, perchloric acid, or hydrogen peroxide may be used according to the nutrient and laboratory protocol.
• Blank correction: Reagents may contain small amounts of nitrogen, phosphorus, sulphur, calcium, or magnesium; a reagent blank is analysed and subtracted.
• Calibration: Instrumental methods require standards prepared in a concentration range covering the sample solution.
• Expression of results: Elemental phosphorus and potassium may be converted to conventional oxide forms:
  
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    P₂O₅ = P × 2.291
    K₂O = K × 1.205

• Quality control: Duplicate samples, certified reference materials, recovery tests, and control charts help detect digestion or measurement errors.

II. Determination of Total Nitrogen in Organic Manures — Kjeldahl Analysis

A. Determination of total nitrogen in organic manures

The Kjeldahl method determines nitrogen by digesting the manure with concentrated sulphuric acid, converting organic nitrogen into ammonium sulphate, liberating ammonia with alkali, and measuring the ammonia by distillation and titration.

• Principle: During digestion, organic nitrogen is converted mainly into ammonium ions:
  
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    Organic-N → (NH₄)₂SO₄

  
  Potassium sulphate raises the boiling point, while a copper or selenium catalyst accelerates oxidation.
• Digestion: Digest a known mass, commonly 0.5–1.0 g, with concentrated H₂SO₄ until the solution becomes clear or pale green. The clear digest contains ammonium sulphate.
• Alkalisation: Add excess sodium hydroxide to convert ammonium ions into ammonia:
  
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    NH₄⁺ + OH⁻ → NH₃ + H₂O

• Distillation and absorption: Distil the liberated NH₃ into boric acid solution. The absorbed ammonia is titrated with standard hydrochloric or sulphuric acid.
• Calculation: If V is the blank-corrected acid volume in millilitres, N is acid normality, 14.007 is the atomic mass of nitrogen, and W is sample mass in grams:
  
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    Nitrogen (%) =
    (V × N × 14.007 × 100) / (1000 × W)

• Worked example: For W = 0.500 g, V = 12.0 mL, and N = 0.0100:
  
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    N (%) = (12.0 × 0.0100 × 14.007 × 100) / (1000 × 0.500)
          = 0.336%

• Nitrogen forms: Conventional Kjeldahl nitrogen includes organic nitrogen and ammonium nitrogen, but normally excludes nitrate and nitrite. A salicylic-acid or reduction modification is needed when oxidised nitrogen forms are significant.
• Sources of error: Incomplete digestion, ammonia loss before absorption, insufficient alkali, and inaccurate standardisation of the titrant cause low or high results.

B. Applications and limitations

Kjeldahl analysis is widely used for manure quality assessment because most manure nitrogen is organic or ammoniacal.

• Application: It is suitable for farmyard manure, compost, poultry litter, green manure, and animal waste after homogenisation.
• Reporting: State whether the result is on an as-received or dry-weight basis; a manure containing 0.336% N on dry matter cannot be directly compared with a fresh basis value.
• Limitation: Volatile ammonia may be lost during storage, drying, or alkaline distillation, so samples should be sealed and analysed promptly.
• Safety: Concentrated sulphuric acid and hot digestion mixtures require a fume hood, acid-resistant vessels, protective clothing, and controlled cooling.

III. Determination of Total Phosphorus in Organic Manures — Digestion and Measurement

A. Determination of total phosphorus in organic manures

Total phosphorus is measured after oxidising organic matter and converting all phosphorus compounds into soluble orthophosphate, followed by colorimetric, gravimetric, or instrumental measurement.

• Principle: Acid digestion transforms organic phosphorus into orthophosphate:
  
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    Organic-P → PO₄³⁻

  
  The digest is filtered or diluted, and phosphorus is determined in an aliquot.
• Digestion: Digest a known sample mass with nitric acid, or with a nitric-perchloric acid mixture where permitted. Complete destruction of organic matter is required for total phosphorus.
• Color development: In the molybdenum blue method, orthophosphate reacts with ammonium molybdate in an acidic medium to form a phosphomolybdate complex, which is reduced to a blue-coloured compound.
• Measurement: Measure absorbance using a spectrophotometer, commonly near 880 nm, against reagent blank and phosphate standards.
• Calibration: Prepare standards from a known phosphate stock solution. Plot absorbance against phosphorus concentration and use the calibration equation to obtain the sample concentration.
• Calculation: If C is phosphorus concentration in the final solution in mg L⁻¹, V is final volume in litres, D is any dilution factor, and W is sample mass in milligrams:
  
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    P (%) = (C × V × D × 100) / (W × 1000)

• Oxide conversion: Agricultural reports often express phosphorus as P₂O₅:
  
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    P₂O₅ (%) = P (%) × 2.291

• Alternative method: Phosphorus may be precipitated as ammonium phosphomolybdate or magnesium ammonium phosphate and weighed, but gravimetric procedures are slower and require careful precipitate washing and drying.

B. Applications and limitations

Phosphorus analysis indicates the manure’s contribution to crop phosphorus supply and helps prevent excessive phosphorus accumulation in soil.

• Application: Poultry manure and composted livestock manure may contain appreciable phosphorus because of feed residues, bedding ash, and concentrated excreta.
• Interference control: Arsenate, silicate, high iron, and turbidity can interfere with molybdenum colour development; digestion blanks and matrix-matched standards reduce this risk.
• Result interpretation: A result of 0.50% P corresponds approximately to:
  
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    P₂O₅ = 0.50 × 2.291 = 1.146%

• Limitation: Total phosphorus does not show immediately available phosphate, organic phosphorus, or water-soluble phosphorus separately. Fractionation requires additional extraction procedures.

IV. Determination of Total Potassium in Organic Manures — Flame Emission

A. Determination of total potassium in organic manures

Total potassium is determined by extracting potassium from an acid-digested manure solution and measuring its flame emission intensity with a flame photometer or atomic absorption spectrometer.

• Principle: Potassium atoms introduced into a flame are excited and emit light at a characteristic wavelength, approximately 766.5 nm. Emission intensity is related to potassium concentration.
• Digestion: Digest the dried, ground manure with nitric acid or another validated acid mixture. Dilute the clear digest to a known volume and filter insoluble particles.
• Calibration: Aspirate potassium standards, for example 0, 5, 10, 20, and 40 mg L⁻¹ K, and prepare a calibration curve:
  
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    Emission signal = slope × K concentration + intercept

• Calculation: If C is potassium concentration in the measured solution, V is final volume in litres, D is dilution factor, and W is sample mass in grams:
  
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    K (%) = (C × V × D) / (10 × W)

  
  when C is expressed in mg L⁻¹.
• Oxide conversion: Potassium fertilizer recommendations commonly use K₂O:
  
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    K₂O (%) = K (%) × 1.205

• Worked example: If a diluted sample contains 20 mg L⁻¹ K, final volume is 0.100 L, dilution factor is 10, and sample mass is 1.00 g:
  
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    K (%) = (20 × 0.100 × 10) / (10 × 1.00) = 2.00%

B. Applications and limitations

Potassium measurement is important because potassium salts in manure are generally more soluble than organically bound nitrogen and phosphorus.

• Application: It is useful for comparing nutrient values of cattle manure, poultry litter, compost, and digestate.
• Interference: Sodium and other easily ionised elements may affect flame emission. Instrument settings, ionisation buffers, and appropriate dilution help control matrix effects.
• Sample preparation: Ash-rich samples may produce solutions with high salt concentrations; dilution must keep the signal within the calibration range.
• Limitation: Total potassium indicates the quantity present but not the rate at which potassium will be released after soil application. Most soluble potassium is readily available, but the actual field response depends on soil texture, rainfall, and crop removal.

V. Determination of Sulphur in Organic Manures — Sulphate Measurement

A. Determination of sulphur in organic manures

Sulphur is determined after converting organic sulphur to sulphate, which is then measured by gravimetric precipitation or by turbidimetric estimation.

• Principle: Oxidative digestion converts organic sulphur into sulphate:
  
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    Organic-S → SO₄²⁻

  
  Sulphate reacts with barium ions to form insoluble barium sulphate:
  
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    Ba²⁺ + SO₄²⁻ → BaSO₄(s)

• Gravimetric method: Add barium chloride to the acidified digest, allow BaSO₄ to mature, filter it through ashless paper or a sintered crucible, wash, ignite, and weigh.
• Calculation: If m is the mass of BaSO₄ in grams and W is sample mass in grams:
  
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    S (%) = (m × 32.065 × 100) / (233.39 × W)

  
  32.065 is the atomic mass of sulphur and 233.39 is the molar mass of BaSO₄.
• Turbidimetric method: Form a fine BaSO₄ suspension and measure turbidity, usually near 420 nm, against sulphate standards. The method is faster but depends strongly on particle size and mixing time.
• Result expression: Report elemental sulphur as % S; do not confuse sulphur with sulphate. Sulphate contains only about 33.5% sulphur by mass.

B. Applications and limitations

Sulphur analysis identifies the manure’s potential contribution to sulphur nutrition and helps evaluate materials containing gypsum, sulphide residues, or protein-rich organic matter.

• Application: Compost made from plant residues may supply sulphur through decomposing proteins, while some amendments supply sulphur mainly as sulphate.
• Interference control: High organic colour, suspended solids, and phosphate can affect turbidimetric readings; a clear digest and reagent blank are essential.
• Limitation: Total sulphur does not distinguish sulphate, sulphide, elemental sulphur, and organically bound forms. Their oxidation and plant availability differ substantially.
• Environmental relevance: Excess sulphur salts can increase soil electrical conductivity, so sulphur results should be considered with manure salinity and moisture data.

VI. Determination of Calcium and Magnesium in Organic Manures — EDTA Complexometry

A. Determination of calcium and magnesium in organic manures

Calcium and magnesium are determined in an acid digest by complexometric titration with EDTA, either separately or together.

• Principle: Ethylenediaminetetraacetic acid, abbreviated EDTA, forms stable 1:1 complexes with calcium and magnesium:
  
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    Ca²⁺ + EDTA⁴⁻ → Ca-EDTA²⁻
    Mg²⁺ + EDTA⁴⁻ → Mg-EDTA²⁻

• Sample digestion: Digest the manure with nitric acid, dilute to a known volume, and filter. The digest should contain calcium and magnesium in soluble ionic form.
• Total calcium plus magnesium: Buffer an aliquot at approximately pH 10 with ammonium chloride-ammonia buffer and use Eriochrome Black T. The endpoint changes from wine-red to blue.
• Calcium alone: Raise the aliquot to about pH 12 so magnesium precipitates as magnesium hydroxide, then titrate calcium using murexide or calcon indicator.
• Magnesium calculation: Determine total (Ca + Mg) first, then determine calcium separately:
  
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    Mg amount = Total (Ca + Mg) amount − Ca amount

• Calculation: If V is EDTA volume in litres, M is EDTA molarity, 40.078 is calcium atomic mass, and W is sample mass in grams:
  
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    Ca (%) = (V × M × 40.078 × 100) / W

  
  The magnesium calculation uses 24.305, its atomic mass, with the corresponding EDTA volume.
• Instrumental alternative: Atomic absorption spectrometry can measure calcium and magnesium directly after dilution, often providing better selectivity for complex manure digests.

B. Applications and limitations

Calcium and magnesium results describe the liming and secondary-nutrient value of manure while also helping assess salt and ash composition.

• Application: Calcium-rich manure or compost may contribute to soil calcium and reduce exchangeable aluminium toxicity, whereas magnesium supports chlorophyll formation and enzyme activity.
• Interference: Iron, manganese, copper, and high phosphate can interfere with EDTA titration; masking agents, selective pH control, or instrumental confirmation may be necessary.
• Endpoint requirement: The indicator colour change must be observed under consistent lighting, and the EDTA solution must be standardised against a primary calcium carbonate standard.
• Limitation: Total calcium and magnesium do not equal immediately exchangeable or plant-available fractions. Availability depends on carbonate content, particle size, decomposition, and soil pH.
• Reporting convention: State results as elemental Ca and Mg, preferably on a dry-weight basis, and include the digestion and analytical method used.